Recently, the BESIII Collaboration carried out a precision study of the inclusive charmed-baryon decay Λc⁺ → ΛX. The study reports the first measurement of the longitudinal polarization of the Λ hyperon produced in this decay and tests CP symmetry using two distinct observables, the decay branching fraction and polarization. BESIII also performed a high-precision measurement of the absolute branching fraction of this inclusive decay, improving the precision by about a factor of four over the previous result. The work has been published in Physical Review Letters under the title "Precision Studies and Searches for CP Asymmetries in the Inclusive Decay Λc⁺ → ΛX."
The matter-antimatter asymmetry of the Universe is one of the central questions in modern particle physics. CP violation is one of the essential conditions for explaining this asymmetry, yet the known CP violation within the Standard Model is insufficient to provide a complete explanation. Compared with meson systems, studies of CP violation in baryons, especially charm baryons, remain very limited, and no experimental evidence for CP violation has yet been observed in charm-baryon decays. The Λc⁺ is the lightest charmed baryon. Its three-quark structure gives rise to weak-decay dynamics richer than those of mesons, making it an important laboratory for studying baryon weak decays and searching for possible CP-violating effects beyond Standard Model expectations.
This study focuses on the inclusive charmed-baryon decay Λc⁺ → ΛX. The composition of the accompanying final state X is not specified; instead, all allowed final states containing a Λ are considered inclusively, providing a global view of the decay properties. Using 4.5 fb⁻¹ of data collected by BESIII at center-of-mass energies from 4.600 to 4.699 GeV, the Collaboration measured for the first time the longitudinal polarization of the Λ hyperon produced in this inclusive decay and tested a polarization-related CP-asymmetry observable. At the same time, the absolute branching fraction of Λc⁺ → ΛX was measured with high precision, improving the precision by a factor of four over the previous BESIII result and providing clear constraints for searches for as-yet unobserved decay modes containing a Λ. By searching for CP violation through both the inclusive branching fraction and polarization, the study probes the net CP asymmetry after summing over different exclusive decay modes. No significant CP-violation signal is observed at the current precision. These results provide new experimental information for understanding charm-baryon weak-decay dynamics, testing the Standard Model, and enabling future searches for CP violation in baryons with higher sensitivity.
DOI: 10.1103/cxm9-mr48
Link: https://journals.aps.org/prl/abstract/10.1103/cxm9-mr48