Search for the Rare Decay B->K nu nubar

TL;DR

This paper reports a search for rare B meson decays involving a kaon and two neutrinos, setting upper limits on their branching fractions using data from the BaBar experiment, which tests the Standard Model and probes new physics.

Contribution

First search for B->K nu nubar decays using semileptonic tagging, providing the most stringent upper limits to date on their branching fractions.

Findings

Upper limit of 1.3 x 10^{-5} for B+ -> K+ nu nubar

Upper limit of 5.6 x 10^{-5} for B0 -> K0 nu nubar

Partial branching fractions measured in specific di-neutrino mass ranges

Abstract

We present a search for the rare decays B+ -> K+ nu nubar and B0 -> K0 nu nubar using 459 million BBbar pairs collected with the BaBar detector at the SLAC National Accelerator Laboratory. Flavor-changing neutral-current decays such as these are forbidden at tree level but can occur through one-loop diagrams in the Standard Model (SM), with possible contributions from new physics at the same order. The presence of two neutrinos in the final state makes identification of signal events challenging, so reconstruction in the semileptonic decay channels B -> D(*) l nu of the B meson recoiling from the signal B is used to suppress backgrounds. We set an upper limit at the 90% confidence level of 1.3 x 10^{-5} on the total branching fraction for B+ -> K+ nu nubar, and 5.6 x 10^{-5} for B0 -> K0 nu nubar. We additionally report partial branching fractions in two ranges of di-neutrino mass squared for B+ -> K+ nu nubar.