A few thoughts about the 750 GeV diphoton bump

The big news in particle physics during the winter 2015-2016 is certainly the excess found by the ATLAS and CMS Collaborations in the invariant mass of two high-energetic photons in the data acquired in 2015 at a centre-of-mass energy of 13 TeV. If you don’t know what I’m talking about, then it’s time to catch up: the Resonaances blog and Boston Review offer two nice summaries.

The main questions we-as-physicists want to answer are: is this real? if so, what it is? As an experimentalist, I should try to answer the one, but the truth is: we don’t know yet and we need more data. However, in the latest update given at the Moriond conference, the statistical significance of this excess-over-background increased a little, as one would naïvely expect if this was due to a real particle. Many colleagues are arguing that, based on basic statistics arguments, we are in principle already able to claim an evidence, if not a discovery – if you sum in quadrature the local significances found by the two separate experiments, the figure is √(3.4^2 + 3.9^2) = 5.2 standard deviations, thus higher than the golden standard of 5σ. I can’t deny I’m excited about this, but I know that extraordinary claims require extraordinary evidence. If this is real, it would be so big that the discovery of the Higgs boson would pale in comparison. It would be comparable to the discovery of the constituents of the proton (quarks and gluon) or of the muon (who ordered that?).

So what’s the story? Albert Einstein once said that subtle is the Lord, malicious he’s not. Or is He? If we are producing this particle via the gluon-gluon fusion (by far the most likely process in proton-proton interactions), then it should also mostly decay to a pair of gluons, which in turn originate two particle jets. But the problem is that both experiments are almost blinded by light (sorry for the pun) in that mass region because of the super-high production rate of the di-jet process. Moreover, in run1, the di-photon analyses focused on a lower or a higher mass regions, almost overlooking this intermediate range. Now that we know that something might be there, with hindsight both ATLAS and CMS have looked better in that region, and in fact they have found some more events – ATLAS has in fact a 2σ peak in 2012 data, which could be just a fluctuation, or actually a positive fluctuation of this process.

I’m an experimentalist, but I’ve always been fascinated by theory. Even though I’m certainly not going to write any serious hep-ph paper about the interpretation of the 750 GeV excess, I would like to share with my readers two wild ideas. Who knows, maybe there is some truth in my craziness (or just ignorance):

  •  We know that there is a U(1) interaction (electromagnetism), a SU(2) interaction (weak force), a SU(3) interaction (strong/colour force)..how about SU(4)? Is there any argument against the existence of a strongly coupled SU(4) interaction, not necessarily embedding the already-known forces? What would such a force look like?
  • My favourite scenario remains the spin-2 interpretation, although I understand that the non-observation of a similar excess in the di-lepton channel strongly contains this possibility. Anyway, what I’ve asked to some colleagues (without getting any answer so far) is whether it would be possible to have a Brout-Englert-Higgs (BEH) mechanism for spin-2 particles. In my head, the graviton being massless would play the role of the photon, and this allegedly new particle may be a massive spin-2 relative of the Z boson (I assume it’s charge-neutral). Given that a tensor has more degrees of freedom than a vector, I imagine that there may be other states associated with the these two. Would such a theory non-renormalizable? Then what: is renormalization an absolutely necessity, or is it just a mathematical trick? And if such a theory would not yet be the ultimate one, but just an intermediate step, should we really care about normalization?
    Even if quite unrelated to this idea, it seems that this spin-2 BEH symmetry breaking resonated somehow in the heads of three famous theorists (arXiv:1602.07993). In their scenario, the Higgs boson “triggers” the appearance of gravity, or alternatively, at very high energies the change in the shape of the Higgs potential “turns off” gravity at once.

If you are interested in this saga, stay tuned and expect some update by this summer!

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