How the Large Hadron Collider found the Higgs boson

Chris - So does that mean then that when the LHC was conceived, because the challenge was, 'can we get at the Higgs boson', You didn't actually know if it would have enough energy to find the Higgs?

Lyn - Yeah, this was the important thing about the LHC I think. First of all, the question was did the Higgs boson exist? Because this is very difficult to explain how the Higgs boson imposes mass on the other particles. It's not actually the Higgs boson, it's the Higgs field. The idea of a field, a gravitational field, which is extremely weak and actually doesn't fit in the Standard Model at all. That's one of the mysteries. But the electromagnetic field, the field of magnetism, for instance, these are all fields, and every field has got a quantum of the field, a particle associated with the field. So for the electromagnetic field, it is the photon, the particle of light which is the quantum of the field. Now, the postulate was there is a field called the Higgs field, which pervades all of our universe, and it is the interaction of particles with the Higgs field that gives them mass.

Lyn - The photon, the particle of light, doesn't interact with the Higgs field at all. So a photon has got zero mass, which doesn't mean to say it's not useful because it brings us light from the sun, it operates our mobile phones. But this Higgs field, the stronger it interacts, the higher the masses of the individual particles. So associated with the Higgs field, there should be the quantum of the field, which is the Higgs boson. So to prove the existence of the Higgs field, we had to create the Higgs boson. That point, all of this is very fine, but nobody had any way of predicting the mass of the Higgs boson. When we designed the large Hadron Collider, first of all, we had experience from the previous proton antiproton collider of how to build it. But we designed it with enough energy so that if the Higgs boson existed, we were bound to find it. We built it with the energy or the mass of the Higgs boson that we discovered, but we built it so that if it existed, we were bound to find it. But the question is, did it exist or not?

Chris - I mean, that's why we're all in a job though, isn't it? That's why we do science, because you think big, you have a theory, you then design experiments to test it. But how did you know that you could make a Higgs boson? What was the physics that led you to believe if we smash protons together, we'll get this mysterious particle that might be the quantum that underpins this field that gives things mass. What was the thinking behind that?

Lyn - The way that these colliders work of course, when you smash protons together. I explained the proton is a composite object. It's like smashing two oranges together. Inside the oranges you've got the pips. So you get an awful lot of junk, but occasionally you get very hard collisions between the pips. And the proton is a little bit like that. Inside the proton there are the quarks. So when you do that, then you produce energy, which then can transform itself into the Higgs boson for a very tiny amount of time, and then it disappears back into the Higgs field. So the idea is you convert energy into mass by accelerating these particles up to very high energies and colliding them together, and you look at the debris of the collisions.

Chris - How did you know what to look for? Because if you don't know something exists, how did you know what the footprints of the Higgs boson transiently popping into existence in that debris that you created from those collisions? How would you know what it looked like?

Lyn - If it existed that was rather easy to predict the decay modes that it would have. There are some decay modes, even though they're extremely rare. One very clean channel, for instance, is the Higgs decays into two photons, and the two photons have to go back to back to conserve energy and momentum. So you'd see two photons coming out of a collision that you would measure their energy, and the sum of their energy would be the mass of the Higgs boson. So that was one way. And there are other signatures as well where you get four electrons coming out. So these were predictable decay modes, so we knew what to look for and to build the detectors to detect them actually.

Chris - And how quickly after kicking off with the LHC was it apparent that you were into the regime you needed and that you were beginning to see promising detections that suggested you were onto something and that we were going to get the Higgs?

Lyn - So once we switched on the LHC, then the big issue was getting the luminosity. The luminosity is the number of collisions per second that we could produce. Because the Higgs is so very rare, we would've to do many, many things to see the Higgs Boson. And when we switched on the LHC, actually, one thing you may or may not remember is that as when we were commissioning the LHC, we had an accident which cost us about a year when one of our superconducting joints failed and produced a big mess. Anyway, we had to fix that, but that gave us an extra year to get everything ready. So when we actually switched on the LHC it came on very quickly. And the first real year of operation that we were in a regime where we could expect to see first signs of the Higgs. And there was a seminar at CERN on July 4th, 2012. This seminar showed there are two detectors on the LHC, which are capable of seeing the Higgs boson, they're called ATLAS and CMS, two huge collaborations that are taking data off the machine. And on that day, neither of the collaborations knew what the other one was going to show. They showed data that showed a small peak in the mass spectrum, at 125 GeV. Any one of those detected, the signal would've been very, very interesting, but not sufficient. But when you put the two together in real time in the lecture theatre and Peter Higgs was present in that seminar, then it was absolutely obvious that the Higgs boson had been discovered because the two results just matched perfectly. So that was a great day that I will never forget actually.

Chris - And was presumably Peter Higgs either awe struck or did he sit there looking smug? What was the reaction of the future Nobel Prize winner when these results went up?

Lyn - Peter is a very modest guy. He's a theoretical physicist. He just could not imagine the efforts that the whole world community had put into finding his particle. He did say, I thought I would never ever see this in my lifetime. And that's imprinted on a t-shirt I have now.