The Physics behind the paper behind “Colliding Particles”

This post is also at The Guardian, and is expanded in Chapter 1.7 of Smashing Physics.

This is a bit of a niche post but there was recently a review in Physics World of these videos I’m in about research at the Large Hadron Collider (LHC). While generally positive, the review pointed out that although the videos are partially based around a particular scientific paper about how one might find the Higgs boson (referred to in the films as the “Eurostar paper”), they don’t really explain the physics behind it, being focussed more on “how science works” than a specific result. Fair comment. So here is my attempt to explain the physics behind this paper to an intelligent but non-specialist audience.

I’ll concentrate on explaining the new ideas in the paper rather than giving a summary of why the Higgs is interesting or what the LHC is. I wrote something about that at the end of this article for the BBC, and might try again at some point. But I’ll assume some familiarity with what the LHC is. For now, you need to know that if the Higgs boson exists, and if its mass (which we normally express in units of energy using E=mc²) is what seems to be the most likely value (around 120 GeV*)  then lots of them will be produced at the LHC, and the realtrick is to be able to pick them out from all the other things going on when the LHC collides its protons.

Now you see it, now you don’t

If the Higgs is there, it is responsible for giving mass to all the other particles because of the way it “couples” or sticks to them. Because it couples to the mass of the particle, it is very likely to decay into the heaviest particle it can. These decays happen super-quickly so all we see is the things into which it has decayed. We have to work out from them that there was, briefly, a Higgs boson in our detector.

For a 120 GeV Higgs, the heaviest thing it can decay to is a pair of bottom (b) quarks. In fact they also then decay (after travelling a few 100 microns or so) and each one gives a spray, or “jet” of hadrons. Hadrons are particles made of quarks, like protons and neutrons but in this case more commonly pions. We see these particles in our detectors, and have to reconstruct the fact that two b-quarks decayed, and that they came from a Higgs decay.

So: a Higgs boson is produced, it decays to two b-quarks, and they give two jets of hadrons.

Problem is, lots and lots of b-quarks and jets of hadrons are produced at the LHC, mostly nothing to do with a Higgs. Before our paper, it looked like this background noise would completely swamp the signal and we would have to rely on other, rarer, Higgs decays to find it. Not only did this make finding the Higgs harder, but seeing the decays to b-quarks is actually pretty important in proving that whatever you might have found really is a Higgs boson!

Fast movers

The idea we had was to look at those collision events where the Higgs is not just made, but is made and given a lot of kinetic energy – i.e. moving very fast, at relativistic speeds. This happens in about 5% of the Higgs production events we were looking at, so we still “waste” a lot of Higgs bosons. But we lose even more of the background, because the background jets are usually moving much more slowly.

Something happens when you look at these fast-moving Higgs decays. The faster the Higgs moves, the smaller the opening angle between the two b-quarks it decays to. In fact very often the jets from the two quarks merge into a single jet. This is a problem if, like the previous studies, you are looking for two b-quark jets as your tell-tale signature that there was a Higgs boson. In our paper we actually used it to our advantage. By looking at the substructure of this jet, you can see evidence for the two b-quarks and the Higgs decay, get rid of even more background, and measure the mass of the Higgs boson well enough to make it stand out over the remaining background. This basically meant that what had looked like a hopeless case was recovered as a promising way of finding the Higgs at the LHC.

Credits and Further Reading

The idea of looking at jet substructure to find the decays of fast-moving particles was put forward in this paper by Mike Seymour and this paper I wrote with the super-famous Brian Cox and Jeff Forshaw. Gavin Salam (also in the films) had a better way of doing it, and together with Adam Davison and Mathieu Rubin we were the first to apply it to the Higgs boson. The idea has lots of other applications at the LHC and several other people (and me, and Gavin) have since written papers applying it to things like supersymmetry searches and top quarks. Also, with colleagues in the ATLAS experiment, Adam and I followed up the paper with a study using a realistic simulation of the detector, which confirmed that it really should work. And of course now we are starting to look at real data!

Mike Paterson makes the cool films.

*GeV = Giga electronvolt. The energy an electron would have if you accelerated it through 1,000,000,000 volts.