Scattering light by light at 750 GeV at the LHC

Abstract

We consider the possibility that the diphoton excess at 750 GeV is caused by a new scalar resonance produced in photon fusion. This scenario is parametrized by only one relevant effective couplings and is thus minimal. We show that this setup can reproduce both the production rate and width of the resonance, and is not in conflict with the 8 TeV limits on the diphoton cross section. The scenario also predicts event rates for WW, ZZ, Zγ final states. We suggest for one to perform precision measurements by studying lightby-light scattering with intact protons detected in forward detectors. We construct a simple model that shows that the required couplings can be achieved with new vectorlike, uncolored fermions (with a strong Yukawa coupling to the resonance) which may also account for the required width.

The production of a diphoton resonance via photon-photon fusion

“…We therefore present a precise evaluation of the exclusive γγ luminosity, accounting for all physical effects, including the probability of no additional underlying event activity, or so-called survival factor. For photon-mediated processes, this turns out to be quite close to unity, but it cannot be ignored; the treatment of this latter factor is either absent or approximate in other studies such as [3,4]. The uncertainty is found to be very small, at the percent level, allowing the corresponding exclusive cross section to be accurately predicted.…”

“…[3,4]. While these give a reasonable estimate of the expected rates, at the ∼ 10-50 % level, a more precise calculation may be needed for future higher statistics studies, provided the excess survives.…”

“…One interesting, and in some sense natural possibility, given that this possible excess is only seen so far in the γγ decay channel (and not, for example, in the dijet mass spectrum), is that the resonance may couple dominantly to photons, with the coupling to gluons and other coloured particles being either suppressed or absent entirely. This has been discussed in [3,4] (see also [5][6][7][8][9][10][11][12][13][14][15]), where the γγ induced process in both the usual inclusive and the exclusive modes has been considered. In the latter case the incoming photons are emitted coherently from the colliding protons, which then remain intact after the collision; this will naturally occur in a non-negligible fraction of events.…”

What is the γγ resonance at 750 GeV?

“…Hence, they roughly cancel out in the gain factors r. We will focus mostly on gg and bb induced processes, which represent the extreme cases as they give the minimum and maximum value of C, and also lead to a large gain in parton luminosity going from 8 to 13 TeV, as needed to fit the data. On the other hand, S production from γγ (see also [10][11][12]) is disfavoured by the small value of r γγ , which has a small uncertainty, because partonic photons are dominantly emitted from u quarks, and their pdf evolution is under good theoretical control.…”

“…By inspecting the RG equation and using eq. (3.9) to solve for y f , the constraints from the absence of Landau poles have JHEP03(2016)144 10) with t CUT = ln(Λ CUT /M ) and Λ CUT the scale where the couplings become non-perturbative. Considering that Γ(S → γγ)/M places a lower bound on Y / C f , we see that by increasing the Casimir C f all bounds, besides the one from the Yukawa RG, are either unaffected or made stronger.…”