Saturday, April 23, 2016

Excitement over the 750 GeV diphoton anomaly

From PRL Synopsis: Explaining a 750 GeV Bump
Fig 1 from Petersson and Torre.
An item in Nature brings to our attention excitement in the particle phyisics world with four possible interpretations of an apparent anomaly in production of photon pairs from proton-proton collisions.  This is covered in detail in the April 16 issues of Physical Review Letters. Alas I don't subscribe to this so can only see the papers in Arxiv pre-prints.  But there is a handy synopis here.

I say apparent anomaly because at present these excess photon pairs are only significant at about 3 standard deviations  (3.9 and 2.6 depending on the experiment) and when you are looking for anomalies in enormous amounts of data you have to be cautious. But if they are real then additional data will settle the matter and it's interesting to see what the theorists come up with.

The point is that, if they are confirmed, this would be the first time that CERN has detected something that is not predicted by the Standard Model.  The theorists models also suggest potentially fruitful areas to look for new data that are within the scope of CERN's current experiements. 

For example Nakai et al in "Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton" discuss "a scenario that a pseudo-Nambu-Goldstone (pNG) boson of a new QCD-like theory is produced by gluon fusion and decays into a pair of the standard model gauge bosons. Despite the strong dynamics, the production cross section and the decay widths are determined by anomaly matching condition. The excess can be explained by the pNG boson with mass of around 750 GeV. The model also predicts exotic hadrons such as a color octet scalar and baryons which are within the reach of the LHC experiment."

I am of course in no position to judge the validity of the competing interpretations. I'm pleased to note that a number of them reference Jeffrrey Goldstone's seminal work since Goldstone taught me quantum physics when I was an undergraduate at Trinity. I remember vividly bumping into him in the College Library when I returned to look something up, and he was back from MIT, with an astonished but friendly "what are you doing here?"

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