What to make of the HCP ATLAS and CMS Higgs results

Kyoto, where HCP was held. Via Wikipedia.

The HCP conference that ended about a week ago turned out to be a bit disappointing/unsettling. Just before I start, let me say that all that follows here is my own opinion: it has absolutely nothing to do with the stance of the ATLAS collaboration on anything. Of course, being part of the collaboration myself, I do have inside information. But I assure you that what is being written here could easily have been written by someone without that information. This is the only thing that I can and should do, because any finding that hasn’t made it outside the collaboration hasn’t been thoroughly checked yet. I cannot be certain of anything that hasn’t passed the rigorous verification of our editorial board, just like someone outside the collaboration.

In any case, here’s what went down at HCP concerning the Higgs boson. Both ATLAS and CMS did present new results for one specific mode of decay of the Higgs boson: Higgs to two taus. ATLAS didn’t manage to update their own ditau result back in July, but CMS did. Interestingly, CMS had a change of heart for their ditau analysis: they changed the method and ended up with very little improvement in sensitivity with respect to their July result, despite several technical improvements and some more data. They haven’t been very open on exactly why that is, and the credibility of their result from July have been called into question.

The new ATLAS result presented at HCP is quite comparable to the new CMS result for the ditau search. It is just a little bit less sensitive. Both experiments are not able to exclude the existence of the ditau decay for the new Higgs-like particle, but they are not able to confirm it exists either despite seeing a small excess.

I have to admit to a bit of schadenfreude towards the situation. I’m particularly happy to know that ATLAS isn’t so far behind in the search for pairs of taus coming from Higgs bosons. We can catch up with CMS for the next conference.

I may have made that point before, but observing a new decay mode for the new Higgs-like particle is a big deal. That means we have a new handle to characterize it, and we can learn more about its interactions with other particles. This is a crucial step in establishing if the particle actually is the Higgs boson.

Now, there are three decay modes that have been observed: two photons, two Z bosons and two W bosons. The diphoton result leads the sensitivity and the mass measurement in both collaborations. The ZZ result also provides a good mass resolution, but that decay mode is more rare. The WW decay mode doesn’t have a good mass resolution because of neutrinos in the decays of the W bosons. It is however, quite abundant and can provide polarization measurements. I should mention that the simplest analyses are the diphoton search and the ZZ search, especially when both Z decay to pairs of light leptons. By light leptons, I mean electrons and muons. Electrons and muons are really easy to reconstruct in the detector (compared to other objects like taus for example), so ZZ to four light leptons is the easiest way to look for that particular decay mode.

What is a bit weird is that neither ATLAS or CMS have updated their diphoton result, and only CMS updated their ZZ result. Given that these are the easiest analyses to update, why haven’t they been updated? There are several possible answers. The first is that the groups doing these analyses have made a radical change in methodology and they haven’t manage to iron out everything on time. It is also possible that they see something suspicious that they can’t quite explain. The first case would probably mean that the next results for these analyses are going to see strong improvement in sensitivity, while the second case may result in either a very exciting outcome or a very boring one.

This is very little to go on though. I don’t want to start rumours. It seems that every time the Internet gets excited about a new scientific announcement, the story ends up in harsh disappointment. Remember the arsenic-based lifeforms? Or the faster-than-light neutrinos? And now there’s the Curiosity rumour? Curb your enthusiasm Internet. I know you are quite young, but you will grow to be a very bitter adult if you don’t stop reading too much into things.

Of course, I really should mention, if only briefly, that there was one very exciting result presented at HCP. LHCb observed a really rare b-meson decay for the first time, and it turns out that this decay is produced just the right amount to be compatible with the Standard Model and incompatible with supersymmetry. This is a big deal, and I will try to talk about it more in a future post.

I know that some people think that the most exciting part is over with the new particle being discovered at the LHC. But what I am trying to say in this post is that it is truly only the beginning. It is worth staying excited about it. We are learning about the most fundamental aspects of the Universe, and no one on Earth before us has ever been able to claim to know these things. Not getting too excited and staying rational when finding unexpected things is part of the process that led us here. It is only then that we can assert the reality of these unexpected things. Let’s keep it up, and let’s try to make it a more common human trait.

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