First Search for Exclusive Diphoton Production at High Mass with Tagged Protons in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

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 $9.4{\mathrm{fb}}^{-1}$ 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 match 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% confidence level are $|{\zeta }_1|<2.9\times {10}^{-13}{\mathrm{GeV}}^{-4}$ and $|{\zeta }_2|<6.0\times {10}^{-13}{\mathrm{GeV}}^{-4}$.

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 axionlike resonance that decays into two photons.

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 nonelastic region used in the background yield correction, thus with a data-to-prediction ratio constrained to unity. Shown also are the theoretical contributions to the LBL signal from the SM and from an anomalous source whose yield at the parameters given is close to the sensitivity reached in this study.

Figure 3

Invariant mass distribution of the diphoton pairs for the elastic selection region with events satisfying $1-|\mathrm{\Delta }{\varphi }_{\gamma \gamma }/\pi |<0.005$ 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.

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 ${\zeta }_2$.