New horizon in particle physics: first observation of CP violation in baryon decays

Abstract

Recently, the LHCb Collaboration achieved the observation of CP violation (CPV) in baryon decays through the process of $\Lambda_b^0\to pK^-\pi^+\pi^-$, reporting an asymmetry of $(2.45\pm0.46\pm0.10)\%$ with a significance of 5.2$\sigma$. This marks a breakthrough and a milestone in particle physics, six decades after the first observation of CPV in mesons. It will be helpful to understand the matter-antimatter asymmetry in the universe. In addition to the global CPV, local CPV is also observed by LHCb in the low mass region of $m_{p\pi^+\pi^-}<2.7$GeV as $(5.4\pm0.9\pm0.1)\%$ with a significance of 6.0$\sigma$. Intriguingly, this measurement aligns well with a theoretical prediction of $(5.6-5.9)\%$ based on a CPV dynamics using the data of $N\pi\to p\pi^+\pi^-$ scatterings. Since baryons contain one more quark than mesons, the dynamics of baryon decays are significantly different from those of mesons. Therefore, the first observation of baryon CPV by LHCb opens a new horizon in the studies of dynamics of the strong interaction.