The hosts explain why neutrinos are the chosen messenger for studying distant cosmic particle accelerators, compared against two alternatives: protons and gamma rays. Protons are charged and get deflected by magnetic fields on their way to Earth, so their arrival direction does not point back to the source. Gamma rays travel in straight lines but are light, so they interact with the intergalactic medium and require space-based telescopes to detect, while neutrinos pass straight through the atmosphere, intergalactic space, and even the Earth itself, making them a cleaner directional probe despite being hard to catch.
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The messengers originate when an accelerated cosmic-ray proton interacts with nearby gamma rays or matter, producing a shower of pions that decay into electrons, gamma rays, and neutrinos.
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Electrons are ruled out as messengers for the same reason as protons: being charged, they get pushed around by magnetic fields before reaching Earth.
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About 5% of the original proton's energy ends up in the neutrino, so a proton accelerated to 10^20 electron volts corresponds to neutrinos in the 10^18 to 10^19 electron volt range, letting researchers work backward to estimate the source's energy.
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The target neutrino flux they describe is on the order of one per square kilometer per year, which sets the scale for why a detector the size of IceCube is needed.
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22:28And that is where we start thinking about what else could we see, right? There's a particle accelerator that is flinging these protons and these charged particles at extremely high velocities. What else could we see in that region that could come at us? There's two possibilities. This is a diagram showing some of the interactions that could be happening where the particle is accelerating, at that electric core. At the origin point. Yeah, at the origin point, right? This is a proton that's of cosmic ray origin. It interacts with, let's say a gamma ray that's nearby, like all of the light and all of the random stuff that's going on. And it creates a particle of showers, pions, those pions then decay into electrons, gamma rays,
23:12and neutrinos. Those are the three that we could kind of look for. Electrons have the same problem. They're charged. So they're gonna be moving around as well. Gamma rays are interesting, right? Because gamma rays, they are uncharged. So they could be coming straight. And neutrinos are interesting because they're uncharged. The problem with gamma rays though is they're light. So, and let me just take a quick step. So the idea is we're trying to figure out what these giant particle accelerators, like what are they? We're trying to understand what they are. And so we can detect them through the things they shoot across the universe. And we've just gone through a list of the things we could detect that are not neutrinos, that
23:53they shoot across the universe, and why it doesn't really work. Yeah, it doesn't really work. Protons, which is the actual thing, if they came without interacting with anything, that doesn't really work because they're moving around through the galaxy. Gamma rays could work because at least they're coming straight at us. But again, they kind of get slowed down sometimes by the intergalactic medium. They're light, so they interact with the intergalactic medium. And you gotta put a telescope in space in order to actually see this thing. How big of a detector are you gonna make in space? So there's an engineering problem. There's an engineering problem there as well because the Earth's atmosphere doesn't like gamma rays. Very good for us here on Earth because we don't want gamma rays. But detecting them is kind of a hassle.
24:35Neutrinos, on the other hand, they're gonna shoot right through. Not just the Earth's atmosphere, not just all of intergalactic medium, but the Earth itself. This makes them kind of a nice target to look for. There's also an approximate energy connection because if the proton itself is at 10 to the 20 electron volts, in that interaction that I showed you with those particles, about 5% of the energy gets deposited into the neutrino. So we can reverse engineer back to the total initial energy starting point. Yeah, we can probabilistically say that the original event had some amount of energy. And so this is where we get into the kinds of neutrinos that we're trying to look at.
25:19If we want something that's 10 to the 20, then maybe we'll look at something that's around 10 to the 18, 10 to the 19 electron volts for the neutrinos. And that's kind of our target. So we now have a flux, which is one per square kilometer per year, and we have a kind of target energy that we wanna take a look at. In our quest to again now find a way to figure out these highly energetic objects that are extra, extra galactic,