CMS-BPH-23-001

CMS-BPH-23-001 ; CERN-EP-2025-196
Measurement of the ratio of the $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $ and $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mu^{+}\nu_{\!\mu} $ branching fractions using three-prong $ \tau $ lepton decays
CMS Collaboration
24 October 2025
Submitted to Phys. Rev. Lett.
Abstract: The ratio between the $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $ and $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mu^{+}\nu_{\!\mu} $ branching fractions is measured using a data sample of proton-proton collisions collected by CMS at a center-of-mass energy of 13 TeV in the years 2016-2018 and corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The $ \mathrm{J}/\psi $ meson is identified through its $ {\mathrm{J}/\psi} \to\mu^{+}\mu^{-} $ decay and the tau lepton is reconstructed in the hadronic three-prong final state. The measured ratio of branching fractions in this tau decay mode, $ \mathcal{R}_{\mathrm{J}/\psi} ^{\text{had}} = $ 1.04$_{-0.44}^{+0.50} $, is combined with the previous analysis based on the $ \tau^{+}\to\mu^{+}\nu_{\!\mu}\overline{\nu}_{\!\tau} $ leptonic decay channel, leading to $ \mathcal{R}_{\mathrm{J}/\psi} = $ 0.49 $ \pm $ 0.26. As this result is consistent with the standard model prediction of 0.258 $ \pm $ 0.004, no evidence of lepton flavor universality violation is found.
Links: e-print arXiv:2510.21559 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $ s_\mathrm{BDT} $ distribution. The data are shown by closed circles, with vertical bars representing the statistical uncertainties. The simulated $ \mathrm{H_b}\to{\mathrm{J}/\psi} \mathrm{X} $ contribution is normalized to match the yield in data after applying all event selection criteria except the BDT. The $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{X} $ processes are normalized before the $ \mathcal{R}_{\mathrm{J}/\psi} $ fit. The ratio between the data and the sum of expected signal and background contributions is shown in the lower panel. The statistical uncertainty in the sum of the simulated contributions is shown as a gray band.
png pdf	Figure 2: Distribution of the SR unrolled $ m(\rho_1) $ vs.\ $ m(\rho_2) $ distribution for the 2016 (upper left), 2017 (upper right), and 2018 (lower) data-taking periods. The dashed vertical lines indicate bins in $ m(\rho_1) $. The data (black markers) are compared to the postfit expected yields of the $ \mathrm{H_b}\to{\mathrm{J}/\psi} \mathrm{X} $ background estimated from data and simulated $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{D}_\mathrm{s}^{(\ast)+} $, and $ \mathrm{B}_{c}^{+}\to\text{others} $ processes (stacked histograms). The ratio between the data and the sum of postfit signal and background is shown in the lower panel. The total uncertainty in the expected event yield is shown by the gray band.
png pdf	Figure 2-a: Distribution of the SR unrolled $ m(\rho_1) $ vs.\ $ m(\rho_2) $ distribution for the 2016 (upper left), 2017 (upper right), and 2018 (lower) data-taking periods. The dashed vertical lines indicate bins in $ m(\rho_1) $. The data (black markers) are compared to the postfit expected yields of the $ \mathrm{H_b}\to{\mathrm{J}/\psi} \mathrm{X} $ background estimated from data and simulated $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{D}_\mathrm{s}^{(\ast)+} $, and $ \mathrm{B}_{c}^{+}\to\text{others} $ processes (stacked histograms). The ratio between the data and the sum of postfit signal and background is shown in the lower panel. The total uncertainty in the expected event yield is shown by the gray band.
png pdf	Figure 2-b: Distribution of the SR unrolled $ m(\rho_1) $ vs.\ $ m(\rho_2) $ distribution for the 2016 (upper left), 2017 (upper right), and 2018 (lower) data-taking periods. The dashed vertical lines indicate bins in $ m(\rho_1) $. The data (black markers) are compared to the postfit expected yields of the $ \mathrm{H_b}\to{\mathrm{J}/\psi} \mathrm{X} $ background estimated from data and simulated $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{D}_\mathrm{s}^{(\ast)+} $, and $ \mathrm{B}_{c}^{+}\to\text{others} $ processes (stacked histograms). The ratio between the data and the sum of postfit signal and background is shown in the lower panel. The total uncertainty in the expected event yield is shown by the gray band.
png pdf	Figure 2-c: Distribution of the SR unrolled $ m(\rho_1) $ vs.\ $ m(\rho_2) $ distribution for the 2016 (upper left), 2017 (upper right), and 2018 (lower) data-taking periods. The dashed vertical lines indicate bins in $ m(\rho_1) $. The data (black markers) are compared to the postfit expected yields of the $ \mathrm{H_b}\to{\mathrm{J}/\psi} \mathrm{X} $ background estimated from data and simulated $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{D}_\mathrm{s}^{(\ast)+} $, and $ \mathrm{B}_{c}^{+}\to\text{others} $ processes (stacked histograms). The ratio between the data and the sum of postfit signal and background is shown in the lower panel. The total uncertainty in the expected event yield is shown by the gray band.
png pdf	Figure 3: Summary of $ \mathcal{R}_{\mathrm{J}/\psi} $ measurements from LHCb leptonic $ \tau $ decays [8], CMS leptonic $ \tau $ decays [14], CMS hadronic $ \tau $ decays, and the CMS combination. The total uncertainties in the measured values are indicated by the horizontal lines. The vertical red line is the SM prediction.

Tables
png pdf	Table 1: Systematic uncertainties considered in this analysis. The upper and lower entries in the table are the uncertainties affecting the shape and normalization of the processes given in the third column. A given uncertainty treated as correlated between analysis channels is marked with a check mark (\checkmark).

Summary

In summary, the branching fraction ratio $ \mathcal{R}_{\mathrm{J}/\psi} = \mathcal{B}(\mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau}) / \mathcal{B}(\mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mu^{+}\nu_{\!\mu}) $ has been measured using $ {\mathrm{J}/\psi} \to\mu^{+}\mu^{-} $ decays for both the numerator and denominator, and the three-prong hadronic $ \tau $ decay for the numerator. Proton-proton collision data corresponding to an integrated luminosity of 138 fb$ ^{-1} $ for the numerator and 59.7 fb$ ^{-1} $ for the denominator, both collected at $ \sqrt{s} = $ 13 TeV by the CMS experiment, are used. This measurement is the first contribution to lepton flavor universality tests in $ \mathrm{B}_{c}^{+} $ meson decays with hadronic $ \tau $ decays. The ratio is measured to be $ \mathcal{R}_{\mathrm{J}/\psi} ^{\text{had}}=$ 1.04$_{-0.44}^{+0.50} $. Combined with our previous measurement using leptonic $ \tau $ decays from Ref. [14], the result is $ \mathcal{R}_{\mathrm{J}/\psi} = $ 0.49 $ \pm $ 0.26. No deviation from the standard model prediction ( 0.258 $ \pm $ 0.004 ) is observed.

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