The rarest B⁰ decay ever observed

The LHCb collaboration has observed the rare baryonic decay B⁰ → pp, as first presented at the European Physical Society conference in Venice in early July. The branching fraction was measured at the level of about 1.3 per 100 million decays, which makes this decay mode the rarest decay of a B⁰ meson ever observed. It is also the rarest observed hadronic decay of all beauty mesons.

Our knowledge of baryonic B decays has increased considerably in the last few years. The LHCb experiment, which is primarily designed to search for new physics in CP-violation and rare decays of particles with heavy flavour, has been pursuing a programme to study the decays of B mesons to final states containing baryons. Among the recent achievements it is worth emphasising the first observation of a baryonic B⁰_s decay, B⁰_s → p Λ K^–, and that of B⁰ and B⁰_s decays to pp plus a pair of light charged mesons. The B⁰_s is the last of the B meson species for which a baryonic decay mode had yet to be observed. The large data samples available at LHCb have made it possible to study the two-body baryonic final-state decays, which are suppressed with respect to higher-multiplicity decay modes.

A search for the rare decays B⁰ → pp and B⁰_s → pp had previously been performed by LHCb with 2011 data only, obtaining evidence for B⁰ → pp. The collaboration now used the full 3 fb^–1 data sample collected during the first run of the LHC, approximately three times more data than in the previous search, to confirm the evidence for this decay. An excess of B⁰ → pp candidates with respect to the background-only hypothesis is now observed with a statistical significance of 5.3 standard deviations. The hint of a B⁰_s → pp signal reported in 2013 is, however, not confirmed, and an upper limit for the corresponding branching fraction has been set.

The measured B⁰ → pp and B⁰_s → pp branching fractions are compatible with the latest theoretical calculations. The observation of the latter will allow a quantitative comparison of various QCD-inspired models describing baryonic B decays.