Category: LHC Background Info

The rumors about the Higgs particle at the Large Hadron Collider [LHC] have begun again, and since that’s all anyone is going to want to talk about until we actually get the news for real, at the ICHEP conference in Melbourne in a couple of weeks, we may as well get started.

[This is especially true since we learned last year that some well-known non-particle-physicist bloggers have information pipelines directly into the experiments.  It is perhaps inevitable that there are scientists who see it in their best interest to subvert the scientific process.]

The current hot rumor is that the LHC experiments ATLAS and CMS have seen, in the new 2012 data, very roughly what they saw last December in the 2011 data, at least as far as the signal from a Higgs decaying to two photons (particles of light) in the mass range of 125 GeV/c².  Note I am just repeating what I have seen on other blogs; as a matter of policy, I do not report secret information on this website, and I make no comment on the validity of any rumors.

Suppose this were true; what would it mean?  Well, it’s pointless to try to speculate about exactly how many standard deviations you would obtain by combining data in various ways — if I tried, the only thing I could guarantee you was that I’d get it wrong.  So rather than kick arbitrary numbers around based on guesswork, let’s talk about the larger picture.

There are two ways to think about the 2011 and 2012 data. One is to think about them as data sets you should combine together to extract the maximum information.  That’s what you would ideally do if you had access to all of the data and knew all the subtleties about how to do this.  It is not going to be that easy. Among many issues, one is that the data are taken at different energies, 8 TeV per proton-proton collision this year versus 7 TeV last year.  So in combining them you are making a theoretical assumption about how the production rate for Higgs particles changes as you change the energy from 7 TeV to 8 TeV.  That’s something you may or may not want to do, and at best it makes things rather complicated when you try to interpret the result.

Another way to think about it, as I discussed last week — and I think this will turn out to be more useful as a rule of thumb — is that last year’s data was good for excluding the Standard Model Higgs particle over much of the range for its mass that was available before 2011.  From a range of 115 to 800 GeV/c² we’re now down, after 2011, to a range of something like 120 to 128  (where we can argue about the edges, but it doesn’t matter very much) or above 600.  The hints we saw last year at around 125 GeV/c² (see also here, here and here) were very interesting and suggestive, but weren’t convincing, in part because with such a large range of possibilities to explore, the possibility of such hints appearing by random chance was not that low.  That’s due to the so-called “look-elsewhere effect”.  One could only obtain high significance by combining lots of different low-significance measurements together, a technique which can be very problematic, especially when some of these measurements are very difficult.

Now we come to 2012.  The LHC experiments ATLAS and CMS have just about the same number of collisions so far in 2012 as they had in all of 2011; and since the energy is a bit higher, the production rate for Higgs particles should be a bit higher (by several tens of percent.)  So what we’re going to see in the current 2012 data is a bit like a do-over of 2011… except that now the range of possible Higgs masses that needs to be explored is much smaller.  The smaller range means that if the same hints were seen in the same place as they were seen in 2011, the probability of this happening in the search region by random chance would be much lower than in 2011.  More precisely, this means the difference between the naive significance of any excess in the data, and the significance after accounting for the look-elsewhere effect, will not be very important.  Look-elsewhere was a big effect in 2011; but after the 2011 results, it will not be a big effect in 2012.  Thus (roughly) if you view the 2011 data as needed to narrow down the search region to a small window, you can roughly take the significance of any reported 2012 excess almost at face value.

So suppose we did see hints in the 2012 data that roughly resemble those in the 2011 data?  Then I think you’ll see most everyone agree that the evidence for something real in the data is pretty strong.  Of course we’ll need to see this in both the ATLAS and CMS experiments to have full confidence, but let’s assume that’s the case.  What would be next?

• First, is what’s being seen in the data a Higgs particle at all?  Could it be something else?
• Second, if it is a Higgs particle, is it the simplest possible type of Higgs particle — the Standard Model Higgs particle — or is it something more complicated?

I’ve described in some detail previously the strategy used to answer these questions, so I refer you to that article.

Generally, I suspect that the July 2012 data won’t be enough (even if you did try to combine it with the 2011 data) to shed clear light on these questions.  But the strong theoretical prejudice on the first question will clearly be that “what’s being seen is a Higgs particle of some type”; after all, the theory of the weak nuclear force requires there be something Higgs-like somewhere, and the Higgs in this mass range was predicted to show up first as a bump in the two-photon search.  I don’t think you’ll find many people who will think it likely that it’s not a Higgs particle, though due diligence will of course be needed to make sure.

The theoretical prejudice on the second question is much weaker, however; we really have no idea whether the Higgs is of Standard Model type or not.  But it seems likely we’ll need the full year’s data set before we start making much progress on this crucial issue (though surprises are possible, if the Higgs is sufficiently different from a Standard Model-type Higgs).  And indeed, even if the Higgs looks somewhat Standard-Model-Higgs-like by the end of the year, our knowledge will still be vague; through Phase 2 of the Higgs search, we’ll be continuing to address the issue for the rest of the decade, making more and more precise measurements of the properties of the Higgs, looking for any deviation from the Standard Model’s predictions.

All of this assumes the rumors are correct.  IF it is true that ATLAS and CMS see something of roughly the same size as last year, in exactly the same place in their plots, then that would mean that evidence for the existence of some type of Higgs particle in the 125 GeV/c² range had firmed up considerably.  And that would be Very Big News.  Well — we’ll find out soon enough.