One of the greatest mysteries in physics is why our Universe is made almost entirely of the normal matter that forms us and everything we touch. Every matter particle has a corresponding antiparticle with the same mass but the opposite charge and, according to our best theories, the Big Bang should have produced matter and antimatter in nearly equal amounts. Today, however, almost no antimatter remains. 

In a recent paper, the CMS Collaboration describes a study exploring one ingredient that could help to explain this imbalance, a subtle difference in the behaviour of matter and antimatter known as charge-parity (CP) violation. 

Studying neutral beauty mesons, which are made of a beauty antiquark and a down-type quark, is one of the best ways to investigate CP violation. This is because neutral beauty mesons have a remarkable property – they can spontaneously transform into their own antiparticles and back again. By measuring the tiny difference in how often matter and antimatter versions of these particles decay over time, we can test the Standard Model with unprecedented precision.

The CMS Collaboration analysed proton–proton collision data collected between 2022 and 2025, reconstructing specific decays of about 1.4 million B⁰ mesons, which contain a down quark, and 16 000 B⁰_s mesons, which contain a strange quark. A key challenge was determining the type of each meson at the moment it was produced, before it decayed into a J/ψ meson and a neutral kaon.

To achieve this, CMS employed an algorithm based on state-of-the-art artificial intelligence. The system combines information from muons, electrons, jets associated with a collision event and, for the B⁰_s meson, nearby particles produced in the collision. This significantly improved the experiment’s ability to identify the meson’s initial state compared to previous analyses.

The measured CP violation is in line with the predictions of the Standard Model, and includes the most precise measurement to date of CP violation in the decay of a B⁰_s particle into a J/ψ meson and a neutral kaon. The accompanying study of the B⁰ decay provides an important test of the same underlying physics. Together, these results improve our knowledge of CP violation in beauty particles and place tight constraints on possible contributions from new particles.

As the Large Hadron Collider (LHC) delivers ever-larger datasets during the High-Luminosity LHC era, measurements of CP violation like these will become even more precise. Whether they continue to confirm the Standard Model or uncover subtle deviations from predictions, they will deepen our understanding of this piece of the matter–antimatter puzzle.