Elusive neutrinos and dark matter could shed light on matter’s dominance over antimatter
The Standard Model of particle physics that has reigned as the defining model of the physical world for decades is being called into question by a growing list of natural phenomena that do not seem to fit. Neutrino oscillations, dark matter and energy, and the imbalance between matter and antimatter are pointing to new models of physics that are more true to nature. The EU-funded InvisiblesPlus project reveals tantalising hints to the hidden identities and properties of neutrinos and dark matter candidates and answers questions about the fundamental symmetries of nature.
Neutrinos, antineutrinos and hidden asymmetries
Almost every particle has an antimatter counterpart: a particle with the same mass but opposite charge and magnetic moment. But neutrinos have no electric charge, raising the possibility that neutrinos and antineutrinos are one and the same. “The absence of antimatter in our visible universe could be explained if neutrinos are their own antiparticles. This possibility would imply the existence of new, still unobserved, particles. We have shown that these particle masses may be within experimental reach in present and future experiments, and have designed new strategies to look for them. Our novel proposal implies that new particles, holding the key to the absence of antimatter in the Universe, may be discovered shortly,” notes Belen Gavela, InvisiblesPlus coordinator. Neutrino oscillations, arising from mixing between their flavour and mass eigenstates, imply that the neutrino has a non-zero mass. Analysis of neutrino oscillation data pointed to new constraints and improved measurements of the values of the neutrino mixing parameters. InvisiblesPlus also provided novel bounds on the absolute value of neutrino masses from analyses of cosmological data. In the T2K neutrino experiment, project members reported differences between the oscillations of the neutrino and antineutrino beams. This measurement of the neutrino-antineutrino oscillation asymmetry could bring scientists closer to the explanation of the existence of our matter-dominated Universe.
Linking the mysteries of antimatter and dark matter
The surprising absence of primordial antimatter in the Universe could have its origin in dark matter. In addition to (sterile) neutrinos, dark matter could be made of axions. “How axions interact with matter and antimatter might provide important clues about the preponderance of matter over antimatter,” notes Gavela. Theoretical studies conducted by project scientists suggested that axions, as potential dark matter candidates, are much heavier particles than expected. In the Axion Dark Matter Experiment (ADMX), the team placed the world's best constraints on axions, ruling out axions of a certain mass range as dark matter. They also tightened constraints on dark matter properties in the XENON experiment. “It is extremely interesting that there is a ‘technical synergy’ between neutrino and dark matter searches. For instance, an experimental technique based on noble elements (such as in the XENON experiment) can serve to directly detect dark matter and discern whether neutrinos are their own antiparticles,” explains Gavela. It is still unknown whether neutrinos acquire mass via the Higgs mechanism. The elementary particles of dark matter are undiscovered. “We still have no clue about the mass origin and the mass values of neutrinos and dark matter. Perhaps the ensemble of ordinary matter, neutrinos and dark matter has to be contemplated all together – as a new periodic table – to understand the origin of mass,” concludes Gavela.
Keywords
InvisiblesPlus, neutrino, dark matter, antimatter, mass, axion, antineutrino
