Cari Cesarotti
speaker
212 appearances
1 recordings
1 series
first heard Sep 2024
last heard Sep 2024
Cari Cesarotti’s voice in public audio — every appearance, attributed to the second.
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Yeah, so this is a great question. And every time, certainly as a PhD student, when I listen to it, I would hear someone describe one machine and be like, well, this is the best collider. And then I would hear someone describe the other one. I'm like, no, this is the best collider. And the answer is they both have strengths.
So when you collide something like an electron, you're basically just colliding electrons. It's a fundamental particle. So electron plus electron combines. Usually we do particles and antiparticles. So you have E plus, E minus come in, collide, produce a charge neutral state.
And all of the energy of these two electrons colliding can be recombined into different massive particles, different momentum particles, as long as the net energy and momentum of the event is conserved. However, for proton-protons, protons are actually a big bag of stuff.
And that bag includes the three quarks that usually we talk about when you first take your nuclear physics course or whatever in school, right? I don't know if many schools have nuclear physics, but chemistry, let's say chemistry. You have up and down quarks, basically are the primary constituents of protons at low energies. But these quarks are tied together with gluons.
And inside the quarks, or sorry, inside the protons, especially as you start cranking these things up to really high energies, is the bag becomes much more complicated. And inside these protons, we have particles that are very cleverly named as partons because they are parts. And particles have to end in on. So partons.
Yes.
He did enough good things, so we can give him a break for this one. But inside this proton, all of these different partons, which include the gluons, which are those bosons that we talked about earlier, and the quarks, they kind of share the total energy of the proton. So at the LHC, we collide protons of 7-ish TeV. And no one particle inside that proton is going to have anywhere near 7 TeV.
tends to happen that the gluons take most of the energy and then quarks also take some of the energy. So when you're really looking at a collision, it's a gluon-gluon collision or a quark-quark collision. Or even in the proton, sometimes you can have quark-antiquark pairs pop into the vacuum or pop out of the vacuum and then disappear again. And sometimes you can collide those.
So you can have a quark-antiquark interaction. And all of these things will share the energy of the proton. So in some ways, that makes for a super interesting collider because the opportunities of what kind of particles you can collide is much bigger, right? You can collide not only up and down quarks, but strange quarks or charm quarks or gluons and some things more rare than others, of course.
But it means that you can see all sorts of interesting signatures come out. The downside is that you never know exactly what the energy of these things are. So that can make your analysis much, much harder. And of course, you don't get that full energy of the protons. So even though the LHC runs at 14 TeV, we don't actually get to see any collision happen at 14 TeV.
It's usually much closer to one or two.
Yeah, often you'll hear these two machines sort of described as electrons are the precision machine because you can get very, everything's very clean, right? Like you just have electron in, electron in, and then you know the energy and the collision can be basically completely reconstructed, assuming what comes out. Versus for protons, it's like smashing cars together. There's debris everywhere.
You don't know exactly what collided into what. And, you know, it's kind of like hitting something with a big hammer and just a bunch of stuff can come out. But knowing exactly where it came from and how it came to be is a much harder question. So protons are called... Yeah, sorry. Electron-electron is precision.
And then proton-proton is often called discovery because you can have all that high energy available to you.
Yeah, so that's a great point. And maybe the way that these things are named is kind of unfair to E plus, E minus machines. So like we were talking about in the beginning with the standard model, part of understanding the standard model is knowing to what degree our predictions are correct, right? Because in science, you never get to say definitively, oh, this number is correct.
You can only measure it to a certain precision. And that's sort of the claim that you can make. So with precision machines, precision machines are standard model machines in the sense that they try to measure standard model things.
But they are also discovery machines in sort of a roundabout way in the sense that if you were to discover something that is not matching the standard model prediction, that's a hint of a discovery. So you don't get to actually physically make the particle and point to it and say, look at this, we did it.
But you get to say, OK, there's a discrepancy in our data, and this could be indicative of new physics. So I guess that's why, you know, we call it precision versus discovery because it can't make, it can't concretely define unambiguously that there is something new going on, but it can absolutely help us sort of know where to look when we go to the higher energies.
Yeah, absolutely. And even if something's not new physics in the sense that there's a new particle or a new degree of freedom that we haven't accounted for, the fact that there could still be new phenomena that we haven't understood is still, of course, a super exciting discovery to make.
Oh, boy. So this is the question of the decade for collider physicists. So there are a couple of different ideas that people want to get into. So we can go as slow or as fast through this part as you want. But to summarize quickly, there are basically three kinds of machines people want to think about making. One is a linear E plus, E minus collider.
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