Abstract
A study of the decays Bs0 → μ+μ− and B0 → μ+μ− has been performed using 26.3 fb−1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in μ+μ− invariant mass is comparable to the Bs0-B0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction mathrm{mathcal{B}}left({B}_s^0to {mu}^{+}{mu}^{-}right)=left({3.2}_{-1.0}^{+1.1}right)times {10}^{-9} and an upper limit mathrm{mathcal{B}}left({B}^0to {mu}^{+}{mu}^{-}right)<4.3times {10}^{-10} at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding mathrm{mathcal{B}}left({B}_s^0to {mu}^{+}{mu}^{-}right)=left({2.8}_{-0.7}^{+0.8}right)times {10}^{-9} and mathrm{mathcal{B}}left({B}^0to {mu}^{+}{mu}^{-}right)<2.1times {10}^{-10} at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the mathrm{mathcal{B}}left({B}^0to {mu}^{+}{mu}^{-}right)-mathrm{mathcal{B}}left({B}_s^0to {mu}^{+}{mu}^{-}right) plane.
Highlights
Data selectionFor data collected during the LHC Run 2, the ATLAS detector uses a two-level trigger system, consisting of a hardware-based first-level trigger and a software-based high-level trigger
Background compositionThe background to the B(0s) → μ+μ− signal originates from three main sources: Continuum background, the dominant combinatorial component, which consists of muons originating from uncorrelated hadron decays and is characterised by a weak dependence on the dimuon invariant mass; Partially reconstructed decays, where one or more of the final-state particles (X) in a bhadron decay is not reconstructed, causing these candidates to accumulate in the low dimuon invariant mass sideband; Peaking background, due to B(0s) → hh decays, with both hadrons misidentified as muons.The continuum background consists mainly of muons produced independently in the fragmentation and decay chains of a b-quark and ab-quark
The numbers of background events contained in the signal region (5166–5526 MeV) are computed from the interpolation of the data observed in the sidebands
Summary
For data collected during the LHC Run 2, the ATLAS detector uses a two-level trigger system, consisting of a hardware-based first-level trigger and a software-based high-level trigger. A full track reconstruction of the muon candidates is performed by the high-level trigger, where an additional loose selection is imposed on the dimuon invariant mass mμμ, accepting candidates in the range 4 GeV to 8.5 GeV. Due to the increased pile-up in 2016 data, an additional selection was added at this trigger stage, requiring the vector from the primary vertex to the dimuon vertex to have a positive component (Lxy) along the dimuon’s transverse momentum direction. The effect of this selection is accounted for in the analysis but has no consequence since stricter requirements are applied in the full event selection (see section 6). Approximately 3.5 × 106 candidates are found in the B(0s) → μ+μ− fit region, with about 1.0 × 106 falling in the blinded range [5166, 5526] MeV
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
