Advancing Basic Science for Humanity
04/23/2020 - First-Ever Glimpse of a Skew in Ghostly Neutrinos Could Be Why We Exist
By Adam Hadhazy
A glimpse inside the Super-Kamiokande detector as it was being filled with water in 2006. The individuals on a boat in the background give a hint of the scale. Image copyright: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo. Used with permission.
Something versus nothing. This age-old and literally existential quandary is all the more boggling because according to some of the best-supported theories in all of science, the cosmic ledger should have firmly settled on the latter—nothing.
Let's start with the big bang theory. It predicts that matter and its doppelganger, antimatter, should have initially formed in equal amounts when our universe came to be 13.8 billion years ago. Matter and antimatter are not exactly live-and-and-let-live; whenever the two substances contact each other, they obliterate each other into pure energy. Thus, a perfect, primordial parity mustn't have been the case in our universe, because after all, here we are.
Doubling down on the mystery is the Standard Model of particle physics. This edifice of modern physics describes how matter and antimatter behave with astonishing accuracy. Yet the Standard Model does not allow for anywhere near the sort of discrepancy between the two kinds of matter that would allow one to thoroughly dominate over the other, as is evidently the case.
All of which is why a new result announced in April is setting scientists' (and maybe philosophers', too) hearts astir. Researchers with the T2K experiment in Japan announced in the journal Nature that they'd detected for the first time a new way in which matter and antimatter might fundamentally behave differently. The difference is between the matter and antimatter versions of ghostly particles called neutrinos—among the least-understood, yet bounteous particles in the universe. If further confirmed, this neutrino discrepancy would go a long way toward explaining why the matter-ful universe and all of its matterous things—planets, people, paramecia—ever arose.
"Neutrinos are just the little particle that could," says Michael Turner, Senior Strategic Advisor with The Kavli Foundation and former director of the Kavli Institute for Cosmological Physics (KICP) at the University of Chicago. "They may well be critical for our existence."
More than two dozen of the many authors on the new paper are researchers at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) at the University of Tokyo in Japan. They, along with their colleagues in the 500-member, 12-country-strong international T2K Collaboration, have reported that a type of neutrino is acting wonkily. Which is saying something, after all, given the neutrino's already-famous quirkiness. The aloof particles barely interact with the rest of the universe's matter; as you sit here reading this, quadrillions upon quadrillions of neutrinos are passing right through your body, and right on through our planet.
Yet in rare instances, when neutrinos do deign to interact with matter, detectors can register this happening through the teensy bit of energy the neutrino imparts. That's what happens inside a giant, stainless-steel tank holding 50,000 gallons of ultra-purified water in the Super-Kamiokande detector. The tank is lined with 13,000 light sensors that see when an interacting neutrino generates brief flash in the water.
Super-K, as it’s known for short, is built in a mine in the Kamioka (the "K" in "T2K") section of a mountain town in western Japan. Kavli IPMU keeps a satellite office there at Super-K, one of the premier neutrino science sites in the world, offsetting what would otherwise be a long commute (easily four hours one-way) for members—including faculty, postdocs, and graduate students—from the Tokyo area.
A one-way commute, however, is actually exactly what neutrinos and antineutrinos do to arrive at Super-K from the Tokai (the "T" in "T2K") campus of the Japan Atomic Energy Agency, in Japan's east. The neutrinos and antineutrinos are generated in the J-PARC (Japan Proton Accelerator Research Complex) facility and then beamed as separate bunches some 295 kilometers (183 miles) underground—straight through Japan at the speed of light—to SuperK. The beams are first measured at J-PARC and again once when they arrive at SuperK.
What scientists are looking for is the extent to which the detectable neutrinos in the beams have changed "flavor" during their transit. Bizarrely, neutrinos come in three forms—dubbed the electron neutrino (not to be confused with plain ol' electrons, which give us electricity), muon neutrino, and tau neutrino—and readily oscillate between the three. This transmogrification is only possible if neutrinos have a modicum of mass, which the Standard Model forbids them from possessing. In this way, neutrinos are one of the most promising avenues for discovering revolutionary new physics not encapsulated by the Standard Model which, for all its successes, is woefully incomplete.
"This is physics beyond the Standard Model," says Turner. "Because we know neutrinos oscillate, they must have mass, so neutrinos are past the Standard Model from the get-go."
The T2K Collaboration reported that the matter and antimatter versions of muon neutrinos oscillated to differing degrees. The difference gives matter a slight, but significant edge over its antimatter counterpart. The finding is not yet at the high statistical level physicists require to declare an unequivocal discovery, but in all likelihood, T2K is onto something.
Showing this asymmetry in matter-antimatter neutrino behavior would be the first evidence of this kind of imbalance in a family of particles called leptons. Researchers have found this asymmetry—technically called charge-parity, or CP violation—in the family of particles called quarks, which form the familiar protons and neutrons in everyday matter. But the amount of CP violation has been far too small to explain matter's dominion. Adding in potential neutrino violations would cover much more of the matter budget shortfall, getting us closer to solving one of science's deepest puzzles.
Neutrinos, after all, are the second-most common particle in the cosmos, after photons (particles of light). Though it's definitely nonzero, we still don't know just how little they weigh. If neutrinos are indeed on the heftier side of estimates, we might be in for a bit of a reckoning when it comes to how substantial, as it were, these host particles actually are compared to what our human senses tell us fills the cosmos. "If the neutrino mass turns out to at the high end of what's possible," says Turner, "then neutrinos will contribute more mass to the universe than do stars."
The connections between the infinitesimal particles and the gargantuan scales of cosmic composition and structure is a key research theme at some of the Kavli Institutes for astrophysics. Understanding the behavior of the fundamental constituents and forces of the universe invariably trickles up to the behavior of the vastest cluster of galaxies, and vice versa.
"This grand convergence of the very big and the very small, of particle physics and cosmology, has been crucial for both fields, and it's very much at the heart of my former institute, KICP, as well as KIPAC at Stanford, and of course at Kavli IPMU," says Turner.
With the latest T2K results, the interrelatedness of the most flitting of particles and the whole of the universe's matter is made stunningly evident.
"Neutrinos are very shy," says Turner, "but they are an amazing window into fundamental physics."