CMS-PAS-FTR-18-013

CMS-PAS-FTR-18-013
Measurement of rare $\mathrm{B} \to \mu^+\mu^-$ decays with the Phase-2 upgraded CMS detector at the HL-LHC
CMS Collaboration
December 2018

Abstract: The sensitivity of the upgraded CMS detector for measuring the rare decays $\mathrm{B_{s}}^{0}\to \mu^+ \mu^-$ and $\mathrm{B^0}\to \mu^+ \mu^-$ in the HL-LHC scenario is studied. The upgraded detector, especially with its improved momentum resolution, and the foreseen total integrated luminosity of 3000 fb $^{-1}$ are expected to enable high precision measurements of the branching fractions of $\mathrm{B_{s}}^{0}\to \mu^+ \mu^-$ and the effective lifetime of the $\mathrm{B_{s}}^{0}\to \mu^+ \mu^-$ decay with reduced systematic and statistical uncertainties. At 3000 fb $^{-1}$ , it will also be possible to observe the $\mathrm{B^0}\to \mu^+ \mu^-$ decay with more than 5 $\sigma$ significance.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The left plot shows the ${\mathrm{B_{s}}^{0}}$ and ${\mathrm{B^0}}$ invariant mass distributions in the Run-2 scenario. The right plot shows the ${\mathrm{B_{s}}^{0}}$ and ${\mathrm{B^0}}$ invariant mass distributions for Phase-2. The ${\mathrm{B_{s}}^{0}}$ distribution is normalized to unity and the ${\mathrm{B^0}}$ distribution is normalized according to the SM expectation.
png pdf	Figure 1-a: The plot shows the ${\mathrm{B_{s}}^{0}}$ and ${\mathrm{B^0}}$ invariant mass distributions in the Run-2 scenario.
png pdf	Figure 1-b: The plot shows the ${\mathrm{B_{s}}^{0}}$ and ${\mathrm{B^0}}$ invariant mass distributions for Phase-2. The ${\mathrm{B_{s}}^{0}}$ distribution is normalized to unity and the ${\mathrm{B^0}}$ distribution is normalized according to the SM expectation.
png pdf	Figure 2: (left)Mass distributions for ${\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}}$ in the Run-2 and Phase-2 scenarios for $\|\eta _f\| <$ 1.4. A single Gaussian is fit to the core of the mass distribution (see text for details). (right) Mass resolution as a function of $\|\eta _f\|$ .
png pdf	Figure 2-a: Mass distributions for ${\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}}$ in the Run-2 and Phase-2 scenarios for $\|\eta _f\| <$ 1.4. A single Gaussian is fit to the core of the mass distribution (see text for details).
png pdf	Figure 2-b: Mass resolution as a function of $\|\eta _f\|$ .
png pdf	Figure 3: Contribution of ${\mathrm{B^0}\to \pi ^- {\mu ^+} \nu}$ background events (with the pion misidentified as a muon) into the signal regions. The ratio of number of ${\mathrm{B^0}\to \pi ^- {\mu ^+} \nu}$ events for Phase-2 to Run-2 is 5/19 in the mass interval 5.2 $< m <$ 5.3 GeV of the ${\mathrm{B^0}}$ signal region.
png pdf	Figure 4: Normalized isolation variable distributions for the ${\mathrm{B_{s}}^{0}}$ signal for the two pile-up scenarios is shown. The blue distribution represents the case with no pile-up while the red one is for average pile-up of 200 interactions per bunch crossing. In the bottom, the ratio between the PU=0 and the PU=200 distributions is also shown.
png pdf	Figure 5: Invariant mass distributions with the fit projection overlayed, corresponding to an integrated luminosity of 3000 fb $^{-1}$ . The left plot shows the central barrel region, $\|\eta _{f}\| <$ 0.7 and the right plot is for 0.7 $< \|\eta _f\| <$ 1.4.
png pdf	Figure 5-a: Invariant mass distribution with the fit projection overlayed, corresponding to an integrated luminosity of 3000 fb $^{-1}$ in the central barrel region, for $\|\eta _{f}\| <$ 0.7.
png pdf	Figure 5-b: Invariant mass distribution with the fit projection overlayed, corresponding to an integrated luminosity of 3000 fb $^{-1}$ for 0.7 $< \|\eta _f\| <$ 1.4.
png pdf	Figure 6: The binned maximum likelihood fit to the background-subtracted decay time distribution for the Phase-2 scenario. The effective lifetime from the fit is 1.61 $\pm$ 0.05 ps.

Tables
png pdf	Table 1: Input sources of systematic uncertainties and the propagated uncertainties on the ${\mathrm{B} \to \mu ^+\mu ^-}$ branching fractions, $\delta {{\mathcal B}}({\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}})$ and $\delta {{\mathcal B}}({\mathrm{B^0} \to {\mu ^+} {\mu ^-}})$ .
png pdf	Table 2: Mass resolutions for ${\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}}$ and ${\mathrm{B^0} \to {\mu ^+} {\mu ^-}}$ , obtained from Gaussian fits to the core of the respective mass distributions (see text for details). The last column shows the ratio between the Run-2 and Phase-2 resolutions.
png pdf	Table 3: Estimated analysis sensitivity for different integrated luminosities. Columns in the table, from left to right: the total integrated luminosity, the median expected number of reconstructed ${\mathrm{B_{s}}^{0}}$ and ${\mathrm{B^0}}$ mesons, the total uncertainties on the ${\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}}$ and ${\mathrm{B^0} \to {\mu ^+} {\mu ^-}}$ branching fractions, the range of the significance of ${\mathrm{B^0}}$ observation (the range indicates the $\pm$ 1 $\sigma$ of the distribution of significance) and the statistical uncertainty on the ${\mathrm{B_{s}}^{0}\to {\mu ^+} {\mu ^-}}$ effective lifetime.

Summary

The inner tracker of the Phase-2 detector provides an order of 40-50% improvement on the mass resolutions over the Run-2 case that will allow precise measurements of the

$\mathrm{B_{s}}^{0}\to \mu^+ \mu^-$ and

$\mathrm{B^0}\to \mu^+ \mu^-$ rare decays. The semileptonic background contribution into the signal regions will be reduced substantially and the improved separation of the

$\mathrm{B_{s}}^{0}$ and

$\mathrm{B^0}$ yields will lower the signal cross feed contamination, which is crucial for the

$\mathrm{B^0}$ observation. With an integrated luminosity of 3000 fb

$^{-1}$ , CMS will have the capability to measure the

$\mathrm{B_{s}}^{0}\to \mu^+ \mu^-$ effective lifetime with an error of about 0.05 ps and to observe the

$\mathrm{B^0}\to \mu^+ \mu^-$ decay with more than 5 standard deviation significance.

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