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Measurement of the $B^0_s\to\mu^+\mu^-$ branching fraction and effective lifetime and search for $B^0\to\mu^+\mu^-$ decays

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Abstract

A search for the rare decays $B^0_s\to\mu^+\mu^-$ and $B^0\to\mu^+\mu^-$ is performed at the LHCb experiment using data collected in $pp$ collisions corresponding to a total integrated luminosity of 4.4 fb$^{-1}$. An excess of $B^0_s\to\mu^+\mu^-$ decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be ${\cal B}(B^0_s\to\mu^+\mu^-)=\left(3.0\pm 0.6^{+0.3}_{-0.2}\right)\times 10^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B^0_s\to\mu^+\mu^-$ effective lifetime, $\tau(B^0_s\to\mu^+\mu^-)=2.04\pm 0.44\pm 0.05$ ps, is reported. No significant excess of $B^0\to\mu^+\mu^-$ decays is found and a 95 % confidence level upper limit, ${\cal B}(B^0\to\mu^+\mu^-)<3.4\times 10^{-10}$, is determined. All results are in agreement with the Standard Model expectations.

Figures and captions

Mass distribution of the selected $ B^{0}_{(s)}\rightarrow \mu^+\mu^-$ candidates (black dots) with ${\rm BDT }>0.5$. The result of the fit is overlaid, and the different components are detailed.

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(Top) Mass distribution of the selected $ B^{0}_{(s)}\rightarrow \mu^+\mu^-$ candidates (black dots) with ${\rm BDT }>0.55$. The result of the fit is overlaid together with the $ B ^0_ s \!\rightarrow \mu ^+ \mu ^- $ (red dashed line) and the combinatorial background (blue dashed line) components. (Bottom) Background-subtracted $ B ^0_ s \!\rightarrow \mu ^+ \mu ^- $ decay-time distribution with the fit result superimposed.

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Supplementary Material [file]

Supplementary material full pdf

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.