Time in Neutrino Oscillations: Can We Catch a Violation? express
Neutrinos are tiny particles that pass through our bodies by the billions every second without touching a single atom. They have a mysterious property: on their way from the Sun or an accelerator, they can change their type, or 'flavor'—for example, turning from an electron neutrino into a muon neutrino. This transformation, called oscillation, depends on the distance the particle has traveled and its energy.
Usually, physicists test whether time symmetry is violated by comparing the behavior of neutrinos and their antiparticles — antineutrinos. But the authors of the new work propose a different approach: take only neutrinos and see how the probability of a muon neutrino turning into an electron neutrino changes with distance to the detector. If this probability depends on distance asymmetrically (that is, the picture differs when going forward versus backward) then time 'flows' differently for neutrinos.
Calculations for two future experiments, DUNE in the US and T2HK in Japan, show that together they could spot such a violation with a significance of about 4 sigma. DUNE works best in neutrino-only mode, while T2HK excels when comparing neutrinos and antineutrinos. The second oscillation maximum that DUNE will observe plays a key role: that's where the effect is most pronounced. The optimal neutrino energy for the search is from 0.68 to 0.92 gigaelectronvolts, roughly the energy of particles accelerated in a small accelerator.
Such a discovery would help determine the CP phase more precisely — a quantity that describes how much the properties of matter and antimatter differ. Currently, it's only known approximately, and new data could narrow the uncertainty. And crucially, it would confirm that at the most fundamental level, time is not fully reversible.
Neutrinos are so elusive that detectors the size of multi-story buildings, filled with tens of thousands of tons of liquid, are built to catch them. And even such a giant catches only a few particles a day.
In Greg Egan's novel 'Schild's Ladder,' physicists encounter a violation of time symmetry in vacuum, leading to catastrophic consequences. Neutrino experiments are a modest but real step toward understanding such effects.