Matter-antimatter asymmetry: a new piece of the puzzle revealed 🧩

The LHCb experiment at CERN has uncovered a fundamental asymmetry in the behavior of baryons.

During the Moriond Conference in Italy, the LHCb collaboration at CERN announced that a new milestone had been reached in our understanding of the subtle yet profound differences between matter and antimatter.


In its analysis of vast amounts of data produced by the Large Hadron Collider (LHC), the international team was able to conclusively demonstrate that particles called baryons—which include protons and neutrons forming atomic nuclei—are influenced by a mirror asymmetry in the fundamental laws of nature, causing matter and antimatter to behave differently.

This discovery provides new clues to explain the arrangement of elementary particles composing matter in the Standard Model of particle physics and to understand why, it seems, matter prevailed over antimatter after the Big Bang.


First observed in the 1960s in mesons, particles made of a quark-antiquark pair, charge-parity (CP) violation has been the subject of numerous studies based on fixed-target experiments or colliders. It was expected that the other major category of known particles, baryons—composed of three quarks—would also exhibit this phenomenon. However, until now, experiments like LHCb had only observed hints of this effect in baryons.

"It took longer to observe CP violation in baryons than in mesons due to the scale of the phenomenon and the volume of available data," explains Vincenzo Vagnoni, spokesperson for the LHCb collaboration. "We needed a machine like the LHC capable of producing a sufficiently large number of beauty baryons and their antimatter counterparts, and we needed an experiment capable of identifying their decay products. It took over 80,000 baryon decays for us to observe, for the first time, a matter-antimatter asymmetry in this category of particles."


It is known that particles and their antimatter counterparts have identical mass and opposite charges. However, when particles decay into other particles—for example, when an atomic nucleus undergoes radioactive decay—CP violation creates a crack in this mirror symmetry. This effect can manifest as a discrepancy in decay rates into lighter particles depending on whether particles or their antimatter counterparts are observed. These discrepancies can be detected using highly sophisticated detectors and analysis techniques.

The LHCb collaboration observed CP violation in a heavier and more short-lived baryon than protons and neutrons: the lambda baryon Λ_b, composed of an up quark, a down quark, and a beauty quark. First, the team sifted through data collected by the LHCb detector during the LHC's first and second operational runs (2009–2013 and 2015–2018, respectively), searching for the decay of Λ_b into a proton, a kaon, and a pair of oppositely charged pions, as well as the decay of its antimatter counterpart, the anti-Λ_b. They then counted the number of decays observed for each of these two particles and calculated the difference.


The analysis showed that the difference between the number of Λ_b decays and anti-Λ_b decays, divided by their sum, deviates from zero by 2.45%, with an uncertainty of about 0.47%. Statistically, the result deviates from zero by 5.2 standard deviations, exceeding the threshold required to claim an observation of CP violation in this baryon decay.

Although CP violation in baryons had long been expected, the complex predictions of the Standard Model of particle physics are not yet precise enough to allow for a thorough comparison between theory and the measurements made by LHCb.

Surprisingly, the degree of CP violation predicted by the Standard Model is several orders of magnitude too small to explain the matter-antimatter asymmetry observed in the Universe. This suggests that there are other sources of CP violation beyond those predicted by the Standard Model. The search for these sources is a key part of the LHC's physics program and will continue at future colliders.

"The more systems in which we observe CP violation, the more precise the measurements, and the more opportunities we have to test the Standard Model and explore physics beyond it," says Vincenzo Vagnoni. "The first-ever observation of CP violation in baryon decays opens the door to further theoretical and experimental investigations into the nature of CP violation, potentially setting new boundaries for physics beyond the Standard Model."

"I congratulate the LHCb collaboration on this brilliant result," says Joachim Mnich, CERN's Director for Research and Computing. "It once again highlights the scientific potential of the LHC and its experiments, providing a new tool to explore matter-antimatter asymmetry in the Universe."