CMS-PAS-BPH-16-004

CMS-PAS-BPH-16-004
Measurement of properties of ${\rm B_s^0}\to\mu^+\mu^-$ decays and search for ${\rm B^0}\to\mu^+\mu^-$ with the CMS experiment
CMS Collaboration
August 2019

Abstract: Results are reported for the ${\rm B_s^0}\to\mu^+\mu^-$ branching fraction and effective lifetime and from a search for the decay ${\rm B^0}\to\mu^+\mu^-$. The analysis uses a data sample of proton-proton collisions accumulated by the CMS experiment in 2011, 2012, and 2016, with center-of-mass energies (integrated luminosities) of 7 TeV (5 fb$^{-1}$), 8 TeV (20 fb$^{-1}$), and 13 TeV (36 fb$^{-1}$). The branching fractions are determined by measuring efficiency-corrected event yields relative to ${\rm B^+}\to{\rm J}/\psi {\rm K^+}$ decays (with ${\rm J}/\psi\to\mu^+\mu^-$), which results in the cancellation of many of the systematic uncertainties. The decay ${\rm B_s^0}\to\mu^+\mu^-$ is observed with a branching fraction of ${\cal B}({\rm B_s^0}\to\mu^+\mu^-) =$ [2.9$^{+0.7}_{-0.6}$ (exp) $\pm$ 0.2 (frag)] $\times10^{-9}$, where frag refers to the uncertainty in the ratio $f_s/f_u$ of the ${\rm B_s^0}$ and the ${\rm B^+}$ fragmentation functions, corresponding to a significance of 5.6 standard deviations. No significant excess is observed for the decay ${\rm B^0}\to\mu^+\mu^-$, and the upper limit ${\cal B}({\rm B^0}\to\mu^+\mu^-) < 3.6\times10^{-10}$ is obtained at 95% confidence level. These measured branching fractions are consistent with standard model predictions, and they supersede previous results from CMS based on the 2011 and 2012 data only. Finally, the ${\rm B_s^0}\to\mu^+\mu^-$ effective lifetime is measured, for the first time in CMS, and is found to be $\tau_{\mu^+\mu^-} = $ 1.70$^{+0.61}_{-0.44}$ ps.
Links: CDS record (PDF) ; inSPIRE record ; HepData record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 04 (2020) 188. The superseded preliminary plots can be found here.

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