CMS-PAS-BPH-23-001

CMS-PAS-BPH-23-001
Measurement of the ratio of the $ \text{B}_\text{c}^{+}\to\text{J}/\psi\tau^{+}\nu_\tau $ and $ \text{B}_\text{c}^{+}\to\text{J}/\psi\mu^{+}\nu_\mu $ branching fractions using three-prong $ \tau $ lepton decays in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
29 August 2024

Abstract: A measurement of the ratio of branching fractions $ \mathcal{R}_{\text{J}/\psi} = \mathcal{B}(\text{B}_\text{c}^{+}\to\text{J}/\psi\tau^{+}\nu_\tau) / \mathcal{B}(\text{B}_\text{c}^{+}\to\text{J}/\psi\mu^{+}\nu_\mu) $ in the decay channel with $ \text{J}/\psi(\rightarrow\mu^{+}\mu^{-}) $ and $ \tau^{+}\rightarrow\pi^{+}\pi^{-}\pi^{+}(+\pi^{0})\bar{\nu}_{\tau} $ (and charge-conjugated processes) is presented. This measurement uses a sample of proton-proton collision data, corresponding to an integrated luminosity of 138 fb$ ^{-1} $, collected at a center-of-mass energy of 13 TeV by the CMS experiment in the LHC Run 2. The ratio is measured to be $ \mathcal{R}_{\text{J}/\psi}= 1.04 _{-0.44}^{+0.50} $, which in combination with the measurement of $ \mathcal{R}_{\text{J}/\psi} $ in the decay channel with $ \text{J}/\psi\rightarrow\mu^{+}\mu^{-} $ and $ \tau^{+}\rightarrow\mu^{+}\nu_{\mu}\bar{\nu}_{\tau} $, yields $ \mathcal{R}_{\text{J}/\psi}= $ 0.49 $ \pm $ 0.25 $ \,\mathrm{(stat)} \pm 0.09\,\mathrm{(syst)} $ in agreement with the value predicted by the standard model of particle physics, 0.258 $ \pm $ 0.004.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Tree level $ \mathrm{b}\to\mathrm{c}\ell\nu $ transition representation: in the SM, the vertex is mediated by a W boson exchange, while in BSM scenarios other contributions could be foreseen.
png pdf	Figure 2: Distribution of the BDT score. The data, represented by black points, is compared to the expectations from simulations (where $ \mathrm{H}_\mathrm{b}\to{\mathrm{J}/\psi} \mathrm{X} $ shown in orange, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \tau^{+}\nu_{\!\tau} $ in purple, $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} \mathrm{D}_\mathrm{s}^{(\ast)+} $ in cyan and $ \mathrm{B}_{c}^{+}\rightarrow\mathrm{others} $ in lime green). The $ \mathrm{H}_\mathrm{b}\to{\mathrm{J}/\psi} \mathrm{X} $ simulated contribution is normalized to match the yield in data, as obtained following the application of all event selection criteria except for the BDT. The $ \mathrm{B}_{c}^{+}\to{\mathrm{J}/\psi} X $ processes are normalized according to their expectations. The ratio between the data and the expected sum of signal and background contributions is shown in the lower panel. The uncertainty displayed is statistical.
png pdf	Figure 3: Distributions of the unrolled $ \rho^0 $ mass variable in the sideband (left) and signal-enriched (right) data regions. In each panel, data (black markers), is compared to the some of the expected yields from data-driven $ \mathrm{H}_\mathrm{b}\to{\mathrm{J}/\psi} \mathrm{X} $ background and simulation-based $ \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}^{+}\rightarrow\mathrm{others} $ (orange, purple, cyan, and lime green histograms, respectively). The normalization and shape parameters for the different contributions, as well as $ \mathcal{R}_{\mathrm{J}/\psi} $, are shown for their best fit values. The ratio between the data and the expected sum of signal and background contributions is shown in the lower panel of each observable. The postfit total uncertainty in the expectation is represented by the hashed band.
png pdf	Figure 3-a: Distributions of the unrolled $ \rho^0 $ mass variable in the sideband (left) and signal-enriched (right) data regions. In each panel, data (black markers), is compared to the some of the expected yields from data-driven $ \mathrm{H}_\mathrm{b}\to{\mathrm{J}/\psi} \mathrm{X} $ background and simulation-based $ \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}^{+}\rightarrow\mathrm{others} $ (orange, purple, cyan, and lime green histograms, respectively). The normalization and shape parameters for the different contributions, as well as $ \mathcal{R}_{\mathrm{J}/\psi} $, are shown for their best fit values. The ratio between the data and the expected sum of signal and background contributions is shown in the lower panel of each observable. The postfit total uncertainty in the expectation is represented by the hashed band.
png pdf	Figure 3-b: Distributions of the unrolled $ \rho^0 $ mass variable in the sideband (left) and signal-enriched (right) data regions. In each panel, data (black markers), is compared to the some of the expected yields from data-driven $ \mathrm{H}_\mathrm{b}\to{\mathrm{J}/\psi} \mathrm{X} $ background and simulation-based $ \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}^{+}\rightarrow\mathrm{others} $ (orange, purple, cyan, and lime green histograms, respectively). The normalization and shape parameters for the different contributions, as well as $ \mathcal{R}_{\mathrm{J}/\psi} $, are shown for their best fit values. The ratio between the data and the expected sum of signal and background contributions is shown in the lower panel of each observable. The postfit total uncertainty in the expectation is represented by the hashed band.
png pdf	Figure 4: Summary of the $ \mathcal{R}_{\mathrm{J}/\psi} $ measurements. The upper row corresponds to the existing LHCb [8] measurement. The second and third row, respectively, correspond to the CMS analysis using leptonic tau decays [14] and hadronic tau decay. The last row represents the combination of the CMS analyses in the leptonic and hadronic final states. The total uncertainties are indicated by the horizontal lines. The SM value is shown as a red perpendicular line.

Tables

png pdf

Table 1:
List of systematic uncertainties. The first category is for the shape and the second category is for the normalization, denoted as ``shape" and ``norm.", respectively, in the table. The presence of a check mark (\checkmark) represents nuisance parameters that are correlated among the ones that are shown in that row. The empty column denotes no corresponding nuisance parameters being assigned. The DD stands for data driven.

Summary

A measurement of the ratio of the branching fractions $ \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}) $ is described. For the numerator, the analysis targets final states with hadronically decaying $ \tau $ lepton into $ \tau^{+}\to\pi^{+}\pi^{-}\pi^{+}(+\pi^{0})\nu_{\!\tau} $. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 138 fb$ ^{-1} $ and collected at $ \sqrt{s} = $ 13 TeV by the CMS experiment during the LHC Run 2. The ratio is measured to be $ \mathcal{R}_{\mathrm{J}/\psi} = 1.04 _{-0.44}^{+0.50} $. This measurement is combined with another that considers the leptonic decay of the tau lepton [14] to result in a measured $ \mathcal{R}_{\mathrm{J}/\psi} $ value of 0.49 $ \pm $ 0.25 (stat) $ \pm $ 0.09 (syst), which is consistent with the SM prediction of 0.258 $ \pm $ 0.004 within 1 $ \sigma $ standard deviation.

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