A Decade of the Higgs Boson and BeyondPostcards from the Energy Frontier by Prof Jon Butterworth
On 4 July 2012, after presentations from the ATLAS and CMS spokespeople summarising the search for the Higgs boson, the then Director General of CERN Rolf Heuer said “I think we have it”.
There was a huge cheer. The crowd agreed.
I was at a press conference event in Westminster, where some of us were being a bit more cagey, much to the frustration of some of the journalists present.
“So have you really found the Higgs boson?”
“Well, we’ve definitely found something new. And it’s definitely a boson. It looks like some kind of Higgs…”
The reason for the caution was that in the Standard Model of particle physics, the Higgs boson is a manifestation of a very specific mechanism for allowing fundamental particles to acquire mass. This has implications for its properties, and we had not measured enough of those properties at that stage.
We had seen the new particle through its decays to photons, and its decays via Z bosons. The latter is crucial, because the mass of the Z and the W bosons, which carry the weak force, is probably the most essential element of the whole deal. The most important piece of information that was still missing back then was whether the Higgs boson could actually decay to matter particles. Collectively known as fermions, these are the quarks and leptons which make up the atoms which make up us.
Over the last decade, we have shown that the new boson does indeed decay to quarks and leptons. Furthermore, to the precision with which we can measure it so far, it does so at the rate predicted by the Standard Model.
This all means it is definitely “some kind of Higgs boson” in the sense that it does explain how fundamental particles acquire mass. And it is the Higgs boson as realised in nature, whatever the Standard Model might say.
And then what?
To understand what has happened since the Higgs discovery, and its repercussions, it helps to think of high-energy physics as an exploration of nature at increasingly high resolution. There is a correspondence between resolution and energy, so as we increase the energies of our colliders, we probe the heart of matter at shorter distances.
There is a particularly important energy – and therefore resolution – associated with the Higgs boson, because of the role it plays in allowing the W and the Z to have mass. At energies below the Higgs mass, the weak force is much weaker than the electromagnetic force (hence the name). Once you get to energies equivalent to the Higgs mass or above, the two forces are very similar in strength, and in a loose sense they are unified. This is a significant transition point in nature, and having revealed the Higgs itself, the LHC is for the first time allowing us to explore in depth above this transition point.
Given that its prediction of the Higgs boson was such a spectacular success, it may seem paradoxical that many, probably most, particle theorists felt it very likely that the Standard Model would fail once we got to energies above the Higgs mass.
Part of the reason for this is that we know the Standard Model must fail at some point, because great though it is there are things it does not explain but which any complete physical theory should. I guess it is reasonable to anticipate that when we break through into a qualitatively new regime of physics, a new and better theory might start to reveal itself. There are also arguments based on the stability of the Higgs mass against quantum corrections which can be used to argue for such a better theory kicking in at energies not too far above the Higgs mass.
So far however, everything measured at the LHC is consistent with the predictions of the Standard Model. There are several marginal discrepancies (see for example “A Growing Anomaly”), some of which are strong enough to draw significant attention, and any one of which could, in future LHC runs (the next one starts on 5 July!) mushroom into some very big news indeed. But so far, nothing passes muster as a solid beyond-the-Standard-Model phenomenon.
To Run 3 and Beyond!
Even though they have not vindicated any new theories, the results of the past ten years represent “new physics”. We are probing the Standard Model in a completely new region, measuring processes never before observed. Personally I get a thrill from seeing nature do stuff we have never seen it do before, and the fact that our theoretical ideas are up to the task of explaining it doesn’t detract from that.
The minimum we will learn from the LHC over the next few years is whether the Standard Model continues to be a good theory at energies much higher, and therefore distances much smaller, than those at which it was developed. If we do explore that far and confirm that, it will be a significant new fact about nature, much more significant than “We looked for all the new ideas theorists came up with and none of them were there”. We have a good theory already, we are measuring new things, and we are quantifying how well the theory predicts them – as well as throwing out multiple ideas that don’t.
It would of course be exciting if, as our precision increases over the next few years, puzzles do emerge and grow. None of the “expected” new ideas have so far shown up, but to paraphrase Isaac Asimov, the most exciting – and certainly more common – phrase to hear in science is not “I think we have it!” but “that’s funny”. Funny things can happen. That’s what it means to explore new territory.
You can read more about the discovery of the Higgs, and what it was like being part of the team, in Jon’s brilliant book Smashing Physics.
Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics. He is @jonmbutterworth on Twitter.