Neutrinos, nearly massless and barely detectable subatomic particles that pour out of stars and nuclear reactors, behave differently from their antimatter counterparts, antineutrinos, report physicists working with a giant particle detector in Japan. The result is far from conclusive, but the asymmetry, known as charge-parity (CP) violation, could help explain how the newborn universe generated more matter than antimatter—and why stars, planets, and people exist today. It should also encourage physicists who are planning even bigger neutrino experiments that aim to conclusively demonstrate the asymmetry and measure it exactly.
“It’s exciting,” says Patricia Vahle, an experimental neutrino physicist at the College of William & Mary who was not involved in the work. “Maybe it won’t be decades until we can discover CP violation. Maybe it’s just over the horizon.”
Neutrinos come in three types—electron, muon, and tau—and, like chameleons, they can change type as they zip along at near the speed of light. For example, an electron neutrino from the Sun can morph into a tau neutrino before reaching Earth. To study such neutrino oscillations, physicists with the T2K experiment fire muon neutrinos or antineutrinos—generated by slamming protons into a graphite target—from the Japan Proton Accelerator Research Complex (J-PARC) in Tokai to SuperKamiokande (Super-K), an underground tank 295 kilometers away that holds 50,000 tons of ultrapure water and is lined with 13,000 light-sensing phototubes.
Rarely, a muon neutrino collides with a nucleus in the water and turns into a muon, which streaks through the water to produce a shock wave of light that casts a telltale circle on the phototubes. Even more rarely, a muon neutrino morphs into an electron neutrino on the journey to Super-K. It then interacts with the water to produce an electron, which casts a fuzzier circle on the phototubes. When T2K runs with antineutrino beams, such interactions produce either an antimuon or a positron, whose signals parallel those generated by the muons and electrons.
After a decade of firing trillions of protons at their target every second, T2K researchers had tallied just 90 electron neutrinos and 15 electron antineutrinos. The counts were big enough for the group to reach a tantalizing conclusion. Muon neutrinos turn into electron neutrinos at a higher rate than muon antineutrinos turn into electron antineutrinos, the 357-strong team reports today in Nature.
The difference in the oscillation rates for the neutrinos and antineutrinos is a violation of CP, a symmetry that says, more or less, that physics should look the same if you swap particles for antiparticles and reverse all their spins. Some CP violation seems necessary, because the infant universe apparently generated more matter than antimatter; otherwise the two would have annihilated each other completely. Since the 1960s, physicists have observed a hint of CP violation involving subatomic particles called quarks, but too little to explain the cosmic imbalance.
Finding CP violation among neutrinos is a hint that bigger sources of asymmetry were at work in the early universe. Neutrinos themselves are too wispy to do the job, but each type of neutrino might be tied to a much heavier so-called sterile neutrino whose interactions would have shifted the balance. Spotting CP violation among ordinary neutrinos would help “shore up” this picture, Vahle says.
T2K’s result is not conclusive: It only rules out the possibility of no CP violation at a 95% confidence level—not even the 99.7% level that physicists require to claim strong evidence, let alone discovery, of an effect. Still, it’s encouraging, says Chang Kee Jung, a member of the T2K team from Stony Brook University. Physicists quantify the amount of CP violation among neutrinos by an angle—like the heading of a needle on a compass—and so far the T2K results suggest that needle points in a direction that maximizes the amount of CP violation, Jung says.
Yet Vahle, the spokesperson for a rival experiment called NuMI Off-Axis νe Appearance (NOνA), urges caution. Since 2014, NOνA researchers have fired muon neutrinos from Fermi National Accelerator Laboratory (Fermilab) to a detector 810 kilometers away in Ash River, Minnesota. So far, their results suggest, with less precision, that the abstract compass points in the direction of no CP violation at all, Vahle says. “I expect the true value will be somewhere in between,” she says.
To help resolve the discrepancies, physicists are planning even bigger experiments for later in this decade. In the United States, scientists plan to shoot neutrinos 1300 kilometers from Fermilab to the Deep Underground Neutrino Experiment, a device containing 40,000 tons of frigid liquid argon to be built 1.6 kilometers underground in Lead, South Dakota. Meanwhile, researchers in Japan recently received approval to replace Super-K with HyperKamiokande, a pair of detectors containing 500,000 tons of water each. And Jung says T2K researchers hope to run until 2025 or 2026, long enough to rule out no CP violation with greater than 99% confidence.