CMS-PAS-BPH-21-006

CMS-PAS-BPH-21-006
Measurement of ${\mathrm B^0_s}\to\mu^+\mu^-$ decay properties and search for the ${\mathrm B}^0\to\mu\mu$ decay in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
July 2022

Abstract: We report a measurement of the ${\mathrm B^0_s}\to\mu^+\mu^-$ branching fraction and its effective lifetime, as well as results of a search for the ${\mathrm B}^0\to\mu^+\mu^-$ decay in proton-proton collisions at $\sqrt{s}=$ 13 TeV at the LHC. The analysis is based on data collected with the CMS detector in 2016-2018 corresponding to an integrated luminosity of 140 fb $^{-1}$ . The measured branching fraction of the ${\mathrm B^0_s}\to\mu^+\mu^-$ decay and the effective ${\mathrm B^0_s}$ lifetime are the most precise measurements to date. No evidence for the ${\mathrm B}^0\to\mu^+\mu^-$ decay has been found. All results are found to be consistent with the standard model predictions.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 842 (2023) 137955. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distributions of the MVA $_{\text{B}}$ output for 2016a (left), 2016b (center), and 2017-2018 (right) data and the corresponding simulations. The blue histograms represent reweighted MC simulations using the XGBoost reweighting method. The MC distributions are normalized to the data integral.
png pdf	Figure 1-a: Distribution of the MVA $_{\text{B}}$ output for 2016a data and the corresponding simulations. The blue histogram represents the reweighted MC simulations using the XGBoost reweighting method. The MC distribution is normalized to the data integral.
png pdf	Figure 1-b: Distribution of the MVA $_{\text{B}}$ output for 2016b data and the corresponding simulations. The blue histogram represents the reweighted MC simulations using the XGBoost reweighting method. The MC distribution is normalized to the data integral.
png pdf	Figure 1-c: Distribution of the MVA $_{\text{B}}$ output for 2017-2018 data and the corresponding simulations. The blue histogram represents the reweighted MC simulations using the XGBoost reweighting method. The MC distribution is normalized to the data integral.
png pdf	Figure 2: The distribution of the B meson $p_{\mathrm {T}}$ after the sPlot background subtraction in data (points with the error bars) and simulation (hatched histogram) for ${\mathrm{B^{+}} \to \mathrm{J}/\psi \mathrm{K^{+}}}$ (left) and ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ (right) events. The MC distributions are normalized to the data integral.
png pdf	Figure 2-a: The distribution of the B meson $p_{\mathrm {T}}$ after the sPlot background subtraction in data (points with the error bars) and simulation (hatched histogram) for ${\mathrm{B^{+}} \to \mathrm{J}/\psi \mathrm{K^{+}}}$ events. The MC distribution is normalized to the data integral.
png pdf	Figure 2-b: The distribution of the B meson $p_{\mathrm {T}}$ after the sPlot background subtraction in data (points with the error bars) and simulation (hatched histogram) for ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ events. The MC distribution is normalized to the data integral.
png pdf	Figure 3: The projections on the dimuon mass axis for the branching fraction fit for the MVA $_{\text{B}} >$ 0.99 category (left) and 0.99 $>$ MVA $_{\text{B}} >$ 0.90 category (right). The blue curves represent the corresponding projections of the final fit model.
png pdf	Figure 3-a: The projection on the dimuon mass axis for the branching fraction fit for the MVA $_{\text{B}} >$ 0.99 category. The blue curve represents the corresponding projection of the final fit model.
png pdf	Figure 3-b: The projection on the dimuon mass axis for the branching fraction fit for the 0.99 $>$ MVA $_{\text{B}} >$ 0.90 category. The blue curve represents the corresponding projection of the final fit model.
png pdf	Figure 4: The profile likelihood scan as a function of ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ (left) and ${\mathrm{B}^{0} \to \mu^{+} \mu^{-}}$ (right) decay branching fractions in 1D (upper plots) and in 2D (lower plot). The contours in 2D enclose the regions with 1-5 standard deviation coverage, where 1 $\sigma$ , 2 $\sigma$ and 3 $\sigma$ regions correspond to 68.3%, 95.4%, and 99.7% confidence level, respectively.
png pdf	Figure 4-a: The profile likelihood scan as a function of the ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ decay branching fraction in 1D.
png pdf	Figure 4-b: The profile likelihood scan as a function of the ${\mathrm{B}^{0} \to \mu^{+} \mu^{-}}$ decay branching fraction in 1D.
png pdf	Figure 4-c: The profile likelihood scan as a function of ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ and ${\mathrm{B}^{0} \to \mu^{+} \mu^{-}}$ decay branching fractions in 2D. The contours enclose the regions with 1-5 standard deviation coverage, where 1 $\sigma$ , 2 $\sigma$ and 3 $\sigma$ regions correspond to 68.3%, 95.4%, and 99.7% confidence level, respectively.
png pdf	Figure 5: The upper limits on the ${\mathrm{B}^{0} \to \mu^{+} \mu^{-}}$ decay branching fraction using the CL $_\text {s}$ method.
png pdf	Figure 6: The UML fit projection on the decay time axis for the signal region 5.28 $< m_{{\mu \mu}} <$ 5.48 GeV.

Tables
png pdf	Table 1: Selection summary before the analysis MVA $_{\text{B}}$ .
png pdf	Table 2: Efficiency corrections for the ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ decays derived using two different methods: the efficiency ratio between data and simulation and XGBoost reweighting in ${\mathrm{B^{+}} \to \mathrm{J}/\psi \mathrm{K^{+}}}$ events. The loose region is defined as MVA $_{\text{B}} >$ 0.9 and the tight one as MVA $_{\text{B}} >$ 0.99.
png pdf	Table 3: Summary of the systematic uncertainties for the ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ and ${\mathrm{B}^{0} \to \mu^{+} \mu^{-}}$ branching fraction measurements.
png pdf	Table 4: Summary of the systematic uncertainties in the ${\mathrm{B_s}^{0}\to \mu^{+} \mu^{-}}$ effective lifetime measurement (ps).
png pdf	Table 5: Expected and observed event yields for each category (postfit). Channel 0 and 1 refer to the ${\| {\eta _{\text {F}}} \|}$ ranges of [0.0,0.7] and [0.7,1.4], respectively.

Summary

A measurement of the branching fraction of the

$\mathrm{B_s}^{0} \to \mu^{+}\mu^{-}$ decay based on the data set of proton-proton collisions at

$\sqrt{s} =$ 13 TeV corresponding to an integrated luminosity of 140 fb

$^{-1}$ has been presented and is found consistent with the standard model (SM) predictions within one standard deviation. The relative combined uncertainty is reduced from 23 to 11% compared with the previous CMS measurement [4] based on the partial 13 TeV data set, while the central value is found to be somewhat higher. The new analysis applied to the data used in Ref. [4] yields a result similar to the original measurement, indicating that the shift in the central value is driven mostly by the new data.

The search for the

$\mathrm{B_d}^{0} \to \mu^{+}\mu^{-}$ decay has not revealed a significant event excess with respect to the dominant combinatorial background prediction. More data will be required to establish its existence and compare the result with the SM predictions.

Compared with the latest LHCb measurement of

$\mathcal{B}(\mathrm{B_s}^{0} \to \mu^{+}\mu^{-}) =$ (3.09

$^{+0.46}_{-0.43}$

$^{+0.15}_{-0.11}$ )

$\times$ 10

$^{-9}$ [6], our result is about 1.2 standard deviations higher, which is going to shift the world average from its current value

$\mathcal{B}(\mathrm{B_s}^{0} \to \mu^{+}\mu^{-}) =$ (2.69

$^{+0.37}_{-0.35}$ )

$\times$ 10

$^{-9}$ [7] to a larger value, more consistent with the SM prediction, thus reducing the overall tension. This implies that the potential beyond-the-SM physics contribution preferred by the global fits to the rare

${\mathrm{b} \to \mathrm{s}\ell^+\ell^-}$ decay data [20] is most likely constrained to a single semileptonic operator

$O_9$ .

The uncertainties in the branching fraction and effective lifetime measurements are dominated by the statistical component, which allows us to expect significant improvements in the precision of the future measurements with the LHC Run 3 data.

The effective lifetime measurement of the

$\mathrm{B_s}^{0} \to \mu^{+}\mu^{-}$ decay is also consistent with the SM predictions and has achieved the level of uncertainty comparable with the lifetime difference between the heavy and light

$\mathrm{B_s}^{0}$ meson mass eigenstates, thus offering sensitivity to potential beyond-the-SM physics effects in the effective lifetime.

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