## Friday, April 22, 2011

### ATLAS memo: 4-sigma diphoton bump at LEP's 115 GeV

An expectedly light Higgs boson could be showing up unexpectedly early at the LHC
Key update: After analyzing a higher amount of data, the 115 GeV Higgs signal has gone away (click)
Jester at Resonaances discusses a leaked internal memo of the ATLAS Collaboration at the LHC. See also Dorigo, Gibbs, Woit, Francis, Kavassalis, Wired, New Scientist, MSNBC.

If the internal note is authentic, which is extremely likely right now (but of course, it is not an official ATLAS document at this point!), and if the authors avoided silly mistakes and wishful thinking, which is not guaranteed (and it is particularly doubtful for those who doubted the 115 GeV LEP claims, because both claims involve Ms Sau Lan Wu - see her 2002 paper), ATLAS has analyzed 63.5/pb of data from 2010 and 2011 and it has observed a γγ resonance (excess of events with two photons) with a significance of 4 standard deviations - i.e. at the 99.994% confidence level.

The total invariant mass of the would-be particle is 115 GeV - the mass of the hypothetical Higgs boson that, according to ancient Greek legends, the LEP collider in the very same tunnel could have discovered a few weeks before it was shut down a decade ago.

There is a problem, however. If the excess were indeed due to the decaying Higgs bosons, theory predicts that it shouldn't have been seen yet! In fact, the observed cross section (probability of the production of the resonance) is about 30 times greater than the prediction from the Standard Model, assuming that the resonance is the Standard Model Higgs Boson, also known as the Weinberg toilet.

The Les Horribles Cernettes (LHC) girls perform their love song, Collider. However, the collider around them is not the LHC but LEP, the previous accelerator in the same tunnel that found hints of a 115 GeV Higgs boson right before it was shut down in 2000.

Various models beyond the Standard Model typically increase the discrepancy 30 to an even larger ratio. Some variations of NMSSM - the Next to Minimal Supersymmetric Standard Model - may decrease the multiplicative discrepancy from 30 to a few - by a factor of ten or more - but it may still way too large. See e.g. Ellwanger 2010 for a very suggestive method to increase the diphoton rate in NMSSM by a factor of six.

Of course, it's entirely plausible that the physicists have missed a possibility that the Higgs is at 115 GeV but because of a subtle feature of the particle, such as compositeness (which I don't like but consider it the most obvious example) or extra matter fields beyond the three generations we know, the diphoton decays are far more often than the Standard Model predicts.

Note that according to the graph above, at 115 GeV, only 1/500 of the decays (the fraction is called the branching ratio) of the Weinberg toilet end up with two photons. The two dominant decays are to bottom quark-antiquark pairs and WW pairs, and even decays to tau-antitau, ZZ, and charm-anticharm are far more frequent than the photon pairs.

ATLAS may soon be nicknamed Telemachus. Music by Don Garbutt

But because of some unexpected subtleties, Nature may ignore the Standard Model and prefer the decay to photon pairs. After all, the Next to Minimal Supersymmetric Standard Model doesn't have to be the last word. Recall that your neighbor who lives next to you doesn't have to be your best friend; it's often the next-to-next guy who is. ;-)

If you forget about the puzzles about the unexpectedly intense diphoton decays of the new would-be particle, the very mass of the particle would be perfectly consistent with supersymmetry, bringing the embarrassing doomsday closer to the anti-supersymmetric bigots of this world.

115 GeV is primarily the value of the Higgs mass that is overwhelmingly favored by supersymmetry. This point was conveyed extremely convincingly by Cassel et al. in 2010 who showed that the degree of required fine-tuning is dramatically minimized for a 115 GeV Higgs. In their statistical sense, a 115 GeV Higgs boson is the most robust and most accurate prediction of supersymmetry that we can make at the present state of knowledge. And it may be confirmed soon.

I guess that many people are going to think about the ways how to modify the existing models in order to predict much higher diphoton cross sections.

Your humble correspondent has predicted 115 GeV to be the most likely Higgs mass in dozens of contexts, so this blog is full of comments about a 115 GeV Higgs. In particular, I recommend you to reread
What a light Higgs boson would mean for particle physics (July 2010)
If the Higgs discovery is confirmed - and 5 sigma is already collected by now if those 4 sigma were real - it will be exciting, indeed. Theorists will have to struggle to explain the anomalously high diphoton branching ratio. But when those detailed complications are resolve, the LHC will have to work hard to discover the expected particles that really matter:

Superpartners at the LHC, by Don Garbutt