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Aidan Randle-Conde | Université Libre de Bruxelles | Belgium

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Higgs Convert

Since 4th July 2012, the physicists at CERN have had a new boson to play with. This new boson was first seen in the searches that were optimised to find the world famous Higgs boson, and the experiments went as far as to call it a “Higgs-like” boson. Since then there has been an intense program to study its spin, width, decay modes and couplings and so far it’s passed every test of Higgs-ness. Whether or not the new boson is the Standard Model Higgs boson is one of the most pressing questions facing us today, as there is still room for anomalous couplings. Whatever the answer is, a lot of physicists will be pleased. If we find that the properties match those of a Standard Model Higgs boson exactly then we will hail it as a triumph of science and a fitting end to the quest for the Standard Model which has taken the work of thousands of physicists over many decades. If we find some anomaly in the couplings this would be a hint to new physics hiding “just around the corner” and tease is with what we may see at higher energies when the LHC turns on again in 2015.

A candidate for a Higgs boson decaying to two tau leptons (ATLAS)

A candidate for a Higgs boson decaying to two tau leptons (ATLAS)

For those who have read my blog for a long time, you may remember that I wrote a post saying how I was skeptical that we would find the Standard Model Higgs boson. In fact I even bet a friend $20 that we wouldn’t find the Standard Model Higgs boson by 2020, and until today I’ve been holding on to my money. This week I found that ATLAS announced the results of their search for the Higgs boson decaying to two tau leptons, and the results agree with predictions. When we take this result alongside the decays to bosons, and the spin measurements it’s seems obvious that this is the Higgs boson that we were looking for. It’s not fermiophobic, and now we have direct evidence of this. We have see the ratio of the direct ferimonic couplings to direct bosonic couplings, and they agree very well. We’d had indirect evidence of fermionic couplings from the gluon fusion production, but it’s always reassuring to see the direct decays as well. (As a side note I’d like to point out that the study of the Higgs boson decaying to two tau leptons has been the result of a huge amount of very hard work. This is one of the most difficult channels to study, requiring a huge amount of knowledge and a wide variety of final states.)

Now the reason for my skepticism was not because I thought the Standard Model was wrong. In fact the Standard Model is annoyingly accurate in its predictions, making unexpected discoveries very difficult. What I objected to was the hyperbole that people were throwing around despite the sheer lack of evidence. If we’re going to be scientists we need to rely on the data to tell us what is real about the universe and not what some particular model says. If we consider an argument of naturalness (by which I mean how few new free terms we need to add to the existing edifice of data) then the Higgs boson is the best candidate for a new discovery. However that’s only an argument about plausibility and does not count as evidence in favour of the Higgs boson. Some people would say things like “We need a Higgs boson because we need a Brout-Englert-Higgs mechanism to break the electroweak symmetry.” It’s true that this symmetry needs to be broken, but if there’s no Higgs boson then this is not a problem with nature, it’s a problem with our models!

The fact that we’ve seen the Higgs boson actually makes me sad to a certain extent. The most natural and likely prediction has been fulfilled, and this has been a wonderful accomplishment, but it is possible that this will be the LHC’s only new discovery. As we move into LHC Run II will we see something new? Nobody knows, of course, but I would not be surprised if we just see more of the Standard Model. At least this time we’ll probably be more cautious about what we say in the absence of evidence. If someone says “Of course we’ll see strong evidence of supersymmetry in the LHC Run II dataset.” then I’ll bet them $20 we won’t, and this time I’ll probably collect some winnings!

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