There are many ways to describe how rarely neutrinos interact with normal matter. Duke’s Kate Scholberg, who is working on it, provided another one. A gamma ray of 10 mega-electron volts passes through 20 centimeters of carbon on average before being absorbed; a 10 MeV neutrino lasts a light year. “It’s called the weak interaction for a reason,” she joked, referring to the weak force-generated processes that produce and absorb these particles.
But there is one type of event that produces so many of these elusive particles that we can’t miss it: a core-collapse supernova, which occurs when a star can no longer produce enough energy to counteract the pull of gravity. We usually see these because of the large amounts of light they produce, but in energetic terms that’s just a rounding error: Scholberg said 99 percent of the supernova’s gravitational energy goes into producing neutrinos.
Within moments of the start of the collapse, gravity forces electrons and protons to fuse, producing neutrons and releasing neutrinos. While the energy needed to produce light is held back by complex interactions with the collapsing star’s outer shells, neutrinos cut through all the intervening matter. At least most do; so many have been produced that their rare interactions matter collectively, though our supernova models don’t quite know how yet.
But our models do say that if we could detect them all, we’d see their flavors (neutrinos appear in three of them) change over time, and we’d see different emission patterns during star incursion, accretion of matter and beyond. supernova cooling. The formation of black holes would suddenly stop their emission, so they could provide a unique window into the events. Unfortunately, there’s the problem that too few of them interact with our detectors to learn much.
The last nearby supernova, SN 1987a, saw a burst of 20 electron antineutrinos about 2.5 hours before the light from the explosion became visible. (Scholberg joked that the Super-Kamiokande detector “generated orders of magnitude more papers than neutrinos.”) But researchers weren’t looking for this, so the outburst was only recognized in hindsight.
That has now changed. Researchers can go to a web page hosted by Brookhaven National Lab and receive an alert if one of the handful of detectors picks up a burst of neutrinos. The Daya Bay, IceCube, and Super-Kamiokande detectors are all part of this program.) When the next burst of neutrinos arrives, astronomers will be on high alert and searching for the source.
“The neutrinos are coming!” Scholberg said. “The supernovae have already happened, their wavefronts are on their way.” She said that it is estimated that there are three nuclear collapse supernovae in our neighborhood every century and that by that measure we “must.”
If that supernova happened in the galactic core, it will put on quite a show. Instead of detecting individual events, the entire ice area monitored by the IceCube detector will glow. The Super-Kamiokande detector will see 10,000 individual neutrinos; “It will light up like a Christmas tree,” Scholberg said.
It’s going to be an impressive show, and I’m sure most physicists (including me) hope they will happen in their lifetime. But if it takes a little time, the show might be even better. There are apparently plans to build a “Hyper-Kamiokande”, which could detect 100,000 neutrinos from a galactic nuclear supernova. Imagine how many papers that would generate.