Strong Constraints on the Rare Decays

Flavor changing neutral current (FCNC) processes are highly suppressed in the standard model (SM) and thus constitute a stringent test of the current description of particle physics. Precise predictions of the branching fractions of the FCNC decays Bs ! þ and B ! þ , BðBs ! þ Þ 1⁄4 ð3:2 0:2Þ 10 9 and BðB0 ! þ Þ 1⁄4 ð0:10 0:01Þ 10 9 [1,2] make these modes powerful probes in the search for deviations from the SM, as contributions from new processes or new heavy particles can significantly modify these values. Previous searches [3–6] already constrain possible deviations from the SM predictions, with the lowest published limits from the LHCb Collaboration: BðB0s ! þ Þ< 1:4 10 8 and BðB0 ! þ Þ< 3:2 10 9 at 95% confidence level (C.L.). In this Letter, we report an analysis of the pp collision data recorded in 2011 by the LHCb experiment corresponding to an integrated luminosity of 1:0 fb . This data set includes the 0:37 fb 1 used in the previous analysis [6]. In addition to the larger data set, improvements include an updated event selection, an optimized binning in the discriminating variables, and a reduction of the peaking background. The data already analyzed in Ref. [6] were reprocessed and, to avoid any potential bias, all the events in the signal region were blinded until all the analysis choices were finalized. The LHCb detector [7] is a single-arm forward spectrometer covering the pseudorapidity range 2< < 5. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector, 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=c to 0.6% at 100 GeV=c. Two ring-imaging Cherenkov detectors (RICH) are used to identify charged particles. 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 alternating layers of iron and multiwire proportional chambers. The trigger consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage (high-level trigger [HLT]) that applies a full event reconstruction. Events with muon final states are triggered using two hardware trigger decisions: the single-muon decision (one muon candidate with transverse momentum pT > 1:5 GeV=c), and the dimuon decision (two muon candidates with pT;1 and pT;2 such that