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The most precise measurements of the Lambda Decay Parameter

2022-09-23 Author:
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The BESIII collaboration has presented the most precise measurements of CP asymmetry and decay parameters with entangled Λ-Λbar pairs. The results are published in the journal Physical Review Letters 129, 131801 (2022) on Sep. 22 2022. 
Modern physics believes that the universe originated from a Big Bang, that create equal amounts of matter and well as antimatter. However, all kinds of experimental evidence indicate that the current universe is dominated by matter rather than antimatter. In 1967, Soviet physicist Sakharov proposed that one of the keys to explaining the matter-antimatter asymmetry was the breaking of charge conjugation and parity (CP) symmetry. To date, small CP violation has been discovered in the K,B and D meson systems, but not sufficient to describe the matter-antimatter imbalance. Hence it is vital to search for additional sources of CP violation. 
For the ground-state hyperons with 1/2 spins, the promising CP-violation signature is the difference between hyperon and antihyperon decay distributions in their parity-violating two-body weak decays. Λ, the lightest baryon with strangeness involved in final states to which many other baryons (Σ00--cb etc.) decays. The Λ decay parameters are relevant to the research about the properties of the Λ baryons, but it is also a crucial input to study Λ polarization in various processes and searched for CP violation in charm and beauty baryons. 
Based on 10 billion J/ψ events collected at BESIII, a detailed study of quantum entangled Λ-Λbar pairs have been performed. The large transverse polarization of Λ makes it possible to simultaneously determine the decay parameters of Λ→pπ-, Λbar→pbar π+ and test CP asymmetry directly. This letter performed a precise CP test in Λ decay using 3.2 million quantum-entangled Λ-Λbar pairs. Results of the Λ decay parameters from different experiments are shown in Fig. 1. The Λ decay parameter α- value obtained in this work agrees with the previous BESIII measurements, but deviates from the most recent result from CLAS by 3.5σ. The Λ and Λbar averaged decay parameter is determined with unprecedented accuracy for the first time. CP in Λ decays is proved to be conserved with an accuracy of 10-3. This is the most precise probe to the CP violation in baryon sector. The results in this analysis are sensitive to the new physics, and play an important role in the measurements of CP violation of strange, charmed and beauty baryons. The next generation of charm factories will greatly improve the accuracy of the CP-violating measurements, and shed light on the mechanism of CP violation in the baryon sector. 
  Fig. 1. Results of Λ decay parameter from different experiments. The green band represents the PDG 2018 value, and the pink band represents the PDG 2022 value.