First search for exclusive diphoton production at high mass with tagged protons in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for exclusive two-photon production via photon exchange in proton-proton collisions,
pp → pγγp 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 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 jζ1j < 2.9 × 10−13 GeV−4 and jζ2j < 6.0 × 10−13 GeV−4.