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Title: A search for the decay of a B meson into a kaon and a tau lepton pair at the BaBar experiment

The flavour changing neutral current (FCNC) process, $B^+$ → $K^+ τ^+ τ^-$ highly suppressed in the Standard Model (SM). This decay is forbidden at tree level and only occurs at lowest order via one-loop diagrams.$B^+$ → $K^+ τ^+ τ^-$ thus has the potential to provide a stringent test of the SM and a fertile ground for new physics searches. Contributions due to virtual particles in the loop allow one to probe, at relatively low energies, new physics at large mass scales. We search for the rare FCNC process $B^+$ → $K^+ τ^+ τ^-$ using data collected by the BaBaR detector at the SLAC National Accelerator Laboratory. The BaBaR data sample corresponds to a total integrated luminosity, at the energy of the Τ(4S) resonance, of 424.4 $fb^-1$ and 471 million $$B\bar{B}$$ pairs. For this search, hadronic $$B_{tag}$$ reconstruction is employed, where one B is exclusively reconstructed via one of many possible hadronic modes. The remaining decay products in an event are then attributed to the signal B, on which the search for $B^+$ → $K^+ τ^+ τ^-$ is performed. Each τ is required to decay leptonically, into either an electron or a muon and the lepton neutrinos. Furthermore, a multi-variate analysis technique (neural network) is used to select for signal events and suppress dominant background modes. No significant signal is observed. The resulting branching fraction is measured to be $$\beta(B^+$$ → $K^+ τ^+)$ = $$1.31^{0:66}_{-0:61}$$(stat.) $$^{+0:35}_{-0:25}$$(sys.) x 10$$^{-3}$$, which is consistent with zero at the 1.9σ level, with an upper limit of 2.25 x 10$$^{-3}$$, at the 90% confidence level.
  1. McGill Univ., Montreal, QC (Canada)
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SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
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US DOE Office of Science (DOE SC), High Energy Physics (HEP)
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United States