CMS-PAS-BPH-23-004

CMS-PAS-BPH-23-004
Measurement of time-dependent $ CP $ violation in $ B_s^0 \to \mathrm{J}/\psi\, \phi(1020) $ decays with the CMS detector
CMS Collaboration
8 April 2024

Abstract: A study of $ CP $ violation in $ B_s^0 \to \mathrm{J}/\psi\, \phi(1020) $ decays is presented, based on the data collected by the CMS experiment at the LHC. The measurement is based on a data sample of roughly 491 000 reconstructed signal events corresponding to an integrated luminosity of 96.5 fb$^{-1}$, collected in proton-proton collisions at 13 TeV in 2017-2018. A time- and flavor-dependent angular analysis of the $ \mu^+ \mu^-\,\mathrm{K}^+ \mathrm{K}^- $ final state is performed to extract the values of the $ CP $-violating phase $ \phi_s $, the decay width difference ($ \Delta\Gamma_s $), and absolute mass difference ($ \Delta m_s $) between the light and heavy $ B_s^0 $ mass eigenstates, their average decay width $ \Gamma_s $, and the direct $ CP $ violation parameter $ \|\lambda\| $. The analysis utilizes a pioneering inclusive flavor tagging framework based on state-of-the-art machine learning techniques, achieving exceptional performance by exploiting both opposite-side and same-side information. The measurement yields $ \phi_s = -$73 $ \pm $ 23 (stat) $ \pm $ 7 (syst) mrad, $ \Delta\Gamma_s = $ 0.0761 $ \pm $ 0.0043 (stat) $ \pm $ 0.0019 (syst) ps$^{-1} $, $ \Delta m_s = $ 17.757 $ \pm $ 0.035 (stat) $ \pm $ 0.017 (syst) $ \ \hslash$ps$^{-1} $, $ \Gamma_s = $ 0.6613 $ \pm $ 0.0015 (stat) $ \pm $ 0.0028 (syst) ps$^{-1}$, and $ \|\lambda\| = $ 1.011 $ \pm $ 0.014 (stat) $ \pm $ 0.012 (syst). These results exhibit a level of precision on par with the most precise single measurements to date and agree with the standard model predictions. A combination with the previous CMS results obtained at 8 TeV is also performed, yielding $ \phi_s = - $74 $ \pm $ 23 mrad and $ \Delta\Gamma_s = $ 0.0780 $ \pm $ 0.0045 ps$^{-1}$. The weak phase $ \phi_s $ is found different from zero by 3.2 standard deviations, indicating evidences for $ CP $ violation in $ B_s^0 \to \mathrm{J}/\psi\, \mathrm{K}^+ \mathrm{K}^- $ decays.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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