CMS-EXO-18-014

CMS-EXO-18-014 ; CERN-EP-2021-191
First search for exclusive diphoton production at high mass with tagged protons in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS and TOTEM Collaborations
12 October 2021
Phys. Rev. Lett. 129 (2022) 011801
Abstract: A search for exclusive two-photon production via photon exchange in proton-proton collisions, ${\mathrm{p}}{\mathrm{p}}\to{\mathrm{p}}\gamma\gamma{\mathrm{p}}$ with intact protons, is presented. The data correspond to an integrated luminosity of 9.4 fb$^{-1}$ collected in 2016 using the CMS and TOTEM detectors at a center-of-mass energy of 13 TeV at the LHC. Events are selected with a diphoton invariant mass above 350 GeV and with both protons intact in the final state, to reduce backgrounds from strong interactions. The events of interest are those where the invariant mass and rapidity calculated from the momentum losses of the forward-moving protons matches the mass and rapidity of the central, two-photon system. No events are found that satisfy this condition. Interpreting this result in an effective dimension-8 extension of the standard model, the first limits are set on the two anomalous four-photon coupling parameters. If the other parameter is constrained to its standard model value, the limits at 95% CL are $ \| \zeta_1 \| < $ 2.88 $ \times $ 10$^{-13}$ GeV$^{-4}$ and $ \| \zeta_2 \| < $ 6.02 $\times$ 10$^{-13}$ GeV$^{-4}$.
Links: e-print arXiv:2110.05916 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	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 (box) of charged fermions or bosons. 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 process producing a scalar axion-like resonance that decays into two photons.
png pdf	Figure 2: Numbers of simulated and observed events for the various selection regions described in the text. The shaded bands show the statistical uncertainties in the simulated backgrounds added in quadrature. All selection regions are sequential from left to right, with the exception of the inclusive region used in the backgrounds yield correction, thus with a data-to-prediction ratio constrained to unity. The signal 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, for data (dots) and MC simulations (histograms). The hatched bands indicate the statistical uncertainties in simulated samples added in quadrature.
png pdf	Figure 4: Two-dimensional limits on the anomalous four-photon couplings, derived from the observed upper limit on the diphoton production cross section. The shaded area depicts the excluded values of the coupling parameters $\zeta _1$ and $\zeta _2$.

Tables
png pdf	Table 1: Description of the selection stages applied on the photon and proton kinematic variables, and used in this study.

Summary

To summarize, the CMS-TOTEM precision proton spectrometer has proven the feasibility of continuously operating a near-beam proton spectrometer at a high-luminosity hadron collider. The first search for the ${\mathrm{p}}{\mathrm{p}}\to{\mathrm{p}}\gamma\gamma{\mathrm{p}}$ process with forward proton tags is presented. The search uses an integrated luminosity of 9.4 fb$^{-1}$ of proton-proton collisions collected at a 13 TeV center-of-mass energy at the LHC during 2016. No events are observed with a pair of proton tracks compatible with the diphoton kinematic properties with an expected background of 0.23 and 0.43 events for the 2 and 3 standard deviations windows, respectively. This provides the first limit for the standard model light-by-light production cross section at a scale of hundreds of GeV, and places limits on anomalous couplings for the four-photon interaction based on an effective field theory extension of the standard model.

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