Neutrinos are detected not directly but through the particles they produce when they occasionally strike a nucleon, interacting via the weak force's W or Z bosons. This collision either creates an electron or another neutrino flavor, along with a spray of other particles, as first captured in a historic photograph from a hydrogen bubble chamber showing a proton, a meson, and a muon shooting out from the collision point. Because these resulting particles are charged, they interact electromagnetically and produce light, which is the signal detectors actually capture.
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The bubble chamber photo described shows the neutrino itself as invisible, entering from the right with no visible track until the collision point where the spray of particles begins.
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25:52we've figured out the signature that's least impacted by the sort of intergalactic medium on its journey, which allows us then to make the most direct correlation back to that initial energy starting point. We know what we're looking for and why we are looking for it. And so then the next obvious question is, okay, how do we detect a neutrino? That's right. It sounds very promising. It sounds very promising, but the same reason why the neutrino is a great candidate for extra galactic astronomy is also a reason why they're notoriously hard to detect. Namely, they don't interact with a lot of things.
26:35That's the whole point of trying to detect them. But at the end of the day, if they don't interact with stuff, how are you going to see them? The reason why I can see light is because the light is readily interacting with the rods and cones in my eye, which we talked about yesterday in yesterday's Nobel Prize. So neutrinos don't do this. I mean, as I said, every second, 65 billion neutrinos from the sun blow through my fingernail. I'm not feeling anything. Unbelievable. There are a few ways that the neutrinos do interact, and here's how it would happen. On the left, you've got sort of a particle picture of how a neutrino would do something. The neutrino is coming from the upper left, and it is interacting with a nucleon.
27:19Nucleons are full of quarks, right? The up and down quarks. There's three of them. That's why there's two that are just sort of skating by, and one of the quarks is going to interact with this neutrino using the W or Z bosons, which is the weak nuclear force. When that interaction happens, we have either one of two choices, okay? Either an electron is going to be born that sort of moves off and carries some of the energy from that initial neutrino, or another type of neutrino is going to be born. Depending on what the interaction was, you can have either an electron coming out or a different
28:01type of neutrino coming out, okay? On the right-hand side, this is the first ever neutrino that was detected as a photograph, okay? This is in a hydrogen bubble chamber. You see on the left there, there's just kind of a source where three things are coming out. That's where the neutrino interaction happened, okay? We're not seeing the neutrino come from the right-hand side of the page because the neutrino's just coming, and then boom, it hits something over there, which caused a spray of particles. That's how we see if a neutrino detection happens. If randomly a spray of particles comes out, and it's the exact type of particle that we expect.
28:42It has a fingerprint. It has a fingerprint. In this case, there's a proton coming out, there's a collision with a meson, and then there's also a mu meson that's coming out, and those things are exactly what we would expect if the neutrino interacted with the hydrogen that's in this bubble chamber. So would it be the case that because they're moving so quickly, we just would see this interaction kind of pop out of nowhere? Mm-hmm, yeah, because the neutrino, we can't detect. We can't detect it. We can only see this, like, yeah, okay. Once the neutrino interacts with something, we can detect the byproducts because the byproducts are charged, so they're gonna be interacting with the electromagnetic field, which then creates light.
29:22What type of light?