First Observation of the Decay

The Bþc meson is the ground state of the bc quark pair system [1]. Studies of its properties are important, since it is the only meson consisting of two different heavy quarks. It is also the only meson in which decays of both constituents compete with each other. Numerous predictions for Bþc branching fractions have been published (for a review see, e.g., Ref. [2]). To date, no measurements exist which would allow us to test these predictions, even in ratios. Production rates for Bþc mesons are about 3 orders of magnitude smaller at high energy colliders than for the other B mesons composed of a b quark and a light quark (Bþ, B, and Bs). All experimental knowledge on the Bþc meson was obtained from measurements at the Tevatron. It was discovered by the CDF experiment in the semileptonic decay, Bþc ! J=c lþ X [3]. This decay mode was later used to measure the Bþc lifetime [4,5], which is 3 times shorter than that of the other B mesons as both b and c quark may decay. Only one hadronic decay mode of Bþc was observed so far, Bþc ! J=c þ. It was utilized by CDF [6] and DØ [7] to measure the Bþc mass [8] to be 6277 6 MeV [9]. In this Letter, we present the first observation of the decay mode Bþc ! J=c þ þ using a data sample corresponding to an integrated luminosity of 0:8 fb 1 collected in 2011 by the LHCb detector [10], in pp collisions at the LHC at ffiffi s p 1⁄4 7 TeV. The branching fraction for this decay is expected to be 1.5–2.3 times higher than that for Bþc ! J=c þ [11,12]. However, the larger number of pions in the final state results in a smaller total detection efficiency due to limited detector acceptance. We measure the Bþc ! J=c þ þ branching fraction relative to that for the Bþc ! J=c þ decay and test the above theoretical predictions. The LHCb detector [10] is a single-arm forward spectrometer covering the pseudorapidity range 2< < 5, designed for the study of particles containing b or c quarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding the pp interaction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system has a momentum resolution p=p that varies from 0.4% at 5 GeV to 0.6% at 100 GeV, and an impact parameter (IP) resolution of 20 m for tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov detectors. Photon, electron, and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a muon system composed of alternating layers of iron and multiwire proportional chambers. The muon system, electromagnetic and hadron calorimeters provide the capability of first-level hardware triggering. The single and dimuon hardware triggers provide good efficiency for Bþc ! J=c þ1⁄2 þ , J=c ! þ events. Here, þ1⁄2 þ stands for either þ or þ þ depending on the Bþc decay mode. Events passing the hardware trigger are read out and sent to an event-filter farm for further processing. Here, a software-based two-stage trigger reduces the rate from 1 MHz to about 3 kHz. The most efficient software triggers [13] for this analysis require a charged track with transverse momentum (pT) of more than 1.7 GeV (pT > 1:0 GeV if identified as a muon) and with an IP to any primary pp-interaction vertex (PV) larger than 100 m. A dimuon trigger requiring pTð Þ> 0:5 GeV, large dimuon mass, Mð þ Þ> 2:7 GeV, and with no IP requirement complements the single track triggers. At the final stage, we either require a J=c ! þ candidate with pT > 2:7 GeV (> 1:5 GeV in the first 42% of data) or a muon-track pair with significant IP. In the subsequent offline analysis of the data, J=c ! þ candidates are selected with the following criteria: pTð Þ> 0:9 GeV, pTðJ=c Þ> 3:0 GeV (>1:5GeV in the first 42% of data), 2 per degree of freedom of the two muons forming a common vertex, vtxð þ Þ=ndf < 9, *Full author list given at the end of the article.