Closing in on neutrino CP violation

Somewhere in the laws of physics, particles must be allowed to behave differently from their antiparticles. If they weren’t, the universe would contain equal amounts of matter and antimatter, and none of us would exist.

Several flavors of quarks have been observed to violate CP symmetry—the combination of charge conjugation and parity reversal—between particles and their antiparticles’ mirror images. (See Physics Today, August 2019, page 14 .) But the extent of the violation is nowhere near enough to explain the imbalance of matter and antimatter in the universe today. To make up the shortfall, researchers are looking for an additional source of CP violation among the particles of the lepton sector: electrons, muons, taus, their antiparticles, and their associated neutrinos and antineutrinos.

Unlike most particles, neutrinos spontaneously change their identities as they travel. (See Physics Today, December 2015, page 16 .) A muon neutrino created in one place, for example, might be detected as an electron neutrino in another. The dynamics of that flavor oscillation can be characterized by three mixing angles—ϑ₁₂, ϑ₂₃, and ϑ₁₃—plus a phase δ_CP that captures the amount of CP violation, if any. A δ_CP of 0 or ±π means that neutrinos and antineutrinos oscillate identically, and CP symmetry is conserved; any other value means that the symmetry is broken.

Those parameters are extremely difficult to measure, because neutrinos are extremely difficult to detect. But now the Tokai-to-Kamioka (T2K) experiment is homing in on a value of δ_CP. By smashing protons into a graphite target at the J-PARC accelerator in Tokai on Japan’s east coast, the researchers create a steady beam of muon neutrinos or antineutrinos. At the Super-Kamiokande detector, 295 km to the west, they measure how many of those neutrinos retain their original flavor and how many transform into electron neutrinos. By comparing the results from neutrino and antineutrino beams, they can estimate δ_CP.

After 10 years of data collection—interrupted, unfortunately, by the Tohoku earthquake and tsunami that devastated Japan in 2011 and by an accident on another beamline at J-PARC in 2013—the T2K team has concluded with 3σ confidence that δ_CP lies somewhere between −3.41 and −0.03. That interval includes the CP-conserving value of −π, so CP conservation is disfavored only at the modest 2σ confidence level. Still, the result suggests that searches for lepton CP violation are probably on the right track and that the next generation of experiments, currently under construction, will give a definitive answer. (T2K collaboration, Nature 580, 339, 2020 .)