Eine der alten Kernfragen der modernen Teilchenphysik.
Zitat From Quarks to Quasars
A CERN breakthrough may reveal why anything exists.
In a breakthrough at CERN’s Large Hadron Collider, physicists have discovered subtle differences in how matter and antimatter decay. It’s the first time this kind of imbalance – known as CP violation – has been observed in baryons, the particles that make up most of the matter around us.
Here’s the problem: According to the laws of physics, the Big Bang should have created equal amounts of matter and antimatter. Since the two annihilate on contact, they should’ve canceled each other out, leaving the universe a flash of energy, and nothing else. But that didn’t happen. Somehow, a tiny excess of matter survived. That sliver became everything: stars, planets, us.
Scientists have long searched for the reason why. CP violation was first seen in other particles called mesons, but baryons – like protons and neutrons – are what really make up the universe. That’s why this new result is so important.
Researchers analyzed 80,000 decays of a particle called the lambda-beauty baryon and found that its antimatter twin decays slightly differently – by about 2.5%. That small difference is statistically strong, with a confidence level of 5.2 sigma, or about 1 in 10 million odds of it being random.
The Standard Model of physics predicts some CP violation, but not enough to explain the universe’s survival. This new result doesn’t solve the puzzle, but it opens the door to new physics that might.
Read the study: "Observation of charge–parity symmetry breaking in baryon decays." Nature, 2025.
Zitat Observation of charge–parity symmetry breaking in baryon decays - LHCb Collaboration
Nature volume 643, pages 1223–1228 (2025)
Abstract
The Standard Model of particle physics—the theory of particles and interactions at the smallest scale—predicts that matter and antimatter interact differently due to violation of the combined symmetry of charge conjugation (C) and parity (P). Charge conjugation transforms particles into their antimatter particles, whereas the parity transformation inverts spatial coordinates. This prediction applies to both mesons, which consist of a quark and an antiquark, and baryons, which are composed of three quarks. However, despite having been discovered in various meson decays, CP violation has yet to be observed in baryons, the type of matter that makes up the observable Universe. Here we report a study of the decay of the beauty baryon to the pK−π+π− final state, which proceeds through b → u or b → s quark-level transitions, and its CP-conjugated process, using data collected by the Large Hadron Collider beauty experiment1 at the European Organization for Nuclear Research (CERN). The results reveal significant asymmetries between the decay rates of the baryon and its CP-conjugated antibaryon, providing, to our knowledge, the first observation of CP violation in baryon decays and demonstrating the different behaviours of baryons and antibaryons. In the Standard Model, CP violation arises from the Cabibbo–Kobayashi–Maskawa mechanism, and new forces or particles beyond the Standard Model could provide further contributions. This discovery opens a new path in the search for physics beyond the Standard Model.
According to cosmological models, matter and antimatter were created in equal amounts at the Big Bang6. Then matter and antimatter mostly annihilated in pairs as the Universe cooled down, with a tiny fraction of matter remaining. The dominance of matter requires the violation of both charge conjugation (C) symmetry and charge conjugation and parity symmetry (CP symmetry) in conjunction with other conditions, as proposed by Sakharov in 1967 (ref. 7). Experimentally, it was established in 1957–1958 that the weak force breaks both parity (P) and C symmetries8,9. The violation of the combined CP symmetry was first observed in strange-meson decays in 1964 (ref. 10). This phenomenon was later also observed in beauty-meson decays in 2001 (refs. 11,12) and in charm-meson decays in 2019 (ref. 13). Here, strange, charm and beauty refer to the flavours of the constituent quarks.
Quark dynamics are described by the Standard Model of particle physics. CP violation arises from the Cabibbo–Kobayashi–Maskawa (CKM) mechanism2. The CKM mechanism uses a complex 3 × 3 matrix to describe how quarks of different generations mix under the weak interaction, which is mediated by the exchange of W± bosons. The structure of this mixing is ultimately linked to the Higgs mechanism, which gives rise to the masses of fundamental particles, including the quarks. The matrix contains a non-zero phase parameter, which provides the only known source of CP symmetry breaking. In general, the CKM mechanism is very successful in describing experimental data for CP asymmetries and decay rates14. However, the amount of matter–antimatter asymmetry explained by the CKM mechanism is vastly smaller than what astronomical observations indicate, presenting an important challenge to the Standard Model and hinting at the presence of further sources of CP violation15. Continuing explorations of CP violation may open new avenues for the discovery of physics beyond the Standard Model.
The lack of observed CP violation in baryons, the predominant form of matter in the visible Universe, remains a puzzle. Similar levels of CP violation in meson and baryon decays are expected due to identical quark-level transitions. Yet, CP violation has so far been detected only in mesons. This discrepancy is especially pronounced in beauty-baryon decays, where large CP asymmetries are anticipated, as seen for beauty mesons. For instance, the beauty-meson decay shows a (23.6 ± 1.7)% CP asymmetry16,17, whereas the corresponding baryon decays , where h denotes a K or π meson, exhibit no such asymmetry with 0.7% precision18. Similarly, three-body beauty-meson decays, such as B+ → π+π−π+, display CP asymmetries of up to 75% (ref. 19), whereas no significant CP violation has been observed in beauty-baryon decays20,21. The decay exhibits a hint of CP asymmetries below 20% with a significance of 3.1 standard deviations, requiring further confirmation22. No CP violation has been observed in strange and charm baryon decays nor in unflavoured baryon decays.
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