The Standard Model of particle physics predicts an asymmetry between matter and antimatter known as charge-parity (CP) violation. But the size of this asymmetry in the Standard Model is not large enough to account for the imbalance and the asymmetry has so far been observed only in certain decays of particles called mesons. In two new studies, physicists from the LHCb Collaboration at CERN’s Large Hadron Collider (LHC) have found evidence of CP violation in decays of baryons and in decays of beauty hadrons into charmonium particles, shedding light on these two pieces of the matter-antimatter puzzle.
View of the LHCb detector. Image credit: CERN.
Experiments, including LHCb, have previously conducted searches for CP violation in baryons, by looking for differences in the way matter and antimatter baryons decay into other particles.
But these searches have essentially come up empty handed so far.
Although one LHCb study has delivered evidence of the process in a particular decay of the bottom lambda baryon, that evidence has not increased in a subsequent study that analyzed a larger sample of such decays.
In the first new study, the LHCb physicists sifted through proton-proton collision data taken during the first and second runs of the LHC to search for different decay modes of the bottom lambda baryon, including its decay into a lambda baryon and two kaons.
They then looked for CP violation in each decay mode by basically counting the number of decays of the bottom lambda baryon and of its antimatter partner and taking the difference between the two.
For the decay into a lambda baryon and two kaons, this difference showed evidence of CP violation with a significance of 3.2 standard deviations.
In the second study, the LHCb team turned its attention to the decay of the electrically charged beauty meson into a J/psi and a charged pion.
The J/psi is a charmonium particle — a meson consisting of a charm quark and a charm antiquark.
Performing a similar analysis to that of the bottom lambda baryon study, and also using data from the first and second runs of the LHC, they found evidence of CP violation in this decay mode of the charged beauty meson, again with a significance of 3.2 standard deviations.
The finding represents evidence of CP violation in decays of beauty hadrons to charmonium particles.
“Our studies mark significant steps towards establishing whether or not CP violation exists in these types of decays,” the authors said.
“Data from the third run of the LHC and from the collider’s planned upgrade, the High-Luminosity LHC, are set to shed further light on these and other pieces of the matter-antimatter puzzle.”
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LHCb Collaboration. 2024. Study of Λ0b and Ξ0b decays to Λh+h’− and evidence for CP violation in Λ0b→ΛK+K− decays. arXiv: 2411.15441
LHCb Collaboration. 2024. First evidence for direct CP violation in beauty to charmonium decays. arXiv: 2411.12178