On the evening of March 24 (local time), during the 59th “Rencontres de Moriond” conference on electroweak interactions and unified theories, the European Organization for Nuclear Research (CERN) announced a groundbreaking discovery by the LHCb (Large Hadron Collider beauty) collaboration. For the first time, the phenomenon of charge-parity (CP) violation was observed in baryon decays. This discovery provides a crucial clue in solving the mystery of matter-antimatter asymmetry in the universe and is hailed as a new milestone in humanity’s exploration of the origin of the universe. The Chinese LHCb experimental team made outstanding contributions, with the particle physics group from Central China Normal University playing a key role in the research.

According to the theory, the Big Bang that occurred 13.8 billion years ago should have produced equal amounts of matter and antimatter. However, the observable universe today is almost entirely dominated by matter. At the core of this cosmic mystery may lie subtle behavioral differences between matter and antimatter — namely, the violation of CP symmetry. Since the paradigm-shifting discovery of parity violation in 1956, scientists first observed CP violation in 1964 in the decay of mesons containing strange quarks, a finding that earned a Nobel Prize in Physics. Over the following decades, CP violation was also confirmed in meson systems containing bottom and charm quarks. However, CP violation in baryons — particles composed of three quarks such as protons and neutrons, which form the fundamental building blocks of visible matter in the universe — had never been directly observed.

Detecting CP violation in baryon systems has long been a major unresolved challenge in particle physics. The Chinese LHCb team, along with international collaborators, analyzed LHCb data to search for CP violation across multiple baryon decay processes. Through multi-channel cross-verification and collaborative efforts across institutions, they successfully identified significant differences — exceeding five standard deviations — in the decay rates of bottom baryons and anti-bottom baryons in a four-body decay involving a proton, a strange meson, and a pair of charged pions. This marks the first observation of CP violation in baryon decays. The discovery fills a gap in particle physics that persisted for over 60 years, offering vital insights for testing the Standard Model’s quark mixing mechanism and understanding the origin of matter-antimatter asymmetry in the universe.

Previously, the Chinese collaborative team had already observed signs of CP violation in a three-body decay of bottom baryons into hyperons and strange meson pairs. This achievement was published online on March 12 as an Editors' Suggestion and Featured Article in Physical Review Letters. On the same day, the American Physical Society’s Physics journal reported on the discovery, stating that the LHCb experiment had provided key insights into the complexities of CP violation.

The LHCb experiment observed a significant difference in the decay yields of (left) bottom baryons and (right) anti-bottom baryons, marking the first discovery of CP violation in baryon decays.

The LHCb international collaboration consists of nearly 1,800 researchers from 100 institutions across 24 countries. Central China Normal University joined the LHCb collaboration in 2013, and currently, the team includes five primary members: Professor Xie Yuehong, Professor Yin Hang, Professor Chen Kai, Associate Professor Zhang Dongliang, and Associate Professor Zhou Xiaokang. They have long focused on heavy-flavor physics and CP violation research. On one hand, they have concentrated on high-precision measurements of CP violation in B mesons, achieving significant results such as the world’s most precise measurement of the Bs meson mixing angle and the first evidence of direct CP violation in B meson decays to charm pairs. On the other hand, since 2016, with funding from the National Natural Science Foundation, they have led exploratory research on CP violation in baryon systems. In recent years, they have worked closely with sister institutions in the Chinese LHCb group to identify a series of bottom baryon decay modes that exhibit high sensitivity to CP violation, engaging in well-organized collaborative efforts. Professor Xie Yuehong, together with graduate students and postdoctoral researchers, has been a core member in studying several major decay processes of bottom baryons, making direct and significant contributions to research planning, data analysis, and paper writing.

The final discovery of CP violation in baryon decays not only confirms the theoretical predictions of the Standard Model of particle physics but also opens up a new path for exploring new physics phenomena beyond the Standard Model. The CP violation in baryons may carry unique "keys" that characterize the different behaviors of matter and antimatter in the early universe, potentially becoming a golden probe for new physics. As large scientific facilities are upgraded, cutting-edge technologies emerge, and experimental performance continues to improve, physicists are expected to analyze CP violation mechanisms with even higher precision. This will help reconstruct the decisive moments of the universe’s birth 13.8 billion years ago, continually pushing the boundaries of human knowledge and gradually uncovering the mystery of matter-antimatter asymmetry in the universe.

LHCb experiment preprint:
https://arxiv.org/abs/2503.16954

LHCb news report:
https://lhcb-outreach.web.cern.ch/2025/03/25/observation-of-the-different-behaviour-of-baryonic-matter-and-antimatter/

CERN news report:
https://home.cern/news/pressrelease/physics/new-piece-matter-antimatter-puzzle

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