There are lots of ways to describe how rarely neutrinos interact with normal matter. Duke's Kate Scholberg, who works on them, provided yet another. A 10 Mega-electron Volt gamma ray will, on average, go through 20 centimeters of carbon before it's absorbed; a 10 MeV neutrino will go a light year. "It's called the weak interaction for a reason," she quipped, referring to the weak-force-generated processes that produce and absorb these particles.
But there's 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 typically spot these through the copious amounts of light they produce, but in energetic terms, that's just a rounding error: Scholberg said that 99 percent of the gravitational energy of the supernova goes into producing neutrinos.
Within instants of the start of the collapse, gravity forces electrons and protons to fuse, producing neutrons and releasing neutrinos. While the energy that goes into producing light gets held up by complicated interactions with the outer shells of the collapsing star, neutrinos pass right through any intervening matter. Most of them do, at least; there are so many produced that their rare interactions collectively matter, though our supernova models haven't quite settled on how yet.
But our models do say that, if we could detect them all, we'd see their flavors (neutrinos come in three of them) change over time, and distinct patterns of emission during the star's infall, accretion of matter, and then post-supernova cooling. Black hole formation would create a sudden stop to their emission, so they could provide a unique window into the events. Unfortunately, there's the issue of too few of them interacting with our detectors to learn much.
The last nearby supernova, SN 1987a, saw a burst of 20 electron antineutrinos be detected about 2.5 hours before the light from the explosion became visible. (Scholberg quipped that the Super-Kamiokande detector "generated orders of magnitude more papers than neutrinos.") But researchers weren't looking for this, so the burst was only recognized after the fact.
That's changed now. Researchers can go to a Web page hosted by Brookhaven National Lab and have an alert sent to them if any of a handful of detectors pick 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 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 estimates are that there are three core collapse supernovae in our neighborhood each century and, by that measure, "we're due."
If that supernova has occurred in the galactic core, it will put on quite a show. Rather than detecting individual events, the entire area of ice monitored by the IceCube detector will end up glowing. The Super-Kamiokande detector will see 10,000 individual neutrinos; "It will light up like a Christmas tree," Scholberg said.
It'll be an impressive show, and it's one that I'm sure most physicists (along with me) hope happen in their lifetimes. But if it takes a little time, the show may be even better. There are apparently plans afoot to build a "Hyper-Kamiokande," which would be able to detect 100,000 neutrinos from a galactic core supernova. Imagine how many papers that would produce.
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