Physicists measure subtle asymmetry in particle decay with record precision
Researchers at CERN have made the most precise measurements yet of how matter and antimatter behave differently in rare particle decays. The findings help test whether the Standard Model of physics is complete, with implications for understanding why the universe contains matter at all.
Originaltitel: Updated measurement of <em>CP</em> violation and polarisation in B<sup>0</sup><sub>s</sub> → J/ψ¯K<sup>∗</sup>(892)<sup>0</sup> decays
<p>A time-integrated angular analysis of the decay B0s→J/ψ¯K∗(892)0, with <em>J</em>/<em>ψ</em> → <em>μ</em><sup>+</sup><em>μ</em><sup><em>−</em></sup> and ¯K∗(892)0→K−π+, is presented. The analysis employs a sample of proton-proton collision data collected by the LHCb experiment during 2015–2018 at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 6 fb<sup><em>−</em>1</sup>. A simultaneous maximum-likelihood fit is performed to the angular distributions in bins of the <em>K</em><sup><em>−</em></sup><em>π</em><sup>+</sup> mass. This fit yields measurements of the <em>CP</em>-averaged polarisation fractions and <em>CP</em> asymmetries for the P-wave component of the <em>K</em><sup><em>−</em></sup><em>π</em><sup>+</sup> system. The longitudinal and parallel polarisation fractions are determined to be <em>f</em><sub>0</sub> = 0.534 ± 0.012 ± 0.009 and <em>f</em><sub>||</sub> = 0.211 ± 0.014 ± 0.005, respectively, where the first uncertainty is statistical and the second is systematic. The <em>CP</em> asymmetries are measured with 3–7% precision and are found to be consistent with zero. These measurements, along with an updated determination of the branching fraction relative to the <em>B</em><sup>0</sup> → <em>J</em>/<em>ψK</em><sup>*0</sup> decay, are combined with previous LHCb results, providing the most precise values for these observables to date.</p>