Scientists Test Asymmetry Between Matter and Antimatter

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An international team, including scientists from HSE University, has collected and analysed data from dozens of experiments on charm mixing—the process in which an unstable charm meson oscillates between its particle and antiparticle states. These oscillations were observed only four times per thousand decays, fully consistent with the predictions of the Standard Model. This indicates that no signs of new physics have yet been detected in these processes, and if unknown particles do exist, they are likely too heavy to be observed with current equipment. The paper has been published in Physical Review D.

Immediately after the Big Bang, matter and antimatter should have formed in equal amounts and annihilated each other in perfect symmetry. Yet the universe has persisted, made up almost entirely of matter—evidence that this symmetry was somehow broken. The cause remains unknown. The Standard Model, the main theory of particle physics, describes the properties of elementary particles and has been confirmed by numerous experiments, but it does not explain the disappearance of antimatter. To search for answers, physicists study the weak interaction, a process in which particles can transform into their antiparticles and back. These transformations are especially sensitive to symmetry violations, making them a convenient tool for testing the limits of the Standard Model.

UTfit, an international team including researchers from HSE University, has carried out the most comprehensive analysis of these processes to date, combining data from dozens of experiments. This includes new results from the LHCb detector at the Large Hadron Collider and Japan’s Belle II experiment, both of which record rare particle decays under different conditions. The scientists focused on charmed mesons—short-lived particles that can spontaneously transform into their antiparticles and back, making them a useful tool for detecting even the slightest differences between matter and antimatter. If such transformations showed even a tiny asymmetry between particles and antiparticles, it would suggest the existence of previously unknown particles or interactions. To process the data, the researchers used a Bayesian approach with Markov chains, which made it possible to account for both statistical and systematic errors and to combine diverse experiments into a single coherent picture.

The results show that meson-to-antimeson transformations are extremely rare—occurring in only about four out of every thousand decays—and that the difference in decay rates between particles and antiparticles is roughly six per thousand. These values align perfectly with the predictions of the Standard Model. The observed CP violation—the very asymmetry between matter and antimatter—is far too small to account for the disappearance of antimatter from the universe.

However, even when no anomalies are detected, such studies help refine the boundaries within which the Standard Model holds and allow researchers to estimate the possible properties of hypothetical new physics. The analysis indicates that if new particles do exist, they must be so heavy that their effects are almost imperceptible at the current level of precision. In other words, such effects could only appear at energy scales that today’s colliders cannot yet reach.

Denis Derkach

'The heavier a hypothetical particle, the weaker its contribution to observed processes at accessible energies,' explains Denis Derkach, Head of the Laboratory of Methods for Big Data Analysis at the AI and Digital Science Institute of the HSE FCS. 'We combined data from dozens of experiments and found that all results remain consistent with the Standard Model. This suggests that if new physics exists, its particles are so heavy that their influence on these processes is nearly undetectable. Noticing even such faint effects will require further accumulation of data and continued improvements in measurement precision.'