What happened to all the antimatter? Matter and antimatter should have been produced in equal quantities following the Big Bang. It is still unclear why most of the universe is made up of matter and very little antimatter. The breach of charge-parity (CP) symmetry, which simply indicates that some processes involving particles act differently from others involving their antiparticles, could be used to explain the excess of matter.
However, the known CP-violating processes are insufficient to account for the universe’s matter-antimatter asymmetry. There must be new sources of CP violation, and they may be lurking in interactions involving the Higgs boson. Higgs-boson interactions with other particles in the Standard Model of particle physics preserve CP symmetry. If researchers discover indications of CP violation in these interactions, they may hold the key to solving one of the oldest riddles in the cosmos.
The ATLAS team evaluated the Higgs-boson interactions with the weak force carriers, the W and Z bosons, in a fresh study of its whole dataset from Run 2 of the LHC, searching for indications of CP violation. The Higgs-boson decays into two Z bosons, each of which converts into a pair of charged leptons (an electron and a positron or a muon and an antimuon), giving rise to four charged leptons. This process was explored by the collaboration. The interactions in which two W or Z bosons combine to form a Higgs boson were also investigated by the researchers. In this instance, the Higgs boson is created along with one quark and one antiquark, resulting in ‘jets’ of particles in the ATLAS detector.
These interactions make for excellent CP violation test cases. When CP symmetry is conserved, the behaviour of the leptons and jets that have been seen should follow the same pattern when particles are swapped for their antiparticles and their flight directions are reversed. However, particles and antiparticles behave differently if CP symmetry is broken.
The optimum observable is a single number that represents all the details about the particles discovered in these processes, according to ATLAS experts. This observable has the unique property that the value measured for antiparticles should be identical to that of the particles but with the opposite sign. The mean value of the ideal observable in the data should be zero if a process conserves CP symmetry. If it doesn’t, the mean value would shift away from zero.
The optimal observable’s observed values were used by ATLAS in its latest study, which is currently available on the arXiv preprint service, to set direct upper and lower bounds on the potential severity of CP violation. After accounting for any experimental effects, the researchers determined the frequency with which each value of the ideal observable appeared in the data.
With the use of this measurement, ATLAS was able to evaluate the theoretical underpinnings by comparing the data to theoretical expectations in a model-independent manner. For the first time, a measurement of a Higgs-boson decay into four leptons has allowed physicists to look for possible evidence of CP violation without heavily depending on Standard Model predictions other than CP symmetry.
All of the findings appear to be consistent with the Standard Model prediction, providing yet another significant endorsement of the natural science paradigm. But this is just the beginning. Small CP-violating signals are still compatible with the data, and ATLAS is already gathering fresh collision data at previously unheard-of energies that will boost the precision of these measurements and help us narrow in on the existence of the Higgs boson.
