Saturday, December 12, 2015

Can the \(750\GeV\) gamma-gamma bump be real?

Two hours before the seminar, the auditorium was already half-full. By 2:40, it was full. The web chat room for the event was here – click and participate. (Find the URL in the HTML source.)

A Dutch variation of this blog post is available.

Astrophysicist Mario Livio has joined the community of people who spread the rumor about a bump that will be presented on Tuesday between 3 pm and 5 pm:
Rumor from #LHC @CERN potential detection of excess at \(700\GeV\) [most recently sometimes said to be \(750\GeV\)] decaying into 2 photons (3 sigma in both @ATLASexperiment & @CMSexperiment)

BTW the schedule for Tuesday December 15th:
............ ............
15:00-15:40 CMS, Jim Olsen (Princeton U., USA)
15:40-16:20 ATLAS, Marumi Kado (Lab. de l'Acc. Lin., FR)
I think that by now, everyone who wanted to know something about the 2015 results has heard or seen this rumor so let me officially declassify it.

All this knowledge could have come from a single source and the source may hypothetically be a prankster. But I have some feelings that the sources are actually numerous and independent at the root. So I think it's more likely than not that an excess of this kind will be reported. There may be other interesting (and maybe more interesting) excesses announced on Tuesday but I won't discuss this possibility in this blog post at all.

Exactly four years ago, we were shown pictures such as this one. When you collide proton beams and look for final states that include two photons (or "diphotons", as physicists like to call it when they want to pretend that they speak Greek), you may find a 3-sigmaish bump around \(125\GeV\) – which is the value of the invariant mass \((p_1^\mu+p_2^\mu)^2\) of the two photons – already with the data that was available by the end of 2011.

The Higgs boson had to be somewhere on the mass axis and the remaining values had been largely excluded so it was pretty much unavoidable that this excess did mean that the Higgs boson existed and had the mass around \(125\GeV\) – we were more likely to call the mass \(126\GeV\) at least throughout 2012 but the most accurate values are close to \(125.1\GeV\) now. Additional channels strengthened in 2012. The Higgs was seen to decay to \(ZZ^*\), a pair of massive electroweak gauge bosons (one of them must be virtual because they're too heavy), and some lepton and quark pairs later.

Now, on Tuesday, we are likely to be told that there is a very similar \(\gamma\gamma\) excess near the invariant mass of \(700\GeV\) – which happens to be 4 times the top quark mass, if you haven't noticed. There is a 3-sigma excess in the ATLAS data and a similar 3-sigma excess in the CMS data. By the Pythagorean rule, the "combined" excess has the significance of \[

\sqrt{3^2 + 3^2} = \sqrt{18} \approx 4.24.

\] Even the combination is insufficient to be a discovery but the excess is pretty strong. But if your prior probability that a "similar particle" should exist is low, 4.2 sigma is a pretty weak piece of evidence even though it's formally just a "1 in 100,000" risk that it's a fluke. Numerous 4-sigma signals have gone away.

In fact, there were apparently even 4-sigma bumps in diphotons that have gone away. Look at this April 2011 blog post about an ATLAS diphoton memo. At that time, it suggested that the Higgs boson could have existed and have the mass of \(115\GeV\). It was the most convincing value of the mass at that moment, I think – but you may also verify that I have never made any statements about being "comparably certain" about the \(115\GeV\) Higgs as I made about the \(125\GeV\) Higgs in December.


The somewhat lighter \(115\GeV\) Higgs would have meant a more direct support for some of the most popular models with supersymmetry "right around the corner". Consequently, I think that it is probably not a coincidence that a Finnish company named Darkglass Electronics introduced a compressor called Supersymmetry \(115\GeV\) a bit later. By a compressor, they probably mean some gadget (a pedal!) for electric guitars but I am really confused what the product actually does LOL. They must have followed particle physics – or know someone who does. Or do you seriously believe that someone could have chosen this bizarre name that so accurately picks the excitement of a light Higgs in Spring 2011?

Too bad. If the Higgs were found at \(115\GeV\), I am sure that the pedal would be a huge bestseller. ;-)

Note that \(115\GeV\) was an intriguing value of the mass also because a decade earlier, the LEP collider saw a very weak hint of a Higgs boson at that mass – which was coincidentally the maximum mass that LEP was able to probe. At any rate, we know that the \(115\GeV\) hints went away (the same was true for hints around \(140\)-\(145\GeV\) from the Tevatron etc.) and a new bump ultimately began to emerge around \(125\GeV\) later in 2011 and it was declared a discovery on July 4th, 2012.

When the ATLAS and CMS see a very similar bump in the same channel, it may turn out to be a fluke – much like the \(115\GeV\) bump etc. In that case, it's not too interesting. Flukes have always taken place and they will never be banned. However, the signals may also be real – after all, 4.2 sigma isn't quite negligible evidence.

Because the particle decays to two photons, it must be a boson. I think that the simplest guess would be that it's a new Higgs boson. Supersymmetric models predict that there exist at least five faces of the God particle. A new gauge boson would marginally increase the probability that supersymmetry is right – \(700\GeV\) is an allowed mass, of course. But this new particle wouldn't be a superpartner yet so I wouldn't say that "supersymmetry will have been discovered" as soon as this new scalar boson were proven to exist.

However, a new Higgs boson would surely be exciting, anyway. We would enter the epoch "Beyond the Standard Model". It would mean that despite all the talk about the Standard Model's nearly eternal validity and the nicknames The Core Theory and similar ideology, the completed Standard Model (with all parameters basically known) will have only been valid for some modest four years! ;-) This completely real possibility is the right perspective from which you should evaluate the question whether the modest, technical, seemingly temporary name "The Standard Model" is appropriate for the particular quantum field theory. It surely is appropriate because we are aware of no good reasons why such a theory couldn't fall very soon.

There is an extra problem about any new particle of mass \(700\GeV\) that decays to two photons. What is the extra problem? The extra problem is that this is a pretty low mass and at these low masses, the increase of the LHC energy from \(8\TeV\) to \(13\TeV\) doesn't substantially improve the visibility of the new particle. So a new \(700\GeV\) particle visible in the 4/fb of the 2015 data should have been visible in the 20/fb of the 2012 data, too! But no one saw a big (or any) bump near \(700\GeV\) in the 2012 data, I think, although I am simply unable to find good diphoton graphs from the 2012 data that go this high.

This ATLAS diphoton graph only goes to \(600\GeV\), with an over-two-sigma bump around \(530\GeV\).

The CMS graph goes up to \(3.5\TeV\) and is very chaotic around \(700\GeV\) or so. For a cleaner CMS paper, see the comments. Update: on Dec 15th in the morning, I found a TRF blog post and CMS paper in it that discuss an \(8\TeV\) excesses of 2.56 and 2.64 sigma between \(700\) and \(800\GeV\) on page 15.

If they are going to announce a bump in the simple \(\gamma\gamma\) channel, it will "probably" look like a pair of flukes due to the tension with the 2012 data. But the channel may be a more complicated one – with other particles produced simultaneously with the diphoton. If that's so, the process that creates the \(700\GeV\) may involve some heavier particles for which the increase from \(8\TeV\) to \(13\TeV\) is much more useful. Some multi-\({\rm TeV}\) particle (which is easy to produce now but was very hard in 2012) may decay to a product that decays further – we have decay chains or cascades etc.

The new particles that exist before the photons may belong e.g. to hidden valleys. What are hidden valleys? Matt Strassler has given this definition:
A unexpected place …
… of beauty and abundance …
… discovered only
after a long climb …
Some of the TRF readers may say: Could you please be a bit more specific about the steps 1-4, Dr Strassler? ;-) More seriously, the definition's being vague is pretty much a point of the hidden valleys. Hidden valleys are any collections (hidden sectors) of new particles and fields influencing experiments at accessible energies that look pretty useless at the beginning and that only play role in the middle of some decay chains. Something more important stands at the beginning of the decay chains; but the chains always end with the Standard Model and not new particles.

To some extent, to focus on hidden valleys means to abandon the minimality – or Occam's razor, if you wish. I would say that minimality is overrated but it's still more sensible to spend more time with models that look simpler than with the contrived ones. One simply shouldn't try to construct completely arbitrary Rube Goldberg machines and test whether they're the right theories of Nature.

On Tuesday, they may announce something much more convoluted than just the "diphoton final state". Perhaps the events will suggest that there is a heavier particle such as the \(2\TeV\) \(W'\)-boson – or a superpartner, if you want to be really ambitious – that decays into some decay products including a new \(700\GeV\) Higgs which decays into two photons. I believe that the number of possibilities is too high here and I won't be able to pick a reasonable candidate of the sort.

Even if these two 3-sigma excesses were the only result in tension with the Standard Model on Tuesday, it should be a very good idea to watch the 2-hour talks carefully.

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