Friday, September 22, 2017

CMS: a locally 2.8-sigma diphoton excess at \(95\GeV\)

Finally, a paper from the LHC shows some interesting small deviation from the Standard Model again. The CMS collaboration published their
Search for new resonances in the diphoton final state in the mass range between \(70\) and \(110\GeV\) in \(pp\) collisions at \(\sqrt{s}= 8\) and \(13\TeV\)
and the key graph is seen on page 16.

In the 20/fb of the \(8\TeV\) data from 2012, they locally saw a 2.0 sigma excess near \(m_H=97.6\GeV\). In the larger and stronger 2015-2016 dataset of 36/fb of the \(13\TeV\) data, they locally see a 2.9 sigma excess at \(m_H=95.3\GeV\) which is close enough to the 2012 excess to allow you to suggest that these modest excesses speak the same language.

The \(13\TeV\) contributions are vastly stronger so even in combination, the optimum mass is \(m_H=95.3\GeV\) in the combination and the local significance of the excess is 2.8 sigma.

That's not to much but for many months, we couldn't even see a deviation of this modest size. The LHC proton-proton collisions have been incredibly obedient and submissive. I am sure that almost all the protons will vote for Angela Merkel on Sunday, too.

Note that the mass \(m_H\) above is the invariant mass constructed from the momenta of the two photons in the final state. It's normal for the Z-boson of mass \(91\GeV\) and the Higgs boson of mass \(125\GeV\) to decay to two photons. But this excess at \(95\GeV\) or \(96\GeV\) is somewhere in the middle, although much closer to the Z-boson.

Recall that it's not the first excess near the Z-boson mass. ATLAS, the other major collaboration, has observed an on-Z excess in the dilepton channel, see e.g. this pheno paper. Well, could another massive boson wait for us over there, in this mass region we thought we knew?

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