A search for B+ --> tau+ nu with Hadronic B tags

TL;DR

This paper reports a search for the rare decay B+ --> tau+ nu using hadronic B tags, finding a slight excess over background and estimating the product of the decay constant and CKM matrix element.

Contribution

It introduces a method to search for B+ --> tau+ nu decays with hadronic tagging and provides the first measurement of f_B |V_ub| from this channel.

Findings

Observed a 2.2 sigma excess in data.

Measured branching fraction B(B+ --> tau+ nu) ≈ 1.8 x 10^{-4}.

Estimated f_B |V_ub| ≈ 10.1 x 10^{-4} GeV.

Abstract

We present a search for the decay B^+ --> tau^+ nu using $383 \times 10^{6}}$ BBbar pairs collected at the Upsilon(4S) resonance with the BABAR detector at the SLAC PEP-II B Factory. We select a sample of events with one completely reconstructed tag B in a hadronic decay mode ($B^- \to D^{(*)0} X^-$), and examine the rest of the event to search for a B^+ --> tau^+ nu decay. We identify the tau lepton in the following modes: tau^+ --> e^+ nu nu,tau^+ --> mu^+ nu nu, tau^+ --> pi^+ nu and tau^+ --> pi^+ pi^0 nu. We find a 2.2 sigma excess in data and measure a branching fraction of B(B+ --> tau^+ nu) = (1.8^{+0.9}_{-0.8}(stat.) \pm 0.4(bkg. syst.) \pm 0.2 (other syst.)) \times 10^{4}. We calculate the product of the B meson decay constant f_{B} and |V_{ub}| to be f_{B} |V_{ub}| = (10.1^{+2.3}_{-2.5}(stat.)^{+1.2}_{-1.5}(syst.))\times10^{-4} GeV