CMS-PAS-EXO-18-014

CMS-PAS-EXO-18-014
First search for exclusive diphoton production at high mass with intact protons in proton-proton collisions at $\sqrt{s} = $ 13 TeV at the LHC
CMS Collaboration
July 2020

Abstract: A search for exclusive two-photon production via photon exchange in proton-proton collisions, $pp\to p\gamma\gamma p$ with intact protons, is presented. The data correspond to an integrated luminosity of 9.4 fb$^{-1}$ collected in 2016 at a center-of-mass energy of 13 TeV under LHC standard luminosity conditions. Events are selected with a diphoton invariant mass above 350 GeV and with both protons intact in the final state to reduce dominant backgrounds from strong interactions. The events of interest correspond to those where the momentum loss observed for the intact, forward-moving protons equals the invariant mass deduced for the central, two photon system. No events are found that satisfy this condition. Using an effective dimension-8 extension of the standard model, this result allows the first limits to be set on anomalous four-photon coupling parameters, with $\|\zeta_1\| < 3.7\times10^{-13} $ GeV$^{-4}$ and $\|\zeta_2\| < 7.7\times10^{-13} $ GeV$^{-4}$.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The process for diphoton production via photon exchange with intact protons in the final state. Several couplings may enter the four-photon shaded area such as a loop of a fermion or a charged boson. The model can be extended with intermediate interactions of new physics objects, such as a loop of a heavy charged particle or an $s$-channel exchange.
png pdf	Figure 2: Number of simulated and observed events for the various selection regions described in the text. The shaded bands show the statistical uncertainty on the MCs added in quadrature. All selection regions are sequential from left to right except for the inclusive region which is a background control region. The signal selection region is denoted as "Tight $\xi _{\gamma \gamma}^{\pm}$''.
png pdf	Figure 3: Invariant mass distribution of the diphoton pairs for the elastic selection region with events satisfying $1-\|\Delta \phi _{\gamma \gamma}/\pi \| < 0.005$ as described in the text. The hatched bands indicate the statistical uncertainty on simulated samples added in quadrature.
png pdf	Figure 4: Two-dimensional limits on anomalous quartic gauge coupling diphoton production mechanisms given the upper limit observed on the light-by-light production cross section. The limits are shown in terms of the coupling parameters $\zeta _1$ and $\zeta _2$.

Summary

To summarize, in its 2016 operation the CMS-TOTEM precision proton spectrometer (CT-PPS) has proven the feasibility of continuously operating a near-beam proton spectrometer at a high-luminosity hadron collider. For the first time the $\gamma\gamma\to\gamma\gamma$ process was sought with the requirement of forward proton tags, using 9.4 fb$^{-1}$of luminosity collected at a 13 TeV center-of- mass energy at the LHC. No events were observed with a pair of proton tracks compatible with the diphoton kinematics, above a background prediction of 0.23 and 0.43 events for the 2$\sigma$ and 3$\sigma$ windows, respectively. This provides the first limit at the electroweak scale for the SM production cross section, and places limits on anomalous couplings for the four-photon interaction based on an effective field theory extension of the SM.

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