Measurement of the relative branching fraction B(B_s ^0 → ψ(2S)φ)/B( B_s^0→ J/ψφ ) with the ATLAS experiment at √s = 13 TeV.

Abstract

In this thesis we present the result of the relative branching fraction of the two hadronic B_s ^0 modes B_s ^0 → ψ(2S)φ and B_s ^0 → J/ψφ. In order to guarantee identical topologies we only look at the modes where J/ψ → µ^+µ^−, ψ(2S) → µ^+µ^− and φ → K^+K^−. The first observation of the rare decay of B_s ^0 → ψ(2S)φ was reported by the CDF collaboration at the Fermilab Tevatron in 2006 and the relative branching faction was reported to be B(B_s ^0 → ψ(2S)φ)/B( B_s ^0 → J/ψφ) = 0.52±0.13(stat) ± 0.04(syst) ± 0.06(BR) [1]. This measurement was repeated by the D0 collaboration in 2009 and LHCb in 2012 measuring 0.53 ± 0.10(stat) ± 0.07(syst) ± 0.06(BR) [2] and 0.489 ± 0.026(stat) ± 0.021(syst) ± 0.012(BR) [3], respectively. Looking at run-2 data from 2016 collected by the ATLAS detector at √s = 13 TeV, we present a measurement of the relative branching fraction of B(B_s ^0 → ψ(2S)φ) / B(B_s ^0 → J/ψφ) = 0.55 ± 0.04(stat) ± 0.09(syst) determined through the relative efficiency-corrected yields of the two decay modes and the world average branching fractions of the charmonium mesons to dimuons. Mass measurement of the B_s ^0 has also been performed yielding m_B_s ^0 = 5367.22 ± 0.24(stat) ± 1.92(syst) MeV/c^2 being in well agreement with the current world average.