Higgs boson in massless-particle coupling shock, and other stories

If the Higgs boson is responsible for the mass of fundamental particles, how can we see it via massless photons? And what is it doing on stage in Hammersmith anyway?

Higgs to two photons
A Higgs boson decaying to two photons via a quantum loop. Jon Butterworth/JMB

In the build-up to the LHC results announced on Tuesday, I was in a position similar to that when I wrote this article in the summer. I knew my collaboration (ATLAS) had a suggestive but inconclusive result, and I did not know what CMS, our opposite numbers, had.

When CMS unveiled their result, it could have been a stronger hint-of-Higgs than ours, a weaker one, or it could have contradicted ours. That's uncertainty for you, and as Suzanne Moore says, we'd better deal with that.

It turned out the CMS result was a slightly weaker, but rather similar, hint; certainly not incompatible with ATLAS. And so we go on, with the odds shifted in favour of a Higgs boson existing at a mass of about 125 GeV (or 125 times the proton mass), but with bets still very much being taken.

In the summer, that was also what happened. But the result this week was more interesting. In the summer, the "hint" was in the WW decay mode of the Higgs. For reasons I described here, this mode is not very good for telling us the mass of any candidate Higgs boson there might be. The neutrinos, from the decaying W bosons, carry away too much information that we don't see.

The WW decay mode still features, and in both ATLAS and CMS it contributes to the hints. But the main interest this week was focussed on two other ways the Higgs can decay, and therefore show up in our detectors.

Both of these decay modes can tell us the mass of the Higgs, should it be there, and both are weird, for different reasons.

The first is the Higgs decay to two photons.

The weirdness here is that the Higgs boson is famous for - or was deduced from, or invented to explain, according to taste - mass. Fundamental particles get mass by interacting with it. By the same token then, the Higgs will generally decay to heavy things. The more massive they are, the more likely the Higgs will decay to them, because it interacts most strongly with them. Conversely, things with no mass don't interact with the Higgs.

So why photons? Photons are quanta of light. They have no mass.

Indeed, the Higgs decays to photons very rarely. If the Higgs boson mass is about 125 GeV, and you make 10,000 of them, less than ten will decay to two photons. Most will decay to bottom (beauty), quarks. But these are very hard to distinguish from other collision debris which don't involve a Higgs. Quarks, even beautiful ones, are cheap at the LHC. Pairs of high energy photons, not surrounded by other stuff, are much rarer and can be measured more accurately. Fabiola, the head of ATLAS, spent quite some time on this in her presentation on Tuesday.

But since the photon's mass is zero, the Higgs really ought not to decay to photons at all. And indeed it does not, directly. It has to go through a loop of some other particle*, as in the cartoon above.

This is fine. In quantum mechanics, anything that can happen does. There might even be other new particles we've never seen going round that triangle, though in the standard model it's usually a W boson or a top quark.

But this is not really what the Higgs is for. More specifically, before I credit a boson with being responsible for mass, I want to see it interact with mass directly, not via a quantum loop.

Enter the other decay modes - more on those soon, I hope, starting with two Z bosons.

Before stopping this piece though (written on a plane again) I want to thank Brian Cox and Robin Ince for handling our garbled appearances at Hammersmith Apollo "Uncaged Monkeys" so well, and the audiences for reacting enthusiastically despite the hiccups (technical and beverage-induced). I would also like to thank whoever took this video on the Tuesday of the start of the link to the UCL CERN flat.

I still can't quite believe that Brian is standing on the stage there talking about gauge theory to 4000 people and skyping to CERN to talk to miscellaneous scientists**, including the head of our research council.

But it's a lovely dream.

* this is rarer than direct decays because of the small couplings, see
perturbation theory if you want to know more.

** Claire, Andy, Hilal, Pauline, John and Jordan.