Neutrinos and Antineutrinos Change Flavors at Different Rates

image credit: Lucy Reading-Ikkanda/Quanta Magazine.

The T2K experiment in Japan reported in Nature (April 15, 2020) that neutrinos have a higher probability of oscillating than antineutrinos.

There is great excitement about this result. Until now CP symmetry violation has not been observed in non-quark elementary particle systems.

Summaries from science journalists:

Natalie Wolchover: Neutrino Asymmetry Passes Critical Threshold

Dennis Overbye: Why the Big Bang Produced Something Rather than Nothing

You can find another summary here.

In this long-running experiment T2K so far reached 3-sigma level of statistical significance. This is not enough to claim “discovery” but the fact that statistical significance is growing with additional data is a very good sign. The next generation of experiments such as DUNE will most likely to confirm these results at 5-sigma level.

Here’s the abstract of the T2K scientific paper

“The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 1964, and CP violation in the weak interactions of quarks was soon established. Sakharov proposed that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis. Leptonic mixing, which appears in the standard model’s charged current interactions, provides a potential source of CP violation through a complex phase δ_CP, which is required by some theoretical models of leptogenesis. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments. Until now, the value of δ_CP has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δ_CP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δ_CP, which is cyclic and repeats every 2π, is [−3.41, −0.03] for the so-called normal mass ordering and [−2.54, −0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.”