Search for rare decays of the B$0\atop{s}$ meson with the DØ experiment

Abstract

This document presents the searches for the flavour-changing neutral current decays B$0\atop{s}$ → μ+μ- and B$0\atop{s}$s → φμ+μ- . A data set with integrated luminosity of 300 pb-1 of proton-antiproton collisions at √ s = 1 . 96 TeV collected with the DØ detector in Run II of the FERMILAB Tevatron collider is used. The former decay mode is particularly sensitive to supersymmetric extensions of the Standard Model. For the latter mode, a measurement of the branching ratio could validate the prediction of the Standard Model. In the absence of an apparent signal, a limit on the branching fraction B(B$0\atop{s}$ → μ+μ-) can be computed by normalising the upper limit on the number of events in the B$0\atop{s}$ signal region to the number of reconstructed B ± → J/ψ K ± events. An upper limit on the branching fraction of B(B$0\atop{s}$ → μ+ μ- ) ≤ 3.7 × 10-7 at a 95% CL is obtained. This limit can be used to constrain models beyond the Standard Model. In models where the lightest supersymmetric particle is considered to be a dark matter candidate the limit aids in restricting the dark matter scattering cross section on nucleons. For the decay B$0\atop{s}$ → φμ+μ- also no signal has been observed and an upper limit on the branching ratio normalised to B$0\atop{s}$ → J/ψ φ events of B$0\atop{s}$ B(B$0\atop{s}$ → φ μ+ μ-)/B(B$0\atop{s}$→J/ψφ) < 4 . 4 × 10-3 at a 95% CL is obtained. In addition, the rare decay B$0\atop{s}$ → ψ (2 S ) φ has been observed. To measure a branching ratio, the B$0\atop{s}$ → J/ψ φ mode was used for normalisation, while B± → ψ (2S) K± and B± → J/ψ K± modes were used as control samples. The relative branching ratio has been measured to be B(B$0\atop{s}$ → ψ (2S) φ)/ B(B$0\atop{s}$ → J/ψ φ ) = 0 . 58 ± 0 . 24 (stat) ± 0 . 09 (sys).