Production and Decay of Omega_c^0

We present an analysis of inclusive Omega_c^0 baryon production and decays in 230.5 fb^-1 of data recorded with the BABAR detector at the PEP-II asymmetric-energy e+ e- collider at the Stanford Linear Accelerator Center. Omega_c^0 baryons are reconstructed in four final states (Omega- pi+, Omega- pi+ pi0, Omega- pi+ pi+ pi-, Xi- K- pi+ pi+) and the ratios of branching fractions for these final states are measured. We also measure the momentum spectrum of the Omega_c^0 baryons in the e+ e- center-of-mass frame. From the spectrum, we observe Omega_c^0 production from B decays and in ccbar events, and extract the two rates of production.

In this letter, we present a study of the Ω 0 c baryon, reconstructed in four decay modes: Ω − π + , Ω − π + π 0 , Ω − π + π + π − , and Ξ − K − π + π + [12]. We measure the ratios of branching fractions for these modes, normalizing to B(Ω 0 c → Ω − π + ). The previous most precise measurements of these ratios are from an analysis of approximately 45 events from six Ω 0 c decay modes [3]. We then measure the spectrum of the Ω 0 c momentum in the e + e − center-of-mass frame (p * ) and observe significant production of Ω 0 c baryons in the decays of B mesons. The data for this analysis were recorded with the BABAR detector at the Stanford Linear Accelerator Center PEP-II asymmetric-energy e + e − collider. The detector is described in detail elsewhere [13]. A total integrated luminosity of 230.5 fb −1 is used, of which 208.9 fb −1 were collected at the Υ(4S) resonance (corresponding to 232 million BB pairs) and 21.6 fb −1 were collected 40 MeV below the BB production threshold.
Simulated events with the Ω 0 c decaying into the relevant final states are generated for the processes e + e − → cc → Ω 0 c X and e + e − → Υ(4S) → BB → Ω 0 c X, where X represents the rest of the event. The pythia simulation package [14] is used for the cc fragmentation and for B decays to Ω 0 c , and the geant4 [15] package is used to simulate the detector response. To investigate possible background contributions, additional samples of generic Monte Carlo (MC) events are used, equivalent to 990 fb −1 for Υ(4S) events (e + e − → Υ(4S) → BB), plus 320 fb −1 for cc continuum events (e + e − → cc) and 340 fb −1 for light quark continuum events (e + e − → qq, q = u, d, s).
The reconstruction of an Ω 0 c candidate begins by identifying a proton, combining it with an oppositely charged track interpreted as a π − , and fitting the tracks to a common vertex to form a Λ candidate. The Λ is then combined with a negatively charged track interpreted as a K − (π − ) and fit to a common vertex to form an Ω − (Ξ − ) candidate. For each intermediate hyperon (Λ, Ξ − , Ω − ) we require the invariant mass to be within 4.5 MeV/c 2 of its nominal value (corresponding to approximately 4, 3, and 3 times the detector resolution, respectively). We form π 0 candidates from pairs of photons in the electromagnetic calorimeter, requiring the energy of each photon to be above 80 MeV and the combined energy to be above 200 MeV. We require the invariant mass of the π 0 candidate, computed at the event primary vertex, to be in the range 120-150 MeV/c 2 .
Each Ω − (Ξ − ) candidate that passes the requirements is then combined with one or three additional tracks that are identified as pions or kaons as appropriate. For the Ω − π + π 0 final state, we also combine the hyperon and π + with a π 0 . The Ω 0 c candidate daughters are refit to a common vertex with their masses constrained to the nominal values. From this fit we extract the decay vertices and associated uncertainties of the Ω 0 c and the intermediate hyperons, the four-momenta of the particles, and the Ω 0 c candidate mass. For each intermediate hyperon we require a positive scalar product of the momentum vector in the laboratory frame and the displacement vector from its production vertex to its decay vertex.
To further suppress the background, we compute the c candidate, where the index i refers to the likelihood variables x i , and p i (x i ) are the probability density functions for signal (S) and background (B). For a given Ω 0 c candidate L has a value between 0 and 1. The likelihood variables x i are the logarithm of the Ω − or Ξ − decay length significance, which is defined as the distance between the production and decay vertices divided by the uncertainty on that distance; the momentum of the Ω − or Ξ − in the e + e − rest frame; the total momentum of the mesons recoiling against the Ω − or Ξ − in the e + e − rest frame; and, for the Ω − π + π 0 mode, the π 0 momentum in the laboratory frame. These variables (particularly the decay length significance) cover the expected range effectively with a limited number of bins. The distributions of these variables for the signal hypothesis are derived from signal MC simulations, and for the background hypothesis from generic MC events in which contributions from real Ω 0 c are excluded. Separate distributions are used for each final state when measuring ratios of branching fractions, and for each momentum range when measuring the momentum spectrum.
To measure the ratios of branching fractions, we require that p * > 2.4 GeV/c in order to suppress combinatoric background. Since the kinematic limit for Ω 0 c produced in B decays at BABAR is p * max = 2.02 GeV/c, only Ω 0 c produced in the cc continuum are retained. We also require that the value of L for each candidate is greater than a threshold L 0 , chosen to maximize the expected signal significance for a given final state based on simulated events. We perform an unbinned maximum likelihood fit to the mass distributions shown in Fig. 1. The signal lineshape is parameterized as the sum of two Gaussian functions with a common mean; the background is parameterized as a first-order polynomial. In the fits to the data, the signal yield is a free parameter; the widths and relative amplitudes of the two Gaussian functions are fixed to values determined from a fit to simulated signal events. The mean mass is also a free parameter, except for the Ξ − K − π + π + final state where we fix it to the central value obtained in Ω 0 c → Ω − π + in order to ensure proper fit convergence. The masses are found to be consistent with one another and with the current world average [1] within uncertainties.
The numbers of signal events are 177±16, 64±15, 25± 8, and 45 ± 12 (statistical uncertainties only) for the final states Ω − π + , Ω − π + π 0 , Ω − π + π + π − , and Ξ − K − π + π + , respectively. These correspond to statistical significances of 18, 5.1, 4.2, and 4.3 standard deviations, respectively, where the significance is defined as √ 2∆ℓ and ∆ℓ is the change in the logarithm of the likelihood between the fits with and without an Ω 0 c signal component. The fitted yields are then corrected for efficiency, which is defined as the fraction of simulated signal events, generated in the appropriate p * range, that are reconstructed and pass all selection criteria. Including the loss of efficiency due to the Λ and Ω − branching fractions, we obtain efficiencies of (8.6 ± 0.6)%, (2.5 ± 0.3)%, (4.3 ± 0.4)%, and (4.7 ± 0.5)% for the four final states, where the uncertainties include systematic effects and are partially correlated. The systematic uncertainties on, and corrections to, the ratios of branching fractions are listed in Table I and discussed further later. We measure the ratios to be We also measure the p * spectrum of Ω 0 c in order to study the production rates in both cc and BB events. Only the Ω − π + final state is used. The same reconstruction, optimization of selection criteria, and fitting procedures described above are applied, except that no requirement on p * is made. Instead, the Ω 0 c candidates are divided into nine equal intervals of p * covering the range 0.0-4.5 GeV/c. We again require L > L 0 and compute the efficiency in each p * interval as before with simulated signal events. In the numerator of the efficiency we count events with measured p * in the appropriate interval, and in the denominator we count events with generated p * in that interval: this definition removes the slight broaden-    (Ω 0 c → Ω − π + π 0 )/B(Ω 0 c → Ω − π + ), R2 ≡ B(Ω 0 c → Ω − π + π + π − )/B(Ω 0 c → Ω − π + ), and R3 ≡ B(Ω 0 c → Ξ − K − π + π + )/B(Ω 0 c → Ω − π + ).