We’re pretty sure that neutrinos come in three different (but equally tasty) flavors: electron, muon, and tau. Each flavor has a slightly different (but tiny) mass, on the order of a million times smaller than the mass of a single electron, and a neutrino can oscillate between these three flavors as it zips along. The original flavor that each neutrino takes depends on how it was created: most often, neutrinos are created through high energy nuclear processes like you'd find going on inside stars. To take one common example, protons and neutrons colliding with each other create pions, which are subatomic particles that decay into a mix of muon and electron neutrinos.
The most common source for the neutrinos that we see here on Earth is the sun, which produces an electron neutrino every time two protons fuse into deuterium. (This happens a lot.) What's much rarer to see are neutrinos that aren't produced close to home—neutrinos that come from outside of our solar system, and even outside of our galaxy. These are called cosmic neutrinos, or astrophysical neutrinos.
Cosmic Neutrinos
Cosmic neutrinos are born, we think, in the same sorts of ultra high energy events out in the universe that also generate gamma rays and cosmic rays. We're talking events like supernova remnant shocks, active galactic nuclei jets, and gamma-ray bursts, which can emit as much energy in a few seconds as our sun does over ten billion years. As you might expect, the resulting neutrinos have stupendously high energies themselves: a million billion electronvolts (1 petaelectronvolt) or so. That works out to be about a tenth of a millijoule, which is a lot for a particle that has an effective size of nearly zero, and it is about equivalent to the kinetic energy of a thousand flying mosquitoes, in case that horrific unit of measurement is of any help to you.
The reason that cosmic neutrinos are important is the same reason that neutrinos themselves are so frustrating to measure: they ignore almost everything. This allows them to carry valuable information across the entire universe. What sort of information can be carried by a particle that we can't even measure directly? Here are three examples:
Since neutrinos aren't affected all that much by even the densest matter, they can escape from the core of a supernova well before the shock wave from the inside of the collapsing star makes it to the outside and releases any photons. By detecting this initial neutrino burst, we can get a warning of as long as a few hours before a supernova would be visible from Earth.
Since neutrinos aren't affected at all by magnetic fields and therefore travel in straight lines, they can help us pinpoint the origin of ultra high energy cosmic rays, which are affected by magnetic fields and therefore can follow winding paths. We know that some cosmic rays come from supernovae, but many of them don't, and we're not sure where the rest of them originate. With energies over a million times greater than the Large Hadron Collider, it would be nice to know where they come from.
The ratio between different flavors of neutrinos may suggest how they were formed. For neutrinos produced by pion decay, we'd expect to see two electron neutrinos for every muon neutrino. If we see different ratios, it would suggest a different formation environment, and particularly weird ratios could even lead to new physics.
Detecting Neutrinos
Since neutrinos don't really interact with anything, there's no reliable way to detect them directly. Whether they're coming from the sun, the atmosphere, or somewhere more exotic, the best we can do is try and spot the aftermath of a very unlucky neutrino smashing headlong into something while we watch.
When a neutrino does smash into something, one of two different things can happen. If the neutrino isn't super energetic, it might just bounce off in a new direction, passing on some of its momentum and energy into whatever it hits (which recoils in response) and occasionally causing that thing to break into pieces. Some neutrino detectors are designed to watch for both of these effects. The other thing that can happen is that the neutrino obliterates itself, dissolving into a subatomic particle that depends on the neutrino's flavor-of-the-moment: an electron neutrino turns into an electron, a muon neutrino turns into a muon, and a tau neutrino turns into a tau particle, stripping electric charge off of whatever it hits as it does so. Some detectors look for this change in charge of the thing that the neutrino ran into (it's the only way of detecting neutrinos with energies less than 1 MeV), but the detector that we're interested in looks for traces of the ex-neutrino's subatomic particle itself.
Detecting particles moving at very high speeds is a (relatively) straightforward thing, at least conceptually. The key is what's called Cherenkov radiation, which is created by charged particles moving through a medium faster than the speed of light in that medium. It's sort of like the sonic boom created by an object travelling through air faster than the speed of sound, except with with light. Here's what the Cherenkov radiation created by a burst of beta particles from a nuclear reactor being pulsed looks like:
So now that we've got some lovely blue glowy-ness to look for, all we need a medium through which charged particles can travel faster than light, and some kind of detection system that can spot the resulting Cherenkov radiation.
Continued in next post...
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