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Studies of B decays to Charmonium at BABAR

Using 22.7 million BBbar events recorded with the BABAR detector, the inclusive branching fractions for the production of J/psi, psi(2S) and Chi_c in B decays are presented. Combining the charmonium state with a K+-, K0, K*+-, K*0 or neutral pion, B decays are reconstructed exlusively and branching fractions are determined. A preliminary study is also presented fot the B -->eta_c K decay mode.


Introduction
Reconstruction and study of charmonium mesons in B decays is a crucial component of the measurement of time-dependent CP-violating asymmetries [1].
The analyses described in the following paper are based on a sample of 20.7 fb −1 collected with BABAR at the Υ (4S) resonance with an additional 2.6 fb −1 collected below the BB threshold. A determination of the B meson branching fractions depends upon an accurate measurement of the number of B mesons in the data sample. The number of BB events is determined by comparing the rate of multi-hadron events in data collected both on and off resonance. The continuum contribution to the on-resonance sample is estimated by rescaling the number of off-resonance hadronic events by the ratio of the number of observed µ + µ − events in the two samples. This procedure yields a total of 22.72 ± 0.36 million BB events.
2 Inclusive decays of B to states containing Charmonium J/ψ candidates are selected by requiring two identified leptons of opposite charge. Electrons are identified based on the ratio E/p of the energy deposited in the calorimeter to the measured momentum from tracking information, the shape of the calorimetric cluster and the ionization in the tracking detectors. Muons are identified by requiring a minimum ionizing signal in the calorimeter. In addition, the shape and penetration of the distribution of hits in the instrumented flux return are used. The number of J/ψ events is determined by fitting the invariant mass distribution to a probability density function obtained from a simulation including contribution from both final state radiation and bremsstrahlung. The fit yields 15739 ± 177 J/ψ → e + e − and 13683 ± 154 J/ψ → µ + µ − signal events (Figure 1). The ψ(2S) candidates are reconstructed in the ψ(2S) → l + l − and ψ(2S) → J/ψπ + π − decays. In the latter case the signal yield is extracted by a fit to the mass difference between the ψ(2S) and the J/ψ reconstructed candidates. We find 552 ± 50 decays to e − e − , 437 ± 44 decays to µ + µ − , 405 ± 37 decays to J/ψ(e + e − )π + π − and 400 ± 34 decays to J/ψ(µ + µ − )π + π − .
The χ c1 and χ c2 candidates are selected by combining a reconstructed J/ψ with a photon. The signal yield is determined by fitting the mass difference between the χ c and the J/ψ candidates. The shape of the signal is extracted from Monte Carlo, with the mass difference between the χ c1 and χ c2 peaks fixed to the PDG value [2]. The fit gives 476 ± 71 χ c1 and 86 ± 59 χ c2 candidates for the J/ψ → e + e − decay and 545 ± 60 χ c1 and 104 ± 56 χ c2 candidates for the J/ψ → µ + µ − decay.
Mode Values for the branching fractions are extracted from the yields [3]. A 90% CL limit is set on the B decay to χ c2 . The results for the inclusive production of J/ψ , ψ(2S) and χ c in B decays are summarized in Table 1, where the first uncertainty is statistical error and the second is systematics.

Exclusive decays of B to Charmonium
The reconstruction of exclusive decay modes containing charmonium presents a very low background in most of the channels. For this reason the lepton identification criteria are loosened for one of the two J/ψ decay products. As in the inclusive analysis, ψ(2S) candidates are reconstructed by their decay to e − e − , µ + µ − and J/ψ π + π − . The χ c candidates are selected through their decay to J/ψγ.
The charmonium states are selected in a window around their expected mass [2] for the decays to leptons. In the decays to J/ψ states the mass difference distribution between the charmonium candidate and the reconstructed J/ψ is used instead.
Selected candidates are then paired with a K + , K 0 s (either π + π − or π 0 π 0 ), K * + (either K + π 0 or K 0 s π + ), K * 0 (either K + π − or K 0 s π 0 ), π 0 or K L to form a B candidate. The two most significant observables used to identify the signal are the difference ∆E in the center-of-mass frame between the reconstructed B energy and half the nominally available energy, √ s/2, and the energy-substituted mass, m ES = s/4 − P * 2 B , where P * B is the center-of-mass momentum of the B candidate. A sample of these distributions is given in Figure 2 for J/ψK S events.
In the case of multiple candidates per event, only the candidate with the smallest |∆E| is selected. For all modes except B → J/ψK L and B → J/ψK * , the number of signal events is determined from the observed number of events in the (∆E, m ES ) region after background subtraction. In addition to the usual combinatorial component, the background distribution shows an excess of events in the signal region. The combinatorial contribution is estimated by using an ARGUS function in the fit to the m ES distribution. The peaking background component is obtained from simulation of inclusive B decays to charmonium, after removing the signal events.
The signal yields for the K * 0 and K * + modes are determined simultaneously from a likelihood fit, which is needed to account for the cross-feed between the K * decay channels.
A different technique is used for the B → J/ψK L decay mode. In this case only the K L direction is measured with information from the calorimeter and the instrumented flux return. Given this direction and the reconstructed charmonium candidate, the K L energy is extracted by using the B  In the above plot the η c candidate is reconstructed through K S Kπ decays.
exclusive modes [4] is summarized in Table 2, while Figure 4 shows a comparison of the new preliminary BABAR results to the current PDG values. In addition to the analyses described above, a very preliminary study has been recently performed by BABAR, on exclusive reconstruction of B decays to η c K modes. The η c has been studied in the K S K + π − , K + K − π 0 and K + K − K + K − (φφ plus non-resonant) decay modes. The energysubstituted mass distribution for the decay η c → K S K + π − is presented as an example in Figure 3, right plot. No value has been extracted yet for the branching fractions in these decay modes.

Summary
Using 22.7 million BB events recorded by the BABAR detector, the inclusive branching fractions for the production of J/ψ , ψ(2S) and χ c are presented. Combining the charmonium state with either a K ± , K 0 , K * ± , K * 0 or π 0 , B decays are reconstructed exclusively and their branching fractions are determined.