# name
Pushing the boundaries of the LHCb rare decays program: search for the Λ0b→ Λγ decay

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Universitat de Barcelona. Facultat de Física

# metadata.dc.description.abstract[].value
[eng]  The main goal of this thesis is to test the Standard Model (SM) of particle physics through the experimental analysis of rare processes, those that are suppressed in the SM. In particular, radiative b and rare strange decays are very sensible to potential contributions from beyond the SM (BSM) physics. The data recorded by LHCb in proton-proton (pp) collisions during the Run 1 (2010-2012) and Run 2 (2015-2018) of the LHC is exploited with this purpose. The LHCb detector is a single-arm forward spectrometer characterised by a precise momentum and impact parameter resolution, a good separation between particles of different species and a very flexible trigger system. A trigger selection dedicated to radiative decays of b-hadrons has been developed for Run 2, based on multivariate techniques exploiting the common features of these decay modes. Two complementary configurations select two- and three-body plus photon candidates, respectively. The results obtained on different Monte-Carlo (MC) signal samples show an improvement with respect to the Run 1 selection of at least 20% in absolute efficiency for all the modes, while for some special cases, as B0 K+π−γ and B+ K∗+γ, larger gains of 70% and 60%, respectively, are achieved. Moreover, the rate is reduced to a half with respect to the Run 1 configuration, which allows to dedicate the remaining budget to exclusive selections for decays with special topologies, expanding even more the reach of the radiative decays physics program. Of special interest among rare strange decays are the K0 f+f−f+f− modes, where f can be either an electron or a muon. As a first milestone towards the study of these channels, the feasibility of observing the topologically similar K0 π+π−e+e− at LHCb has been assessed. Selection efficiencies have been extracted from MC samples, obtaining an expected signal yield in Run 1 conditions of 120+280/ fb−1. No −100 signal peak has been observed in the invariant mass distribution of selected candidates in the 2012 dataset and the possibility of observing this mode with these data has been evaluated with pseudoexperiments, concluding that an observation if feasible using a selection with a separation power similar to that usually achieved by multivariate algorithms. For Run 2, tow complementary trigger selections have been developed to improve its efficiency, reaching an expected yield of 620+290/ fb−1, which guarantees an 0−120 + − + − observation of this mode in Run 2 and allows the first studies of the KS → f f f f modes. In the area of radiative b-decays, the most interesting observable is the polarisation of the emitted photon, which is predicted to be almost 100% left-handed in the SM, while right-handed currents can contribute in BSM models. This observable is accessible through the previously unobserved Λ0 Λγ mode so a search for this decay has been performed using the dataset recorded by LHCb in pp collisions during 2016. Due to its particular topology, a dedicated reconstruction has been developed and used both in the trigger and offline. The candidate selection is based on multivariate methods, which have been trained using MC as a proxy for the signal and data as background. The requirement on the output distribution has been optimised by maximising the probability of achieving an observation of the signal, obtaining a 33% signal efficiency with a 99.8% background rejection. The B0 K∗0γ mode has been used as normalisation channel, which allows to measure the signal branching ratio from the ratio of yields. A simultaneous maximum likelihood fit of the invariant mass distributions has been performed resulting in a Λ0 → Λγ yield of (64 ± 13) events and a measurement of the branching fraction of B(Λ0 → Λγ) = (2.59 ± 0.52 (estad.) ± 0.28 (sist.) ± 0.33 (fΛ0 /fB0 )) × 10−6. The statistical significance of the signal has been evaluated by means of a likelihood profile scan, obtaining a 5.7σ significance, which represents the first observation of the Λ0 → Λγ decay and opens the doors to the study of the photon polarisation in this mode.

# metadata.dc.description.abstract[].value
[cat]  En aquesta tesi s’han estudiat modes de desintegració radiatius d’hadrons b i rars de mesons estranys, amb l’objectiu de testejar el Model Estàndard (MS). S’han utilitzat dades enregistrades pel detector LHCb en col lisions protó-protó (pp) durant el Run 1 (2010-2012) i el Run 2 (2015-2018) de l’LHC. LHCb ´es un espectòmetre d’un sol brac¸ caracteritzat per una alta precisió en la reconstrucció del moment i el paràmetre d’impacte respecte del punt d’interacció, una gran capacitat per separar partícules de diferents espècies i un sistema activador (trigger) molt flexiu. Per al Run 2 s’ha desenvolupat una selecció de trigger dedicada a processos radiatius basada en algoritmes multivariats, explotant-ne les característiques comunes. S’ha obtingut un augment absolut en l'eficiència entorn del 20%. Per a casos amb topologia especial, com B0 K+π−γ i B+ K∗+γ, el guany arriba al 70% i 60%, respectivament. Pel que fa als modes rars de desintegració d’hadrons estranys, s’ha avaluat la viabilitat d’observar el canal K0 π+π−e+e− a LHCb. Utilitzant mostres de Monte-Carlo (MC), s’ha obtingut una senyal esperada de 120+280/ fb−1 en condicions del Run 1. En les dades del 2012 no s’ha observat la senyal −100 degut a la presència d’un gran fons i per mitjà de pseudo-experiments s’ha arribat a la conclusió que l’observació és possible utilitzant una selecció multivariada. Per al Run 2 s’han desenvolupat seleccions de trigger millorant la senyal esperada fins a 620+290/ fb−1. −120 0 Finalment, utilitzant les dades del 2016 s’ha realitzat una cerca del mode de desintegració Λb Λγ basada en el desenvolupament d’una reconstrucció especial i d’una selecció multivariada, que aconsegueix una eficiència del 33% rebutjant al mateix temps un 99.8% del fons. El nu´mero d’esdeveniments de senyal s’ha extret d’un ajust simultani de màxima versemblança de la distribució de massa invariant, obtenint-ne (64 ± 13), amb una significació estadística de 5.7σ, que constitueix la primera observació d’aquest mode de desintegració. Utilitzant el canal B0 → K∗0γ com a normalització, s’ha mesurat també la fracció d’embrancament B(Λ0 → Λγ) = (2.59 ± 0.52 (estad.) ± 0.28 (sist.) ± 0.33 (fΛ0 /fB0 )) × 10−6.

# metadata.dc.title[].value
Pushing the boundaries of the LHCb rare decays program: search for the Λ0b→ Λγ decay