Tesis Doctorals - Departament - Física Quàntica i Astrofísica

URI permanent per a aquesta col·leccióhttps://hdl.handle.net/2445/103124

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Transient sources with LST-1: study of novae, supernovae, and gamma-ray bursts
(Universitat de Barcelona, 2025-09-23) Aguasca i Cabot, Arnau; Bordas Coma, Pol; Ribó Gomis, Marc; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] The branch of astrophysics that studies transient phenomena allows the exploration of astronomical sources that show temporal variability that can extend from a few fractions of a second to scales as long as tens of years. These transient events can give rise to a wide variety of phenomena observed throughout the electromagnetic spectrum. In some cases, these events may even be associated with the detection of gravitational waves or the emission of neutrinos. The detection of transient and highly energetic electromagnetic emission in the gamma-ray range (with energies above 100 MeV) is usually related to drastic changes in the environment of the system that emits this radiation. In some cases, these transitory phenomena can be produced due to their partial or total destruction. During these episodes, the source can become extremely luminous, becoming one of the brightest in the sky, even if it is located at cosmological distances. Despite advances in this field of research in recent years, the mechanisms that produce the emission of gamma rays from transient sources remain enigmatic, regardless of the nature of the source that produces them. Gamma rays in the energy range from tens of GeV to hundreds of TeV can be detected from the ground with Cherenkov telescopes. This Thesis is framed precisely in the study of transient sources using the first prototype of the new generation of Cherenkov telescopes, which will be part of the future Cherenkov Telescope Array Observatory (CTAO). The CTAO will be made up of different types of telescopes distributed over a large area, forming an array of Cherenkov telescopes in order to detect gamma rays in a wide range of energies. CTAO's largest telescopes, called Large-Sized Telescopes (LSTs), are optimized to detect relatively low-energy gamma rays of a few tens of GeV. The first of these telescopes (LST-1) is in the commissioning phase, although at the same time it is already beginning to give the first scientific results. In addition, there are currently three LSTs under construction. This thesis focuses on the study of transient sources in the range of very high energy gamma rays (100 GeV < E < 100 TeV). A detailed study of nova and supernova explosions (SNe) and gamma-ray bursts (GRBs) has been carried out. We studied the emission produced in the RS Ophiuchi source, the first nova explosion ever detected in the very high energy range. We analyzed data obtained concurrently with LST-1 and the Fermi-LAT telescope during this event. We have modeled its emission using the most complete gamma-ray spectrum ever obtained by an event of these characteristics, which includes in addition to the observations taken with LST1 and Fermi-LAT, the data obtained with the Cherenkov MAGIC and H.E.S.S. telescopes. We have also evaluated the prospects for future detections of new explosions with the CTAO, highlighting the excellent capabilities of LSTs to detect this type of event and restricting the relevant physical parameters involved in gamma-ray emission. We searched for very high-energy gamma-ray emission in two nearby supernovae produced during the collapse of a very massive star, labeled SN 2023ixf and SN 2024bch. These supernova explosions were observed with LST-1 and the MAGIC telescopes a few weeks after their discovery. Although neither of the two sources has been detected, we have been able to obtain upper limits to their possible flow of very high-energy gamma rays, emphasizing the strengths and limitations that observations in this range can offer for the understanding of the physical phenomena that give rise to this type of explosion. We studied the very high-energy gamma-ray emission of GRB 221009A, known as the brightest GRB of all time. Observations obtained with LST-1 show an excess of gamma rays with a statistical significance of 4.1σ during the first observations. We have restricted its emission of very high-energy gamma rays and compared it with several theoretical models that can consistently explain the multi-wavelength data obtained with different telescopes for this event. Our results can help resolve the current degeneration between some of these theoretical models compatible with the broadband emission of this event. The results obtained by these three types of sources highlight the excellent capabilities of LSTs for the study of transient gamma-ray sources. In addition, these results provide a first taste of the potential of CTAO to study these types of events.
Beyond Standard Neuthreenos. Fundamental physics in the neutrino sector from three perspectives: oscillations, astrophysics and cosmology
(Universitat de Barcelona, 2025-09-15) Bertólez Martínez, Antoni; Salvadó Serra, Jordi; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] If we look at the matter that forms the Universe with the highest level of detail we have reached, we find fundamental particles. The physical theory that collects these particles, like the “periodic table” did for the chemical elements, and that describes how they interact is the Standard Model (SM) of Particle Physics. Of all the fundamental particles of the SM, there are three of them that we cannot yet fully explain: neutrinos. According to the SM, neutrinos are massless particles. That is, none of the particles that we know is capable of providing mass to the neutrinos, as the Higgs field does with the rest of the particles of the SM. However, in recent decades we have dis- covered and experimentally tested –with a precision that leaves no room for doubt– that neutrinos do have mass. These experiments have measured the so-called neutrino oscillations. These are the first direct experimental proof that the SM, or fundamental particle physics as we know it, is incomplete. That is, new particles or forces are required to explain why neutrinos have mass. This, however, is not the only shortcoming of the SM. There are phenomena, such as Dark Matter (DM) and Dark Energy (DE), that the SM can hardly explain, and that generally require physics Beyond the Standard Model (BSM): new particles or new forces. While the scientific community has thought of many BSM extensions that could solve these shortcomings, as of today the search for them has not given positive results. Since neutrinos are the first particles that have experimentally warned us that the SM is in- complete, why not use them to find out exactly what is missing? This thesis is an effort in this direction. Specifically, it is a compilation of three ways –different and complementary– in which we can use neutrinos to discover what physics is beyond the SM. In the first part, we delve into neutrino oscillations and, specifically, in the search for sterile neutrinos in low-energy experiments. We review the wave packet description and demonstrate that –for our current experimental knowledge– the wave packet width could affect the results of the searches. In the second part, we move from low energies to the highest energies we know, those of cosmogenic neutrinos. These are neutrinos which are produced by the most energetic particles we know of, ultra-high- energy (UHE) Cosmic Rays. We reanalyze the anomalous detections of UHE neutrinos by the ANITA-IV antenna and analyze how future and present neutrino telescopes could restrict possible BSM extensions. Finally, in the third part, we look at how to use cosmology to calculate the mass of the relic neutrinos of the Universe, as well as the robustness of the method against BSM. Furthermore, we use Big Bang Nucleosynthesis to constrain interactions between neutrinos and ultralight DM. Thus, this thesis belongs to the phenomenology of particle physics and the search Chapter 0 | CONTENTS for BSM physics, specifically in the neutrino sector. Its approach is a bridge between theoretical and experimental physics: the analysis of theoretically motivated models based on their consequences in present and future experiments. Specifically, it aims to better understand the potential of these experiments and to rethink the hypotheses conventionally assumed behind their measurements.
Search and study of massive runaway stars in the Milky Way and impact on high-energy binaries
(Universitat de Barcelona, 2025-10-03) Carretero Castrillo, María del Mar; Ribó Gomis, Marc; Paredes i Poy, Josep Maria; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Massive stars are ubiquitous in many astrophysical phenomena. A significant fraction of massive stars are runaways, displaying high velocities with respect to their environment. Due to their formation scenarios, runaway stars play a relevant role in cluster and binary physics. However, the relative contribution of runaway formation scenarios, their role in massive binary evolution, and their relation with binary products hosting compact objects (COs) are not well constrained. In particular, some of these binaries with CO can be high-energy sources. Although some studies have been conducted in the Magellanic Clouds, the massive runaway star population in the Milky Way remains poorly explored. The Data Release 3 (DR3) of the Gaia mission of the European Space Agency (ESA) has provided data with unprecedented astrometric precision, allowing us to derive accurate distances and proper motions that are key to identify runaway stars in the Milky Way. The central aim of this thesis is to improve the understanding of the massive runaway star population in the Milky Way. A secondary aim is to study their connection with high-energy binaries. To achieve these objectives, we developed a novel and self-consistent method to identify runaway stars which avoids the use of velocity thresholds, in contrast to previous works. This allows us to detect runaway stars as well as walkaway stars. The characterization of the runaway stars was done with Gaia-DR3 data, complemented with multiwave- length archival data, dedicated radio observations, and high-resolution IACOB spectroscopic information. We identified 106 and 69 O- and Be-type Galactic massive runaway stars, with half of them being new discoveries. We characterized their spatial and velocity distributions in the Galaxy. We found higher velocities and runaway fractions for the O-type stars, in agreement with predictions for dynamical ejections from clusters. This work resulted in the compilation of two runaway catalogs, including precise distance and velocity estimates. In addition, we studied the interaction of these runaway stars with the interstellar medium (ISM) using WISE data, discovered new stellar bow shocks and bubbles, and derived ISM densities around them. Radio searches around these bow shocks resulted in no new detections, although the results obtained served to model the nature of their potential radio emission. We conducted a large and unprecedented observational study of O-type runaway stars in the Milky Way in space and rotational velocities, as well as in binarity classifications, obtained in a homogeneous way. To this end, we combined our runaway star catalogs with IACOB spectroscopic data. We found that most runaway stars appear to be single, confirming theoretical expectations, while most are slow rotators. The trends observed in the investigated parameter space were used to identify potential imprints of the runaway ejection mecha- nisms in the studied Galactic runaway population, offering valuable constraints on models about runaway origins. We found an interesting sample of runaway binary systems, some of which are high-mass X-ray binaries (HMXBs) or gamma-ray binaries. In particular, we find an overabundance of runaways among the known gamma-ray bina- ries. Within the O-type runaway star sample, we identified some runaway single-lined spectroscopic binary systems, with half of them being HMXBs or candidates to host black holes. The latter exhibit similar properties to the HMXBs within our investigated parameter space. Altogether, these runaway binary systems represent a rich sample for potentially hosting COs. We also searched for very-high-energy (VHE) gamma-ray emission from the HMXB Cygnus X-3 with the MAGIC Telescopes. We analyzed the largest VHE data set and found no significant emission. We provided the most stringent constraints on the VHE flux to date, which could offer insights into the system properties. In conclusion, this thesis has deepened the knowledge of massive runaway stars in our Galaxy. It provides robust runaway star classifications, new empir- ical constraints on runaway formation mechanisms, and interesting runaway binaries potentially hosting COs, which could eventually show high-energy emission. Since massive runaway stars are crucial to many astrophysical phe- nomena, all catalogs compiled in this thesis are openly published to broaden the research impact beyond its immediate scope. Future larger catalogs of massive stars and Gaia DR4 will enable a more comprehensive study of runaway stars, which could allow the discovery of new high-energy binaries through multiwavelength studies, and potentially refine the observed trends, binary evolution models, and ultimately, the understanding of massive binary interaction.
X-ray Signatures of Particle Escape from Pulsar Wind Nebulae
(Universitat de Barcelona, 2024-09-30) Zhang, Xiying; Bordas Coma, Pol; Bosch i Ramon, Valentí; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] This PhD thesis aims at understanding the particle escape and subsequent emission processes taking place in Bow-shock Pulsar Wind Nebulae (BSPWNe), trying to connect a) the observational findings of large X-ray jet-like structures resolved in some of these systems; b) the phenomenology associated to the recently discovered gamma-ray TeV halos, and c) the prediction of similar extended structures in the radio band. To this end, I have studied in detail three BSPWN with prominent extended emission features detected in X-rays showing evidences of such particle escape mechanism. For the first source, PWN J1135-6055, archival Chandra observations have revealed a compact nebula as well as two lateral jet-like structures displaying a highly asymmetric geometry. The origin of these structures is discussed in different scenarios, either considering the development of ”kinetic jets” from particle escape processes, or the presence of ”true pulsar jets” bent by the external medium pressure. The properties of PSR J1135-6055 are reminiscent of those of the Geminga PWN, one of the few BSPWN with a confirmed TeV halo structure. For the second source, PWN B1853+01 located inside SNR W44, a rich X-ray data set composed of archival data from Chandra, XMM-Newton and NuSTAR is analysed. The results reveal the presence of a cometary tail trailing the pulsar, a spectrally hard antennae-like feature ahead of the pulsar, as well as a possible ”X-ray Halo” around the PWN. These findings are discussed and compared with the PWN in G327.1-1.1, known to display also both antennaeand halo-like extended emission in X-rays. PWN in G327.1-1.1 is also known to have a TeV counterpart. For the third source, IGR J11014-6103 (the ”Lighthouse Nebula”), the possibility to detect extended radio emission produced by the particle population powering the jet-like structures detected in X-rays is investigated. If relativistic particles with moderate Lorentz factors, γ ≳ 105, are able to escape the bowshock region, such extended ”Radio Halo”-like structures could be detected with current instruments. This study provides therefore an observational test allowing to constrain the escape mechanism thought to operate in BSPWNe. The results of this thesis provide new constraints on the particle escape mechanism( s) operating in BSPWNe, and argue for a common mechanism leading to the extended features around BSPWNe observed in the X-ray and gamma-ray energy band, and potentially detectable also at radio energies.
La evolución química del disco de la Galaxia a partir de los cúmulos estelares
(Universitat de Barcelona, 2024-06-11) Carbajo Hijarrubia, Juan; Carrera Jiménez, Ricardo; Casamiquela, Laia; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[spa] En esta tesis, estudiamos la evolución química del disco galáctico utilizando cúmulos abiertos como trazadores. Los cúmulos, formados por estrellas nacidas durante el mismo proceso de formación estelar, se mantienen ligados gravitatoriamente. Tienen la ventaja de que podemos medir con precisión su distancia y edad, propiedades difíciles de determinar en estrellas de campo. El estudio se enmarca en el proyecto "Open Clusters Chemical Abundance from Spanish Observatories" (OCCASO), dedicado a medir abundancias elementales precisas. Adquirimos espectros de alta resolución R > 60 000 cubriendo un amplio rango de longitudes de onda, 400 a 900\,nm mediante los instrumentos FIES, HERMES y CAFE en los telescopios NOT, Mercator y CAHA2.2, respectivamente. Hasta la 2023 se han realizado 38 campañas de observación, llevadas a cabo 21 de ellas en el transcurso de esta tesis. Esto ha supuesto aumentar la cantidad de cúmulos observados de 18 a 71. La muestra abarca un amplio rango de edades (100 Ma - 7.25 Ga) y radio galactocéntrico (6.4 - 11.7 kpc). Adicionalmente, se estudiaron dos cúmulos en las regiones externas del disco galáctico utilizando el espectrógrafo MEGARA en GTC. Obtenemos velocidades radiales precisas mediante correlación cruzada con espectros sintéticos de 49 cúmulos (47 pertenecientes a OCCASO y dos observados con MEGARA). La combinación de estas velocidades con la astrometría de Gaia nos permite determinar la cinemática y las órbitas de los cúmulos, encontrando que son compatibles con la cinemática del disco galáctico delgado. El análisis de parámetros atmosféricos y abundancias químicas se efectúa empleando dos métodos: síntesis espectral mediante iSpec y anchura equivalente mediante DAOSPEC y GALA. Determinamos abundancias químicas 25 elementos logrando precisiones en torno a 0.05 dex. Las desviaciones estándar de las abundancias de los cúmulos se encuentran en este mismo rango, denotando homogeneidad química. Combinamos la muestra OCCASO con APOGEE, GALAH y GES, estudios de alta resolución (R > 20 000) si han observado al menos cuatro estrellas en el apelotonamiento rojo de un cúmulo. Estudiamos tanto con OCCASO como con la muestra combinada, las dependencias entre abundancia, posición y edad. Los elementos [Mg/Fe] y [Si/Fe] muestran una dependencia positiva con la edad, teniendo algunos cúmulos jóvenes valores inesperadamente altos. Los elementos de pico del Fe muestran dependencias positivas y negativas. La dependencia entre [X/Fe] y edad de los elementos de captura neutrónica depende su proporción de producción mediante proceso s. El gradiente radial [Fe/H] se aplana a partir de 11.3 ± 0.8 kpc. La pendiente de este gradiente depende de la edad, y en los cúmulos jóvenes (<1 Ga) no encontramos que se produzca el aplanamiento. Esto sugiere que el evento que lo produjo, ocurrió hace más de 1\,Ga y apoya la hipótesis de que el aplanamiento se formó por migración radial hacia el exterior. Los gradientes radiales [X/Fe] de los demás elementos muestran dependencias diferentes. Los elementos α y el [Zn/Fe] muestran gradientes positivos, mientras que los demás elementos de pico del Fe tienen gradientes planos. Los elementos de captura neutrónica muestran comportamientos en general positivos. Obtenemos una cota superior a la dependencia de estos gradientes con la edad, estando muy por debajo de la dependencia que encontramos en el [Fe/H]. El resultado es compatible con que no haya dependencia. Estudiamos el gradiente azimutal, encontrando indicios de una dependencia positiva para cúmulos con edades entre 2 y 7.3 Ga. En el análisis de cúmulos en el disco externo con MEGARA, encontramos valores de [Fe/H] inferiores a lo esperado para su radio galactocéntrico, lo que sugiere que puedan no estar afectados por el aplanamiento del gradiente. Sin embargo, debemos cerciorarnos de que sus valores no se deben a diferencias sistemáticas con OCCASO.
De-noising of gravitational-wave data: the rROF method in the cWB data analysis pipeline
(Universitat de Barcelona, 2024-01-19) Barneo González, Pablo José; Portell i de Mora, Jordi; Figueras Siñol, Francesca; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Since the first experimental evidence for the existence of gravitational waves in 2015, the amount of data in this scientific area has increased enormously. There has also been a great deal of interest in the scientific community in gravitational waves. The interferometers, used to capture these waves, need to achieve a high level of instrumental sensitivity to be able to detect and analyse the weak signals emitted by both distant sources of intrinsically high intensity and nearby sources of much lower intensity. High sensitivity is often accompanied by high levels of noise that difficult data analysis. In nowadays interferometers, large amounts of data are recorded with a high percentage of noise from which we attempt to extract the possible gravitational waves buried therein. In this dissertation we propose to use a denoising method based on the minimisation of the total variance of the time series that constitute the data. Known as the ROF method, it assumes that the largest contribution to the total variance of a function comes from noise. In this way, a minimisation of this variance should lead to a drastic reduction in the presence of noise. This denoising procedure should help to improve the detection and data quality of gravitational wave analysis. We have implemented two ROF-based denoising algorithms in a commonly used gravitational-wave analysis software package. The analysis package is known as coherent WaveBurst (cWB) and uses the excess energy from the coherence between data from two or more interferometers to find gravitational waves. The denoising methods are the one-step regularised ROF (rROF), and the iterative rROF procedure (irROF). The latter is designed as an improvement of the former for those cases where the noise cleaning is excessive and extracts a portion of the signal in an unrecoverable way. We have tested both methods using events from the gravitational-wave catalogue of the first three observing periods of the LIGO-Virgo-KAGRA scientific collaboration. These events, named GW1501914, GW151226, GW170817 and GW190521, comprise different wave morphologies of compact binary systems injected at different noise quality levels. We can see that the analysis of these wavelets with the rROF method is defective as it incorrectly extracts a portion of the signal at the high frequencies. However, the use of the irROF enhancement procedure effectively removes the noise while preserving nearly intact the wavelet function of the signals, providing a significant increase in the signalto- noise ratio values. One of our goals has been to use the irROF denoising method during a data collection period to support on-the-fly signal detection. To this end, we have extended our study by characterising the background noise of one week of data after the application of the irROF method. We have calculated and analysed the detection efficiencies of a selection of signals mimicking various types of gravitational waves. The results obtained so far do not support the effect found in the analysis of individual gravitational waves. However, we have found that further improvements and variations of the irROF denoising method could improve the detection efficiencies. Our work demonstrates that, although the irROF method applied to a period of data does not improve the detection achieved using methods that treat individual wavelets, this improvement can be achieved by further developing and fine-tuning some of the strategies proposed here. The methodology presented here can be used in the implementation of other denoising methods currently in use or under development. The present work provides a set of suggestions and proposals that will allow to increase the detection of these gravitational waves.
Dynamical characterisation of the Magellanic Clouds with Gaia data and the KRATOS simulations
(Universitat de Barcelona, 2024-04-08) Jiménez Arranz, Óscar; Romero Gómez, Mercè; Luri Carrascoso, Xavier; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Els Núvols de Magalhães són les galàxies més visibles a simple vista en el cel nocturn, ja que estan molt a prop de la nostra galàxia i són les més grans del seu entorn. A causa de la seva proximitat, el Gran i el Petit Núvol de Magalhães (LMC i SMC) proporcionen als astrònoms una finestra única per a l'estudi de les complexitats dels sistemes galàctics en interacció, gràcies a la gran quantitat de característiques observables en aquestes galàxies. Situada a una distància d'aproximadament 50 kpc, la peculiar LMC presenta un únic braç espiral i una barra estel·lar asimètrica. La SMC sempre s'ha considerat un satèl·lit de la LMC a causa de la seva proximitat. Es troba a uns 62 kpc de la Via Làctia i a una distància de 20-25 kpc de la LMC. La SMC és una galàxia irregular nana i rica en gas. Amb el llançament de la missió espacial Gaia el 2013, s'ha inaugurat una nova era en l'observació astronòmica, permetent l'estudi detallat de milers de milions d'estrelles. L'objectiu científic principal de Gaia és cartografiar i estudiar les estrelles de la Via Làctia. No obstant això, Gaia recull informació de totes les fonts de llum, la qual cosa inclou també les estrelles dels Núvols de Magalhães, així com asteroides, binàries no resoltes o quàsars. Un element destacat d'aquesta tesi és l'anàlisi cinemàtic del disc de la LMC utilitzant dades de Gaia, on es van obtenir mostres netes (eliminant la contaminació de la Via Làctia) i es van presentar els primers mapes i perfils de velocitat en 3D. Això va ajudar a definir noves restriccions sobre la velocitat de rotació de la barra. Seguint l'èxit del classificador per a la LMC, es va entrenar una altra xarxa neuronal per a separar també les estrelles de la SMC de la contaminació de la Via Làctia. No obstant això, l'estudi de la formació del sistema LMC-SMC no es pot dur a terme només amb observacions, ja que donen una imatge estàtica del procés. També és necessari fer servir simulacions numèriques, típicament centrades a recrear la distribució de gas i corrents estel·lars de l’halo i no en la cinemàtica interna dels Núvols. Aquesta tesi inclou la presentació de KRATOS, 28 simulacions de N-cossos (d'accés lliure) de galàxies tipus LMC i SMC, que possibiliten l'estudi de la formació de subestructures en el disc de la LMC després de la interacció amb la SMC, mitjançant la seva comparació amb les observacions
Disentangling the formation path of lenticular galaxies
(Universitat de Barcelona, 2024-04-12) Tous Mayol, Josep Lluís; Solanes, José M. (José María); Perea Duarte, Jaime D.; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Astronomers have known of the existence of lenticular galaxies (S0) almost as long as they have known that there are other galaxies besides the Milky Way. While it is accepted that spiral galaxies emerge from the collapse of primordial vast clouds of gas and that when they merge they often give place to elliptical galaxies, the origin of S0 galaxies is still subject to debate. A clear bimodality in the properties of these galaxies, which are relatively abundant in low- and high-density environments, suggests that multiple formation mechanisms may be at play. In the field or in small groups, S0 galaxies can evolve from mergers. Conversely, hydrodynamic interactions are expected to transform spirals into S0s within large galaxy aggregations. This thesis is devoted to reviewing the main properties of galaxies classified as S0. Our goal is to gather abundant and robust information about relevant parameters of this poorly understood morphological type and their possible dependence on the environment to constrain their formation. For the first time, we explore the entire optical spectrum of these galaxies, seeking clues to infer their evolution. We start the exploration with single-fibre spectra from the SDSS of a sample of ∼70,000 nearby S0s and their global properties. A principal component analysis (PCA) is used to reduce the high complexity of the spectral data through its projections into a low-dimensional space, thereby facilitating a bias-free, machine-learning-based classification of the galaxies. The procedure reveals that the S0 population consists of two main classes with statistically inconsistent properties. While the bulk population is made by passive lenticulars with inactive spectra, the other is characterized by active galaxies that, despite their early-type morphology, show star formation rates that can be similar to those observed in late-type spirals. The main ionisation source of the active galaxies is star formation. However, in the Seyfert and LINER S0 systems detected in radio and X-ray, activity is driven, respectively, by nuclear accretion and post-AGB stars. Applied to spatially resolved spectra from MaNGA, the PCA can be used to study the radial configuration of activity in the galaxies. The extension of the PCA to these spectra leads us to identify star-forming rings in S0 galaxies typically associated with a positive activity gradient, and assemble the largest catalogue of these objects ever identified through this kind of data. Assessment of the rings indicates that they are relatively abundant (∼30%) in fully-formed S0s with a frequency that sharply increases with the mass of the hosts, but are uncorrelated with the environment. Rings are twice more frequent among the members of the passive class than in the active, and likely feed on residual gas from the disc. These results link the rings with the capture by the S0s of tiny dwarf satellites that closely orbit them. Finally, we examine the radial activity profiles of S0s as a function of their properties. The comparison reveals that the radial activity gradient of these galaxies is tightly related to their PCA and BPT classifications, and star formation status. The passive class often shows low-level, flat activity profiles, while their active counterparts generally have negative activity gradients, associated with high specific star formation rates. Altogether, our results support a scenario where minor mergers actively play a rejuvenation role in the recent evolution of S0 galaxies, while clusters operate in the opposite direction by quenching their activity. The framework we have developed provides a unified picture of activity in S0 galaxies in the optical domain. Combined with physical quantities and line diagnostics, the framework is a valuable tool for interpreting key trends in S0s that should be transferable to other morphologies.
Towards a non-perturbative description of cosmological inflation
(Universitat de Barcelona, 2023-12-13) Cruces Mateo, Diego; Germani, Cristiano; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] From the observation of the cosmic microwave background radiation (CMBR) we know that the universe is isotropic on scales larger than ∼ 75 Megaparsec to at least one part in 100000. The leading paradigm for explaining this observational data is a period of accelerated expansion in the earliest stages of the universe, called cosmological inflation. Inflation also provides a causal mechanism for generating anisotropies on cosmological scales. These anisotropies result from the amplification of the unavoidable vacuum quantum excitations of the gravitational and matter fields due to the accelerated expansion. In particular, it is possible to explain the almost scale invariant power spectrum of the CMBR temperature map with a very simple inflationary regime called Slow Roll (SR) inflation. Quantum fluctuations are usually assumed to be small enough such that they are well described with cosmological perturbation theory. However rare large quantum fluctuations can also be randomly generated during inflation. These large inhomogeneities can lead to the formation of Primordial Black Holes (PBH) and the probability of its generation is related with the amplitude of the power spectrum. Both the amplitude of the power spectrum and the non-gaussianities measured at the CMBR are too small to generate a relevant amount of large fluctuations. To form enough PBH we need to exponentially enhance the amplitude of the power spectrum on scales which are not probed by the CMBR, for which a violation of SR is needed. Although the growth of the power spectrum can be described at leading order in perturbation theory, the description of the tail of the Probability Distribution Function (PDF), where the large inhomogeneities are located, must be done in a non-perturbative way. We study two main approaches that aim to describe inflation in a non-perturbative way, the δN formalism and the stochastic approach. Both approaches are based on gradient expansion, which assumes that the effect of quantum fluctuations whose characteristic wavelength is much larger than the Hubble radius is well described by an ensemble of locally homogeneous and isotropic patches, where spatial gradients are negligible, this approach is valid for any amplitude of local over-densities. In this thesis we show that spatial gradients and the momentum constraint of general relativity play an essential role when we use global coordinates, which are necessary to describe the statistical properties of the inhomogeneities by comparing different patches. The δN formalism and the stochastic inflation represent consistent ways of providing initial conditions to gradient expansion. In the δN formalism, initial conditions on the field fluctuations are perturbatively given. The idea of stochastic inflation is fully non-perturbative. In this approach, the long-wavelength part of the field follows the evolution dictated by gradient expansion and the short-wavelength part act as a random noise continuously changing local trajectories such that the cumulative behaviour of small random kicks can induce non-perturbative effects in the local patch. During this thesis we formulate for the first time a stochastic approach to inflation which includes spatial gradients and the momentum constraint and hence it is in principle able to describe the correct long-wavelength dynamics of inflationary inhomogeneities in a non-perturbative way. To test our results, we show that for any inflationary regime, the stochastic inflation so formulated, precisely reproduces the results of perturbative correlators in all regimes in which perturbation theory is supposed to work. We also elucidate that, in order to take into account the whole non-perturbative dynamics of the local patches via stochastic inflation, the noises must be computed over the stochastically corrected local background rather than over the fiducial deterministic global background, as it is typically done in the literature.
Exotic phases of matter in low dimensional lattices: from quantum liquids to kinetic magnetism
(Universitat de Barcelona, 2023-07-10) Morera Navarro, Ivan; Juliá-Díaz, Bruno; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] In this Thesis we explore exotic phases of matter which arise when different mechanisms compete and favor different ground state configurations. We focus on two classes of low dimensional lattice systems. The first class concerns fluids with both repulsive and attractive interactions, which can result in the appearance of quantum liquid phases. The second class corresponds to systems with geometric frustration, where the minimization of kinetic energy is incompatible with the underlying lattice geometry. In the first part of this Thesis, we show how quantum liquid phases emerge in ultracold atomic systems loaded into high one-dimensional optical lattices. We study different microscopic systems, including two-component bosonic mixtures, single-component dipolar systems, and single- component systems with two- and three-body interactions. For the bosonic mixture, we investigate Bogoliubov’s theory in the weakly interacting regime and derive an effective dimerized theory in the strongly inter- acting regime. Both theories capture the liquefaction of the mixture and predict a phase transition to a dimerized gas. We perform large-scale, unbiased tensor network simulations, which we contrast with our theoretical predictions, and we find an excellent agreement between them in different regimes of parameters. Moreover, we characterize the different phase transitions with our numerical simulations. Then, we demonstrate that a single- component dipolar system liquefies in the strongly interacting regime due to the presence of superexchange processes. We provide an effective theory that captures the liquefaction and the formation of self-bound Mott insulators. In addition, we provide the two-particle excitation spectrum and observe that its structure provides insights into the many-body phases. Finally, we study the universality of quantum gases and liquids in one dimension and provide a self-consistent theory to solve a quantum fluid with two- and three-body interactions. We demonstrate that quantum gases exhibit universal properties, while quantum liquids do not, since the equation of state depends on microscopic details of the theory. Moreover, we show that the long-distance tails of quantum droplets exhibit a universal decay. In the second part of this Thesis, we investigate the effects of kinetic frustration on the many-body properties of spin-1/2 fermionic and bosonic systems. Firstly, we study frustrated ladder geometries, such as the zigzag ladder and the square ladder with a perpendicular magnetic flux. For the zigzag ladder, we find that fermionic (bosonic) systems exhibit an effective attractive (repulsive) interaction between holes and spin flips induced solely by kinetic energy and geometric frustration. In the square ladder with a perpendicular magnetic flux, both fermionic and bosonic systems show effective attractive interactions between holes and spin flips. We analyze the formation of multi-body composites and determine their phase diagram. Moreover, we explore the many-body problem where multi-body composites self-organize to form different many-body phases, including a magnetic polaron gas and a pair density wave. We also discuss various protocols for cold atom experiments that could detect these multi-body composites. Secondly, we examine the impact of kinetic frustration on the magnetic properties of a spin-1/2 fermionic system in a triangular lattice at finite and zero temperature. We demonstrate that the system exhibits a magnetic transition as it is doped away from half-filling. Specifically, the system becomes antiferromagnetic (ferromagnetic) when doped below (above) half-filling. At finite temperature, we observe the system’s tendency to form magnetic orders by inspecting the behavior of the magnetic susceptibility. Next, we explore the formation of magnetic many-body phases at low temperatures, including the magnetic polaron gas the antiferromagnetic spin bag phase. The former is characterized by the formation of magnetization plateaus and the latter by binding of holes. Finally, we provide the temperature dependence of the charge-spin-spin correlation function.
Dark energy in quantum field theory: Implications on modern cosmology
(Universitat de Barcelona, 2023-07-06) Moreno Pulido, Cristian; Solà Peracaula, Joan; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] The Cosmological Constant, Λ, has been a controversial element in theoretical physics and cosmology since its introduction by Einstein in his field equations in 1917. Despite maintaining an irregular reputation over the years, the discovery of the accelerated expansion of the Universe at the end of the 20th century confirmed Λ as a major ingredient in the cosmological puzzle and a part of the standard model of cosmology, the ΛCDM model. Although the ΛCDM model fits well with the overall cosmological data, it still faces several theoretical conundrums and observational issues that require urgent attention. The cause of the accelerated expansion is called Dark Energy, which is mathematically modeled by Λ, but its origin is uncertain, despite the fact that it is commonly assumed to be vacuum energy. General theoretical estimates of the vacuum energy density in the context of Quantum Field Theory differ from observations by as much as 123 orders of magnitude in the most severe case. Additionally, attempts to adjust its value by collecting several contributions to the vacuum budget have been unsuccessful and drove to the well-known problem of fine-tuning. This inability to derive the correct observed value of vacuum energy density in the Universe constitutes the Cosmological Constant Problem one of the biggest mysteries that theoretical physics faces. The problem becomes even more significant when considering the fact that Dark Energy and Matter have an energy density of the same order of magnitude at present, despite the fact that Dark Energy have a constant energy density, while Matter dilutes with cosmological expansion. This is known as the Coincidence Problem. If that were not enough, we also have additional problems from a phenomenological perspective. Specifically, there are cosmological tensions between early Universe and local observations affecting two important parameters in the cosmological model. The first one is H0 (the Hubble function or expansion rate at the current time) and the second one is σ8 (related to the structure formation in the Universe). The discrepancies can reach up to ∼ 4 − 5σ and 2 − 3σ, respectively. Inspired by these significant challenges, the work performed under the direction of Prof. Joan Sol`a Peracaula has followed two different but closely related directions. Firstly, we focused on the renormalization and regularization of the vacuum energy density in the context of Quantum Field Theory through a new formalism based on the traditional adiabatic regularization. We obtained significant and noteworthy results regarding the dynamical behavior of the vacuum energy density, which seems to evolve smoothly with the background expansion in terms of the Hubble function, ρvac(H). These results coincide with the so-called Running Vacuum Models (RVM), which have been around for many years. Secondly, with the cosmological tensions in mind, we tested two models related to the previous theoretical investigations against a large set of cosmological data to constrain the cosmological parameters: 1) The Brans-Dicke model consists in a modification of General relativity by promoting the Gravitational Constant to be a scalar degree of freedom. This can be reformulated as an effective picture of General Relativity possessing a dynamical vacuum component, similar to the RVM. 2) The Ricci-RVM is a variation of the more traditional model in which we replace the dependency in H by R, the Ricci scalar. This has some adventages such as not affecting the usual predictions for Big Bang Nucleosynthesis. To summarize, this thesis presents a rigorous investigation of the departure of the cosmological framework from the ΛCDM model, considering the possibility of Dark Energy being a dynamical quantity. We study this possibility from first principles in the context of Quantum Field Theory, obtaining surprising and unprecedented results in the literature. Additionally, we explore two models against different cosmological datasets and scenarios to obtain a more complete perspective. Our fits show promising results suggesting a possible deviation from the ΛCDM model.
Nonlocal Lagrangian Formalism
(Universitat de Barcelona, 2023-03-31) Heredia Pimienta, Carlos; Llosa, Josep; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] This thesis aims to study nonlocal Lagrangians with a finite and an infinite number of degrees of freedom. We obtain an extension of Noether's theorem and Noether's identities for such Lagrangians. We then set up a Hamiltonian formalism for them. Furthermore, we show that rth-order Lagrangians can be treated as a particular case, and the expected results are recovered. Finally, the method developed is applied to different examples: nonlocal harmonic oscillator, p-adic particle, p-adic open string field, and electrodynamics of dispersive media. We divide this thesis into three blocks: In the first block, we review the Lagrangian and Hamiltonian formalism for first-order and rth-order theories with a finite and an infinite number of degrees of freedom. In addition, we examine Noether’s first and second theorem for such Lagrangians, and we show that, by adding a total derivative to them, their equations of motion remain unchanged. For the particular case of rth-order Lagrangians with an infinite number of degrees of freedom, we focus on the Poincaré symmetry. We find the corresponding canonical energy-momentum tensor and the spin current. Furthermore, we indicate how to obtain the so-called Belinfante-Rosenfeld energy-momentum tensor. Finally, we discuss how Lagrangian and Hamiltonian formalisms are affected by extending them to theories of infinite order. In the second block, we present the nonlocal Lagrangian formalism for a finite and an infinite number of degrees of freedom. We begin by introducing what we mean by time evolution. We then establish the principle of least action and obtain their equations of motion. Furthermore, we show that every nonlocal Lagrangian can be written as a nonlocal total derivative (or quadri-divergence for the case of an infinite number of degrees of freedom). Next, we demonstrate that its equations of motion are not identically zero and give an example to visualize this fact. We then find a sufficient asymptotic condition so that the equations of motion are unaffected by introducing a nonlocal total derivative (or quadri-divergence). We extend Noether's first and second theorems for such Lagrangians. For the case of nonlocal Lagrangians with an infinite number of degrees of freedom, as in the first block, we focus on the Poincaré symmetry and obtain the canonical energy-momentum tensor and the spin current. Thanks to this extension, it allows us to politely infer (or propose) (based on the analogy of the local case) a Lagrange transformation to construct a Hamiltonian formalism for them. Finally, in the third block, we exemplify the formalism developed. To do so, we choose four examples: nonlocal harmonic oscillator, p-adic particle, p-adic open string field, and electrodynamics of dispersive media. In the first two, we obtain the Hamiltonian and the symplectic form. As for the example of the p-adic open string field, we obtain the Hamiltonian, the symplectic form, and the components of the canonical and Belinfante-Rosenfeld energy-momentum tensors. For the latter, only the components of the canonical and Belinfante-Rosenfeld energy-momentum tensors.
From the Precision Era towards the Accuracy Era of Cosmology with DESI
(Universitat de Barcelona, 2022-09-14) Brieden, Samuel; Gil-Marín, Héctor; Verde, Licia; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Despite the successes of the cosmological ΛCDM model and having entered the "Precision Era of Cosmology" there are still open questions. The principal model ingredients, ΛCDM, contribute to ~95% of the total energy density of the universe, but their underlying nature is still completely unknown. This lack of understanding is the main science driver behind many experimental and observational missions as well as theoretical eﬀorts within the field of fundamental physics. Furthermore, diﬀerent cosmological observations favor diﬀerent parameter values, where the most famous discrepancy is the up to (depending on the considered dataset) ~5σ "tension" between model-dependent early-time and direct late-time measurements of the Hubble constant H0. The Dark Energy Spectroscopic Instrument (DESI) survey is one of these campaigns. As the name indicates, it was launched to unravel the mystery of dark energy by measuring millions of distant galaxy and quasar spectra to create the largest, three-dimensional map of the large scale structure of the universe ever obtained. From that map, the DESI collaboration aims to extract both the expansion history and the growth rate of structures history throughout cosmic time. The expansion history is obtained via the so-called standard ruler technique: distances (in function of redshift) are measured in units of a characteristic scale, the standard ruler, which is an imprint of the gravity-pressure waves in early universe leading to the so-called baryon acoustic oscillations (BAO). The growth rate of structures is traced by the measurement of the anisotropy of galaxy clustering along and across the line-of-sight, which is induced by the peculiar velocities of galaxies impacting the redshift measurements from their spectra. As a consequence, distances inferred from these redshifts are distorted, hence this eﬀect is called redshift-space distortions (RSD). Both the BAO and RSD observables deliver a pristine probe of the late-time dynamics of the universe. In the first part of this thesis we present a method to blind the galaxy catalogs to mimic diﬀerent BAO and RSD signals. Upcoming DESI data will benefit from blinding in order to remove the impact of confirmation bias on cosmological results. We explore two blinding shifts at the catalog level, perturbing individual galaxy positions within the galaxy clustering catalog along the line of sight. The first one is a purely geometrical shift based on a diﬀerent expansion law. In the second one redshifts are shifted depending on the galaxy density field mimicking RSD with a modified growth rate. We test both blinding shifts by performing BAO and full shape RSD analyses on original and blinded galaxy mocks. In the second part, we elevate the established way how BAO and RSD analyses are performed towards including another observable, the shape of the clustering signal as function of galaxy separations. While the BAO and RSD incorporate the horizontal and vertical information respectively in the clustering signal, the shape captures the "diagonal" information. We find that this technique called ShapeFit is suﬃcient to obtain cosmological constraints as tight as direct model fits to galaxy two-point statistics while preserving the advantages of model-independence of the standard BAO and RSD analyses. Both parts of this thesis stress the importance of model-agnosticism in the context of large surveys and cosmological tensions. They play a crucial role for the DESI survey cosmological analysis providing a road to transition from the "Precision Era" to the "Accuracy Era" of cosmology.
Effective-theory description of heavy-flavored hadrons and their properties in a hot medium
(Universitat de Barcelona, 2022-07-08) Montaña Faiget, Glòria; Ramos Gómez, Àngels; Tolós Rigueiro, Laura; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] For many decades after the conception of the quark model in 1964, and the development of quantum chromodynamics (QCD) a few years later as the theory governing the strong interaction between quarks and gluons, there was no experimental evidence of the existence of hadronic states beyond the quark-antiquark mesons and the three-quark baryons. In the last two decades, however, with the explosion of data in electron–positron and hadron colliders, many states have been observed that do not fit in this picture, especially in the heavy-flavor sector. Evidence of the existence of the so-called exotic hadrons has recently prompted a lot of activity in the field of hadron physics, with experimental programs in ongoing and upcoming facilities dedicated to the search for new exotic mesons and baryons, and many theoretical efforts trying to disentangle, for instance, compact multiquark structures from hadronic molecules. In this dissertation, we focus on recently seen exotic hadrons with heavy-quark content that may be understood as being generated dynamically from the hadron–hadron interaction. This interaction is derived from a suitable effective Lagrangian and properly unitarized in a full coupled-channel basis. In particular, we discuss the possible interpretation of some of the Ωc* excited states recently discovered at LHCb as being meson–baryon molecular states. We also discuss the dynamical generation of excited open-charm mesons from the scattering of pseudoscalar and vector charmed mesons off light mesons. We show that a double-pole structure is predicted for the D0*(2300) state, as well as for the D1(2430), within the molecular picture, while the Ds0*(2317) and the Ds1(2460) may be interpreted as molecular bound states. Extensions of these calculations to the bottom sector are also presented. Moreover, charmed hadrons are a promising probe of the quark-gluon plasma (QGP) phase that is expected to be created in heavy-ion collision experimental facilities. Charm and anticharm quarks are produced in the early stages of the collision and experience the whole evolution of the QGP, before hadronizing predominantly into open-charm mesons. To describe the experimental data, it is necessary to understand, from the theoretical side, the propagation of the D mesons in the hadronic phase and their interaction with the surrounding medium of light mesons. The approach that we employ in this thesis to study the thermal modification of open heavy-flavor mesons in a hot medium is based on the use of effective theories. By means of an extension to finite temperature of the unitarized effective interactions with the light mesons, we obtain the in-medium spectral properties of the D, D*, Ds, and Ds* ground-state mesons. We also analyze the temperature dependence of the masses and the decay widths of the dynamically generated states. Additionally, we provide results for the bottomed mesons by exploiting the heavy-quark flavor symmetry of the Lagrangian. In order to test the results of the thermal effective theory against lattice QCD calculations, we further employ the temperature-dependent scattering amplitudes and spectral functions to compute charm Euclidean correlators. The spectral properties of charmed mesons at finite temperature can be extracted from lattice QCD data of meson Euclidean correlators, yet relying on a priori assumptions about the shape of the spectral function. Hence, we compare both approaches at the level of Euclidean correlators and find that they compare reasonably well at temperatures below the QCD phase transition temperature. We also present calculations of off-shell transport coefficients in the hadronic phase, such as the drag force and the diffusion coefficients. Contrary to previous approaches in the literature, we implement in-medium scattering amplitudes and the thermal dependence of the heavy-meson spectral properties. The transport coefficients in the QGP phase have been recently computed with lattice QCD and extracted from Bayesian analyses of heavy-ion collision data. We observe a smooth matching with our results at the QCD phase transition temperature
Observational consequences of Black Holes in the Universe: From dark matter candidates to quasars
(Universitat de Barcelona, 2022-06-03) Oncins Fernández, Marc; Miralda Escudé, Jordi; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] The existence of black holes that go beyond the mass given by their stellar origin has been known for some time. On the lower end, a subset of primordial black holes (PBHs) could form all of the dark matter with individually very low masses of up to 10-11 M⊙. On the opposite mass range Super Massive Black holes (SMBHs) have been found at very high redshifts. Using models and instrumental techniques, we study the observational consequences of both of these types of black holes. We start by studying current constraints on PBHs. We look at microlensing, which makes up the largest constraints on PBHs as dark matter, and conclude that despite its usefulness the method will not improve the constraints for lower mass ranges. We make similar analyses of gravitational waves and PBH evaporation, with the former having a lot of potential but requiring the next generation of experiments, and the later being a simple case with few further constraints. Finally, we zero in the last remaining window for PBHs to be all the dark matter around 10-12 M⊙. The PBHs in this mass range would leave little observational results, but a possible way to constrain them would be through their interaction with stars. As stars form in the very early universe, they will accrue a large dark matter density gravitationally bound to the star. If PBHs were the dark matter, they will orbit the star, following a ﬂat eccentricity distribution. There is a chance some of the orbits of the PBHs naturally cross the star. The dynamical friction of a main sequence star on the PBH is enough to bring the PBH to the core of the star within the Hubble time, capturing it. There, the PBH will accrete the star resulting in a black hole the mass of the star. We compute the capture rate of such PBHs by a number of stars. We use stellar models from MENSA ranging from 0.3 to 1 M⊙ for the stars, compute the dynamical friction numerically using two diﬀerent types and take into account the eﬀect of perturbations coming from the rest of the galaxy. The result, which we call Ξ, is generic and can be used to compute the capture rate for any dark matter density and velocity dispersion. For the case z ∼ 20, and our models tells us that we expect stars represented on our stellar models that are very close of the center of their galaxies to end capturing a PBH and being accreted in turn. The capture rate lowers with distance, but it is still relevant at larger distances. This should result in a wealth of subsolar mass black holes that would survive to this day. We finally study the case of quasi stellar objects (QSOs), SMBHs with very high luminosity. The presence of Lymanα nebulae surrounding them is relevant, as we do not know whereas it is generic feature present in QSOs or if they are singular cases. A possible solution is through stacking. The technique allows the use of multiple images to reach depths otherwise impossible. For stacking QSOs, the upcoming survey J-PAS and its predecessor J-PLUS are the most promising. We use stacking in J-PLUS for a total of ∼ 1, 550 QSOs and more than three hundred thousand stars, the later to obtain an accurate recreation of the points spread. We find that J-PLUS cannot reach the magnitude needed, but we reach very high depths for the star stack in line with our projections. We expect to reach the magnitude needed to observe the diﬀuse Lymanα signal with J-PAS.
Variational quantum architectures. Applications for noisy intermediate-scale quantum computers
(Universitat de Barcelona, 2022-06-20) Bravo Prieto, Carlos; Latorre, José Ignacio; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Quantum algorithms showing promising speedups with respect to their classical counterparts already exist. However, noise limits the quantum circuit depth, making the practical implementation of many such quantum algorithms impossible nowadays. In this sense, variational quantum algorithms offer a new approach, reducing the requisites of quantum computational resources at the expense of classical optimization. Disciplines in which variational quantum algorithms may have practical applications include simulation of quantum systems, solving large systems of linear equations, combinatorial optimization, data compression, quantum state diagonalization, among others. This thesis studies different variational quantum algorithm applications. In Chapter 1, we introduce the main building blocks of variational quantum algorithms. In Chapter 2, we benchmark the seminal variational quantum eigensolver algorithm for condensed matter systems. In Chapter 3, we explore how the task of compressing quantum information is affected by data encoding in variational quantum circuits. In Chapter 4, we propose a novel variational quantum algorithm to compute the singular values of pure bipartite states. In Chapter 5, we develop a new variational quantum algorithm to solve linear systems of equations. Finally, in Chapter 6, we implement quantum generative adversarial networks for generative modeling tasks. The conclusions of this thesis are exposed in Chapter 7. Furthermore, supplementary material can be found in the appendices. Appendix A provides an introduction to Qibo, a framework for quantum simulation. Appendix B presents some results related to the Solovay-Kitaev theorem. Extra results from Chapter 5 and Chapter 6 can be found in Appendix C and Appendix D, respectively.
The Dynamical State of Star-forming Regions, from Molecular Clouds to Massive Clumps
(Universitat de Barcelona, 2022-04-26) Lu, Zujia; Padoan, Paolo; Pelkonen, Veli-Matti; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Star formation is a fundamental and still largely unsolved problem of astrophysics and cosmology. Its complexity stems from the complex interaction of turbulence, magnetic fields and gravity, and from the onset of different feedback mechanisms from massive stars, such as stellar winds, ionizing radiation and supernovae (SNe). This complexity makes it hard to develop purely analytical theories, so future progress in this field relies heavily on numerical simulations. The main goals of this thesis are to improve our understanding of the formation, evolution and dynamical state of star-forming clouds. This is pursued primarily through the analysis of numerical simulations. In Chapter 1, Chapter 2 and Chapter 3, I give the general scientific background and introduce the methods used throughout the thesis. Chapter 4 consists of the paper where we investigate the effect of SN explosions on the dynamics of molecular clouds (MCs). This work is based on the analysis of a simulation of magneto-hydrodynamic (MHD), SN- driven turbulence in a large interstellar medium (ISM) volume (250 pc). The position and timing of SNe is computed self-consistently for the first time by resolving the formation of individual massive stars. The main conclusions are that SNe are able to generate the turbulence observed within MCs, and they may also be the main mechanism for their formation and dispersal. Chapter 5 is the paper where we study the global properties of MC clumps, that are usually considered as possible progenitors of massive stars. This is achieved by comparing synthetic dust continuum observations of our SN- driven simulation with Herschel's observations. We generate a very large catalog of synthetic compact sources that have properties consistent with the observations. The comparison shows that the observed clumps are often projection effects, so their mass is usually overestimated by a large factor. In addition, a large fraction of clumps that are believed to contain a protostar, based on their spectral-energy distribution, are probably starless. Chapter 6 consists of the publications where we compute and analyze synthetic N2H+ line profiles of a sub-sample of our catalog of synthetic compact sources. Thanks to the line data, we study the dynamical state of the compact sources. We show that the observations largely overestimate the virial parameter of the most massive clumps. This generates an observational correlation between the mass and virial parameters of massive clumps that, according to our results, is primarily and observational artifact. In Chapter 7, I summarize the main conclusions of the papers presented in the thesis and briefly discuss future research directions.
Primordial black holes and their implications for Inflation
(Universitat de Barcelona, 2021-11-12) Triantafyllou, Nikolaos; Garriga Torres, Jaume; Atal, Vicente; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] The standard cosmological model, ΛCDM, with the addition of an early inflationary phase, provides an accurate description of a nearly flat and homogeneous Universe, at large scales, which expands at an accelerated rate. Despite its vindication, our knowledge of the components that trigger the early formation of structures and drive the accelerated expansion of the Universe, that is, dark matter (DM) and dark energy respectively, is severely limited, given their feeble interactions with the other components of the Universe. A number of candidates from particle physics, e.g weakly interacting massive particles (WIMPs) or axions, have been proposed to constitute DM, but so far there has been no evidence to support their existence. However, the detection of a signal from the merger of a binary of black holes of stellar masses, reinvigorated the interest in an old candidate for DM, namely primordial black holes (PBHs). These black holes behave as the ones sitting at the end of stellar evolution, with the distinctive differences that they may form in significant fractions even well before the appearance of the first stars, with masses that may range from the Planck mass, to the order of MBH ∼ 1012 M. One possible formation mechanism involves perturbations originating from the fluctuations of a scalar field during inflation, that collapse after they re-enter the causal horizon in a radiation or matter domination era. The PBHs could easily form binaries in the early Universe and merge within our Hubble time, rendering them observable by the current detectors LIGO/VIRGO. The work presented in this thesis focuses on how such a population of PBHs could be utilised in order to elucidate certain spectral features of curvature perturbations characterizing the initial state of the Universe. Firstly, the effect of matter and radiation perturbations on the orbital parameter distributions of PBH binaries is studied. These perturbations are shown to provide a source of torque to the binary, particularly when their power spectrum is enhanced at the comoving scale of the binaries, leading to the suppression of the merger rate and subsequent relaxation of constraints on the PBH abundance. Secondly, the effect of primordial clustering on the distribution of orbital parameters of PBH binaries is investigated with the use of a phenomenological model of local non-Gaussianity. It is shown that due to the modal coupling of the perturbations, the merger rate and the stochastic background of gravitational waves (SBGW) sourced by merging PBH binaries, are altered. An immediate result of clustering is that the observational constraint on the abundance of PBHs in DM is relaxed considerably, allowing for significant fractions, even close to totality. Thirdly, the possibility that the SBGW from the mergers of massive PBHs could provide an explanation for the recently detected isotropic signal by the NANOGrav collaboration is considered. The presence of non-Gaussianity, sourced from a phase of constant roll, is essential in order for such massive PBHs to evade the CMB µ-distortions constraints, in which case they may have formed in small abundances, of order 0.1% with respect to DM. The present work aims to provide a more robust modelling of the observational consequences of a population of PBHs in order to gain more insight into the spectrum of primordial perturbations at small scales and therefore into the initial conditions of the early Universe.
Modeling and observations of relativistic outflows in high-energy binary systems
(Universitat de Barcelona, 2021-09-29) Molina, Edgar; Bosch i Ramon, Valentí; Ribó Gomis, Marc; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Some binary systems consisting of a compact object, which can be either a neutron star or a black hole, and typically a non-degenerate companion star, have been shown to emit broadband radiation from radio up to gamma-ray frequencies. These systems are normally classified as X-ray or gamma-ray binaries, depending on the frequency at which their emission has its maximum. Unlike with stars, a big part of the observed emission cannot be explained by thermal radiation, and therefore non-thermal radiative processes need to be invoked. The interactions between the star and the compact object may result in the launching of outflows of plasma originating around the compact object position. These outflows can attain speeds close to the speed of light, and be an efficient site for acceleration of charged particles up to relativistic energies. A part of the non-thermal emission observed from X-ray and gamma-ray binaries comes precisely from the non-thermal radiative cooling of these accelerated particles. Additionally, when the companion star is very massive, it produces a strong stellar wind that interacts with the aforementioned relativistic outflows, modifying both their dynamical and radiative evolution. The main theoretical objective of this thesis is the study the interactions between the outflows of X-ray and gamma- ray binary systems and the stellar wind of a massive companion star. For this purpose, we developed versatile semi- analytical models that give a complete view of these interactions for different kinds of systems. The results of the theoretical modeling include broadband spectral energy distributions and light curves that are directly comparable with the observational data. Radio sky maps are also obtained for the large-scale emission of the outflows. The latter allows to directly visualize the dynamical effect of the stellar wind in the outflow trajectory, which acquires a helical or spiral-like pattern. This modified trajectory gives rise to asymmetries in the light curves at different energy ranges, as well as changes in the spectral energy distributions mostly due to variations of angle-dependent processes influencing the outflow emission. From the observational point of view, this thesis focuses on the analysis of the potential very high-energy gamma-ray emission above 100 GeV of the X-ray binary MAXI J1820+070, as seen by the MAGIC telescopes. The analysis is done through a custom software developed by MAGIC, which allows to reconstruct the arrival direction and energy of a gamma ray from the Cherenkov light emitted by the electromagnetic cascade that the gamma ray generates when it enters the atmosphere of the Earth. The observational results consist on a multiwavelength study of MAXI J1820+070 in the form of light curves and spectral energy distributions that use data from a number of telescopes at radio, optical, X-ray and gamma-ray frequencies. The source is not detected in gamma-rays above 100 MeV, and only flux upper limits can be given for those energies. Nevertheless, the obtained upper limits, together with the observed fluxes at other frequencies, are enough to constrain significantly the properties of a potential gamma-ray emitter in MAXI J1820+070. In conclusion, this thesis deepens in our understanding of the interactions between the stellar wind and the outflows of high-energy binary systems. It shows that these interactions must be taken into account in order to properly characterize the subset of those binary systems hosting a massive companion star, in which a powerful stellar wind is present. In this thesis, it is also shown that observations in high-energy and very high-energy gamma rays of X-ray and gamma-ray binary systems allow to set meaningful limits to the outflow properties, even when the sources are not detected and only upper limits in the flux are obtained.
Approaching nuclear interactions with lattice QCD
(Universitat de Barcelona, 2021-09-09) Illa Subiña, Marc; Parreño García, Assumpta; Universitat de Barcelona. Departament de Física Quàntica i Astrofísica
[eng] Nuclei make up the majority of the visible matter in the Universe; obtaining a first principles description of the nuclear properties and interactions between nuclei directly from the underlying theory of the strong interaction, Quantum Chromodynamics (QCD), is one of the main goals of the nuclear physics community. Although the theory was established nearly fifty years ago, the complexities of QCD at low energies precludes analytical solutions of the simplest hadronic systems, let alone the features of the nuclear forces. Until the beginning of the century, the only way to overcome this handicap in the low-energy regime was to use phenomenological descriptions of nuclei or effective field theories (EFTs). While they have been very successful, these approaches rely heavily on experimental data. In contrast to what happens in the study of nucleon-nucleon interactions, where the amount of experimental data is overwhelming, the study of hadronic systems beyond the up-down quarks sector becomes more limited. This is because hyperons (baryons containing the next lightest quark, the strange quark), are unstable against weak interaction processes, making the experimental study of the interaction between hyperons and nucleons, and among hyperons, very difficult. In this thesis we follow the lattice QCD (LQCD) approach, according to which QCD is solved non-perturvatibely in a discretized space-time via large-scale numerical calculations. Specifically, the interactions between two octet baryons are studied at low energies with larger-than-physical quark masses corresponding to a pion mass of 450 MeV and a kaon mass of 596 MeV. The two-baryon systems that are analyzed have strangeness ranging from 0 to -4 and include the spin-singlet and triplet N-N, Sigma-N (I=3/2), and Xi-Xi states, the spin-singlet Sigma-Sigma (I=2) and Xi-Sigma (I=3/2) states, and the spin-triplet Xi-N (I=0) state. Due to the inherent large noise in multi-baryon calculations (mitigated by the use of unphysical quark masses), the finite-volume energies are extracted using a robust fitting methodology, where in order to reliably estimate the systematic uncertainties, both the fitting form and the fitting range are varied. Then, the corresponding S-wave scattering phase shifts, low-energy scattering parameters, and binding energies when applicable, are extracted using Lüscher's formalism. While the results are consistent with most of the systems being bound at this pion mass, the interactions in the spin-triplet Sigma-N and Xi-Xi channels are found to be repulsive and do not support bound states. Using results from previous studies of these systems at a larger pion mass, an extrapolation of the binding energies to the physical point is performed and is compared with available experimental values and phenomenological predictions. The low-energy coefficients in pionless EFT relevant for two-baryon interactions, including those responsible for SU(3) flavor-symmetry breaking, are constrained. The SU(3) flavor symmetry is observed to hold approximately at the chosen values of the quark masses, as well as the SU(6) spin-flavor symmetry, predicted at large Nc. A remnant of an accidental SU(16) symmetry found previously at a larger pion mass is further observed. The SU(6)-symmetric EFT constrained by these LQCD calculations is used to make predictions for two-baryon systems for which the low-energy scattering parameters could not be determined within the present LQCD study, and to constrain the coefficients of all leading SU(3) flavor-symmetric interactions, demonstrating the predictive power of two-baryon EFTs matched to LQCD.

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