Progress on baryon CP violation at the LHCb experiment

The violation of the combined charge-conjugation and parity symmetries is a fundamental phenomenon in particle physics, providing essential clues to understanding the observed matter–antimatter asymmetry in the Universe. Within the framework of the Standard Model, CP violation originates from a single irreducible complex phase in the Cabibbo–Kobayashi–Maskawa matrix, which governs quark flavor mixing and weak interactions. However, the magnitude of CP violation predicted by the SM is insufficient to account for the baryon asymmetry of the Universe, suggesting the existence of additional CP-violating sources beyond the SM. Baryons, as the dominant constituents of visible matter, provide a unique platform for testing the SM and probing possible new physics. While CP violation has been firmly established in meson systems—such as kaons, D mesons, and B mesons—it remained unobserved in baryon decays for several decades, mainly due to the limited data samples and experimental precision available. The LHCb experiment at the Large Hadron Collider has significantly advanced this field through its high-luminosity data collection, excellent vertex and momentum resolution, and superior particle identification capabilities, enabling a new era in the study of CP violation in baryon decays. This review presents a comprehensive summary of the recent progress achieved by the LHCb Collaboration in measuring direct CP violation in beauty baryon decays. After briefly describing the LHCb detector and the data-taking conditions, the principal experimental methodologies for CP violation measurements—based on yield asymmetries, angular distributions, and control-mode corrections—are introduced. Subsequently, recent LHCb results on charmless two-body, three-body, and four-body Λb0 and Ξb0 decays are discussed in detail. No significant CP violation has been observed in two-body and most three-body modes. A major breakthrough was achieved with the first observation of CP violation in baryon decays, realized in the Λb0→pK−π+π− decay channel, with a significance exceeding five standard deviations. Moreover, localized CP asymmetries have been identified in specific resonance-dominated regions of the phase space, implying strong-interaction-induced phase differences as a possible origin of the observed asymmetry. These findings not only represent a milestone in the exploration of discrete symmetry violation but also provide valuable insight into the interplay between weak and strong interaction dynamics in baryonic systems. With the ongoing Run III of LHCb and the forthcoming high-luminosity LHC phase, which are expected to deliver integrated luminosities of up to 25 and 300 fb−1 respectively, future precision measurements of CP asymmetries in beauty and charm baryons will offer stringent tests of the CKM mechanism and enhance the search for new CP-violating sources beyond the Standard Model.