Figure: Image of the Higgs boson decay event into two Z-bosons, each of which decays into a lepton-antilepton pair. Source: ATLAS/CERN
The European Center for Nuclear Research (CERN) announced the results of testing one of the fundamental symmetries in the experimental work of the ATLAS collaboration. Scientists have been looking for combined parity violations (CP-parity) to answer the question: where did the excess of matter over antimatter come from?
Physicists tested whether CP-parity, that is, symmetry with respect to simultaneous charge conjugation (change of particle sign from plus to minus) and specular spatial reflection, is preserved in reactions involving the Higgs boson. The aim of the study was to find the source of inequality between matter and antimatter, without which it is impossible to explain the emergence of matter in the Universe. The measurements were taken from 2015 to 2018, but their new analysis has only just been completed. The results were negative: no additional CP-violating interactions were ever found.
CP-parity means the identity of the properties of nuclear reactions in which particles and antiparticles participate in mirror reflection of spatial coordinates. Most interactions in nature preserve CP parity. However, CP violation is essential to our understanding of cosmology. From the most convincing and consistent models of the origin of the Universe, it follows that after the Big Bang, both particles and antiparticles were formed. There were a lot of those and others, but the particles were, in order of magnitude, one billionth more. Therefore, almost all particles annihilated with all antiparticles, and the visible Universe arose from one billionth part.
But for such a scenario, it is necessary that in the Standard Model of particle physics the properties of particles differ from the properties of antiparticles. That is, the CP symmetry must be broken. This is partly achieved through the so-called mixing of quarks with each other. However, the CP-parity violation observed due to the mixing of quarks is not enough to give rise to the effects needed in cosmology. Therefore, physicists are looking for other interactions to provide the level of CP violation required for cosmology.
The basic hypothesis tested by the CERN researchers was CP-parity violation in Higgs boson decays. In the Standard Model, all "envisaged" interactions of the Higgs boson with other particles preserve CP parity.
The researchers proposed several channels for the formation and decay of the Higgs boson, which are not in the Standard Model and which would violate CP parity. Physicists have reduced CP-violating processes to so-called optimal observables. These are observables that must be equal to zero if CP-parity is preserved. They are arranged in such a way that particles make a positive contribution to them, and antiparticles make a negative one, so if there is symmetry between particles and antiparticles, the optimal observables are nullified.
The physicists of the ATLAS collaboration considered the processes of the production of the Higgs boson from two W or Z bosons (in these cases, a quark and an antiquark are produced together with the Higgs, as can be seen from the hadron jets). They also studied how the Higgs boson decays into two Z-bosons, each of which decays into a lepton-antilepton pair. Roughly speaking, when particles are replaced by antiparticles and flight directions are reversed while maintaining CP-parity, nothing should change in these processes. And if CP parity is violated, particles and antiparticles will behave differently.
For the study, scientists used data from the Large Hadron Collider accumulated over three years. As a result of processing these data, they found that with a probability of more than 99%, CP parity is preserved during the decay of the Higgs boson. Therefore, the problem of baryon asymmetry in cosmology remains: no one knows why slightly more matter was formed than antimatter.