At the 59th “Rencontres de Moriond EW", the LHCb collaboration announced a landmark discovery: the first observation of Charge-Parity (CP) violation in baryon decays. This breakthrough may provide further insight into one of the universe’s most profound mysteries: why matter dominates over antimatter. The research team led by Professor Zheng Yangheng and Qian Wenbin from the University of Chinese Academy of Sciences (UCAS) played an important role in this achievement.
According to the Big Bang theory, matter and antimatter should have been created in equal amounts 13.8 billion years ago. Yet today, our universe is made almost entirely of matter. CP violation—a subtle asymmetry in how particles and antiparticles behave—is essential to explain this imbalance. While CP violation was first detected in mesons (particles made of quark-antiquark pairs) in 1964 (a discovery awarded the Nobel Prize), it had never been observed in baryons (three-quark particles like protons and neutrons) until now.
The LHCb team, including UCAS scientists, analyzed proton-proton collision data from the LHCb detector and observed a statistically significant difference (exceeding five standard deviations) between the decay rates of bottom baryons and anti-bottom baryons. This definitive sign of CP violation in baryon decays fills a six-decade gap in particle physics. Baryons, the building blocks of visible matter, may hold the key to understanding the universe’s matter-antimatter asymmetry. This discovery not only confirms a key prediction of the Standard Model but also opens new avenues to explore physics beyond it.
Figure 1: LHCb data showing the asymmetry between (left) bottom baryon and (right) anti-bottom baryon decays, marking the first discovery of CP violation in baryon decays.
The LHCb collaboration comprises approximately 1,800 researchers from 100 institutions across 24 countries. Since joining in 2015, researchers from UCAS have made significant contributions, including precision measurements of CP-violation phases, hadron spectroscopy studies, and investigations into heavy-flavor hadron production. Their work has led to several important discoveries, including a new type of CP violation in charmless three-body B decays and the observation of hadronic states like the double-charmed baryon and doubly charged tetraquark.
“To track down CP violation in baryon decays, we first identified all the potential decay channels and worked together with collaborators to analyze each one”, explained Associate Professor Qian Wenbin. “Completing these studies required more than a decade of dedicated work”, said Professor Zheng Yangheng. Beyond the current breakthrough, their team recently reported evidence of CP violation in a specific three-body baryon decay mode, published as an Editor's Suggestion in Physical Review Letters (March 12, 2025), and featured in Physics.
The definitive observation of CP violation in baryons opens a new window for future studies. With further upgrades of the detector, physicists in the LHCb collaboration are now poised to study CP violation with unprecedented precision. These advances could help us reconstruct the pivotal moments 13.8 billion years ago that determined the survival of matter over antimatter, bringing us closer to solving one of science's greatest mysteries.
Further readings:
LHCb preprint on discovery of CP violation in baryon decays：https://arxiv.org/abs/2503.16954
LHCb publication on evidence of CP violation in baryon decays：https://doi.org/10.1103/PhysRevLett.134.101802
News from APS：https://physics.aps.org/articles/v18/56
LHCb news：https://lhcb-outreach.web.cern.ch/2025/03/25/observation-of-the-different-behaviour-of-baryonic-matter-and-antimatter/ and https://lhcb-outreach.web.cern.ch/2024/11/08/
CERN news：https://home.cern/news/press-release/physics/new-piece-matter-antimatter-puzzle
