Evidence ofB+→τ+νdecays with hadronicBtags

Abstract

We present a search for the decay B + → τ + ν using 467.8 × 10 6 BB pairs collected at the Υ (4S) resonance with the BABAR detector at the SLAC PEP-II B-Factory. We select a sample of events with one completely reconstructed B − in the hadronic decay mode (B − → D ( * )0 X − and B − → J/ψ X − ). We examine the rest of the event to search for a B + → τ + ν decay. We identify the τ + lepton in the following modes: τ + → e + νν, τ + → µ + νν, τ + → π + ν and τ + → ρ + ν. We find an excess of events with respect to the expected background, which excludes the null signal hypothesis at the level of 3.8σ (including systematic uncertainties) and corresponds to a branching fraction central value of B(B + → τ + ν) = (1.83PACS numbers: 13.25.Hw The study of the purely leptonic decay B + → τ + ν [1] is of particular interest to test the predictions of the Standard Model (SM) and to probe of new physics effects. It is sensitive to the product of the B meson decay constant f B , and the absolute value of the CabibboKobayashi-Maskawa matrix element |V ub | [2]. In the SM the branching fraction is given by: The process is sensitive to possible extensions of the SM. For instance, in two-Higgs doublet models (2HDM) [6] and in minimal supersymmetric extensions [7] it can be mediated by a charged Higgs boson. A branching fraction measurement can, therefore, also be used to constrain the parameter space of new physics models.The data used in this analysis were collected with the BABAR detector at the PEP-II storage ring. The sample corresponds to an integrated luminosity of 426 fb −1 at the Υ (4S) resonance. The sample contains (467.8±5.1)×10 6 4 BB decays (N BB ). The detector is described in detail elsewhere [12]. Charged particle trajectories are measured in the tracking system composed of a five-layer double-sided silicon vertex tracker and a 40-layer drift chamber, operating in a 1.5 T solenoidal magnetic field. A Cherenkov detector is used for charged π-K discrimination, a CsI calorimeter for photon and electron identification, and the flux return of the solenoid, which consists of layers of iron interspersed with resistive plate chambers or limited streamer tubes, for muon and neutral hadron identification.We use a Monte Carlo (MC) simulation based on GEANT4 [13] to estimate signal selection efficiencies and to study backgrounds. In MC simulated signal events, one B + meson decays as B + → τ + ν and the other decays in any final state. The BB and continuum MC samples are equivalent to approximately 3 times and 1.5 times the data sample, respectively. Beam-related background and detector noise are sampled from data and overlaid on the simulated events.We reconstruct an exclusive decay of one of the B mesons in the event (which we refer to as the tag-B) and examine the rest of the event for the experimental signature of B + → τ + ν. The tag-B reconstruction can be performed by looking at both hadronic B decays and semileptonic B decays. Published results from both BABAR and Belle are summarized in Table I.We reconstruct the...

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