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Researchers discover CP violation in charm meson decays

April 05, 2019

Researchers from the Higher School of Economics and Yandex, as part of the LHCb collaboration at CERN, have been the first to discover CP violation in charm meson decays. On March 21, representatives of the LHCb collaboration spoke about this recent breakthrough at the Conference on Electroweak Interactions and Unified Theories in La Thuile.

This discovery may become a key to solving the mystery of matter-antimatter asymmetry in the universe.

One of the unsolved problems in physics concerns the abundance of matter over antimatter in the universe. During the first split seconds after the Big Bang, matter and antimatter appeared in equal shares. Today, the observed quantity of antimatter in the universe around us is negligibly small. The physicists are trying to understand where it has gone. According to Soviet academician Andrey Sakharov's hypothesis from 1967, the matter-antimatter imbalance could have evolved as a result of CP invariance violation (particle-antiparticle symmetry).

In 1973, Makoto Kobayashi and Toshihide Maskawa proposed that a natural explanation for CP violation effect. According to the Kobayashi-Maskawa theory, in the Standard Model of fundamental interaction, CP violation happens through a single phase. However, the manifestation of this effect in the decay of particles containing various heavy quarks strongly depends on the other features of the Standard Model of elementary particles. CP violation in charm D meson decays was expected to come to around 0.1-0.01%.

The LHCb (Large Hadron Collider beauty) experiment was carried out at CERN to study B mesons, unstable particles, in the decay of which the matter-antimatter asymmetry manifests particularly well. Scholars analyzed the data received in an LHCb experiment in 2011-2018 and found that the total number of decays of anti-D0 mesons exceeded the total number of decays of D0 mesons. The result has a statistical significance of 5.3 standard deviations, exceeding the threshold of five standard deviations used by particle physicists to claim a discovery.

'Studying CP violation is extremely important for understanding the mechanisms of our universe's evolution,' explained Denis Derkach, Senior Research Fellow at the Faculty of Computer Science Laboratory of Methods for Big Data Analysis (LAMBDA), adding: 'The discovery of CP violation in charm meson decays is a big step in the study of this phenomenon in heavy meson decays'.

Furthermore, researchers from HSE and Yandex School of Data Analysis applied AI tools in the study, which improved the quality of LHCb experiment data selection and analysis. Yandex's computation capacities have also been used to model the LHCb experiment events, which is essential for correct interpretation of the physical results.

'Thanks to our team efforts, the effectiveness of the trigger used to select significant events was increased by 40% on average,' said Fedor Ratnikov, Senior Research Fellow at LAMBDA Laboratory. He noted: 'With the use of neural network Bayesian approaches, we have improved the algorithm for detecting the type of particles observed by the detector. We have also developed a "smart" system for quality monitoring of detector operations'.
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National Research University Higher School of Economics

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