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| CMS-PAS-BPH-26-005 | ||
| Measurement of time-dependent $ CP $ violation in $ \mathrm{B}^0_{(\mathrm{s})} \to \mathrm{J}/\psi \mathrm{K}^0_\text{S} $ decays with the CMS detector | ||
| CMS Collaboration | ||
| 2026-05-27 | ||
| Abstract: A measurement of time-dependent $ CP $ violation in $ \mathrm{B}^0 \to \mathrm{J}/\psi \mathrm{K}^0_\text{S} $ and $ \mathrm{B}^0_\mathrm{s} \to \mathrm{J}/\psi \mathrm{K}^0_\text{S} $ decays is presented, using proton-proton collision data collected with the CMS detector at the CERN LHC at $ \sqrt{s} = $ 13.6 TeV during 2022-2025, corresponding to an integrated luminosity of 274 fb$ ^{-1} $. The $ CP $-violating parameters $ S $ and $ C $, describing mixing-induced and direct $ CP $ violation, respectively, are extracted from a fit to the time-dependent $ CP $ asymmetry, using samples of approximately 1.4 million reconstructed $ \mathrm{B}^0 $ and 16 thousand reconstructed $ \mathrm{B}^0_\mathrm{s} $ signal candidates. The production flavour of the B meson is determined using a dedicated flavour-tagging framework comprising opposite-side muon, electron, and jet taggers, together with a same-side tagger for $ \mathrm{B}^0_\mathrm{s} $ decays, all based on state-of-the-art AI architectures. The measured values are $ S_{\mathrm{B}^0} = $ 0.710 $ \pm $ 0.013 (stat) $ \pm $ 0.009 (syst), $ C_{\mathrm{B}^0} = $ 0.013 $ \pm $ 0.011 (stat) $ \pm $ 0.006 (syst), $ S_{\mathrm{B}^0_\mathrm{s}} = $ 0.00 $ \pm $ 0.19 (stat) $ \pm $ 0.00 (syst), and $ C_{\mathrm{B}^0_\mathrm{s}} = - $ 0.18 $ \pm $ 0.23 (stat) $ \pm $ 0.01 (syst). All results are consistent with standard model predictions. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Results of the flavour tagging DNNs score calibration fit on $ {\mathrm{B}^{+}} \to \mathrm{J}/\psi \mathrm{K^+} $ decays for 2024 data, the largest data set. The measured tagging accuracy probability is plotted versus the value predicted by the tagging algorithm. The solid line shows the results of the Platt scaling calibration fit to data. |
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Figure 1-a:
Results of the flavour tagging DNNs score calibration fit on $ {\mathrm{B}^{+}} \to \mathrm{J}/\psi \mathrm{K^+} $ decays for 2024 data, the largest data set. The measured tagging accuracy probability is plotted versus the value predicted by the tagging algorithm. The solid line shows the results of the Platt scaling calibration fit to data. |
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Figure 1-b:
Results of the flavour tagging DNNs score calibration fit on $ {\mathrm{B}^{+}} \to \mathrm{J}/\psi \mathrm{K^+} $ decays for 2024 data, the largest data set. The measured tagging accuracy probability is plotted versus the value predicted by the tagging algorithm. The solid line shows the results of the Platt scaling calibration fit to data. |
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Figure 1-c:
Results of the flavour tagging DNNs score calibration fit on $ {\mathrm{B}^{+}} \to \mathrm{J}/\psi \mathrm{K^+} $ decays for 2024 data, the largest data set. The measured tagging accuracy probability is plotted versus the value predicted by the tagging algorithm. The solid line shows the results of the Platt scaling calibration fit to data. |
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Figure 1-d:
Results of the flavour tagging DNNs score calibration fit on $ {\mathrm{B}^{+}} \to \mathrm{J}/\psi \mathrm{K^+} $ decays for 2024 data, the largest data set. The measured tagging accuracy probability is plotted versus the value predicted by the tagging algorithm. The solid line shows the results of the Platt scaling calibration fit to data. |
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Figure 2:
Mass distribution of the selected candidates, with the result of the fit overlaid. The signal components are shown in blue ($ {\mathrm{B}^0} \to \mathrm{J}/\psi \mathrm{K^0_S} $) and orange ($ \mathrm{B}_{s}^{0} \to \mathrm{J}/\psi \mathrm{K^0_S} $), while the combinatorial background is shown in green. |
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Figure 3:
Reconstructed proper decay time distribution of the selected candidates, COW-weighted for the $ {\mathrm{B}^0} $ (left) and $ \mathrm{B}_{s}^{0} $ (right) hypothesis. Only tagged candidates tagged as $ {\mathrm{B}}^{0}_{(s)} $ (blue) and $ \overline{\mathrm{B}}^{0}_{(s)} $ (red) are shown. |
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Figure 3-a:
Reconstructed proper decay time distribution of the selected candidates, COW-weighted for the $ {\mathrm{B}^0} $ (left) and $ \mathrm{B}_{s}^{0} $ (right) hypothesis. Only tagged candidates tagged as $ {\mathrm{B}}^{0}_{(s)} $ (blue) and $ \overline{\mathrm{B}}^{0}_{(s)} $ (red) are shown. |
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Figure 3-b:
Reconstructed proper decay time distribution of the selected candidates, COW-weighted for the $ {\mathrm{B}^0} $ (left) and $ \mathrm{B}_{s}^{0} $ (right) hypothesis. Only tagged candidates tagged as $ {\mathrm{B}}^{0}_{(s)} $ (blue) and $ \overline{\mathrm{B}}^{0}_{(s)} $ (red) are shown. |
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Figure 4:
Time-dependent $ CP $ asymmetry in $ {\mathrm{B}}^0_{(s)} \to \mathrm{J}/\psi \mathrm{K^0_S} $ decays, with the result of the fit overlaid. The shaded bands indicate the one, two, and three standard deviation confidence intervals of the fitted function. |
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Figure 4-a:
Time-dependent $ CP $ asymmetry in $ {\mathrm{B}}^0_{(s)} \to \mathrm{J}/\psi \mathrm{K^0_S} $ decays, with the result of the fit overlaid. The shaded bands indicate the one, two, and three standard deviation confidence intervals of the fitted function. |
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Figure 4-b:
Time-dependent $ CP $ asymmetry in $ {\mathrm{B}}^0_{(s)} \to \mathrm{J}/\psi \mathrm{K^0_S} $ decays, with the result of the fit overlaid. The shaded bands indicate the one, two, and three standard deviation confidence intervals of the fitted function. |
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Figure 5:
Two-dimensional ($ S $, $ C $) 68.3% CL contours obtained from the CPV measurements compared with the latest SM-based predictions [1] and the latest experimental results from BaBar, Belle, Belle-II, and LHCb [49]. |
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Figure 5-a:
Two-dimensional ($ S $, $ C $) 68.3% CL contours obtained from the CPV measurements compared with the latest SM-based predictions [1] and the latest experimental results from BaBar, Belle, Belle-II, and LHCb [49]. |
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Figure 5-b:
Two-dimensional ($ S $, $ C $) 68.3% CL contours obtained from the CPV measurements compared with the latest SM-based predictions [1] and the latest experimental results from BaBar, Belle, Belle-II, and LHCb [49]. |
| Tables | |
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Table 1:
Calibrated tagging performance evaluated in the $ {\mathrm{B}}^0_{(s)} \to \mathrm{J}/\psi \mathrm{K^0_S} $ data sample for different data-taking periods. The effective dilution $ \mathcal{D}_\text{tag}^2 $ is obtained from the measured tagging efficiency $ \epsilon_\text{tag} $ and tagging power $ P_\text{tag} $. |
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Table 2:
Summary of the systematic uncertainties for the physics parameters. The dashes ($ \text{---} $) indicate that the corresponding uncertainty is either not applicable or it was not evaluated. Statistical uncertainties are also presented to ease comparisons. |
| Summary |
| A measurement of time-dependent $ CP $ violation in $ {\mathrm{B}^0} \to \mathrm{J}/\psi \mathrm{K^0_S} $ and $ \mathrm{B}_{s}^{0} \to \mathrm{J}/\psi \mathrm{K^0_S} $ decays has been presented, using pp collision data collected with the CMS detector at $ \sqrt{s} = $ 13.6 TeV during 2022--2025, corresponding to an integrated luminosity of 274 fb$^{-1}$. The $ CP $-violating parameters $ S $ and $ C $, describing mixing-induced and direct CPV respectively, are extracted from fits to the time-dependent $ CP $ asymmetry constructed from approximately 1.4 million $ {\mathrm{B}^0} $ and 16 thousand $ \mathrm{B}_{s}^{0} $ signal candidates. The initial $ {\mathrm{B}}^{0}_{(s)} $ meson flavour at production is determined using a novel tagging framework based on the Particle Transformer architecture, comprising opposite-side muon, electron, and jet taggers together with a same-side tagger for the $ \mathrm{B}_{s}^{0} $ mode. Background contributions are statistically subtracted using the COW technique, which avoids any explicit modeling of background distributions in the physics observables. The measured values are $ S_{{\mathrm{B}^0}} = $ 0.710 $ \pm $ 0.016, $ C_{{\mathrm{B}^0}} = $ 0.013 $ \pm $ 0.012, $ S_{\mathrm{B}_{s}^{0}} = $ 0.00 $ \pm $ 0.19, and $ C_{\mathrm{B}_{s}^{0}} = - $ 0.18 $ \pm $ 0.23. All results are consistent with SM predictions. The simultaneous analysis of both modes within a single experiment provides a coherent probe of the CKM flavour sector and offers direct sensitivity to penguin-induced shifts in the interpretation of $ \sin{2\beta} $, contributing to the broader programme of precision tests of the SM in the $ {\mathrm{B}^0} $ and $ \mathrm{B}_{s}^{0} $ meson system, and enables further constraints on possible BSM contributions to quark flavour-changing processes. |
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