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We evaluate the cross section for diffractive bremsstrahlung of a singlephoton in the $pp \to pp \gamma$ reaction at high energies and at forwardphoton rapidities. Several differential distributions, for instance, in $y$,$k_{\perp}$ and $\omega$, the rapidity, the absolute value of the transversemomentum, and the energy of the photon, respectively, are presented. We comparethe results for our standard approach, based on QFT and the tensor-pomeronmodel, with two versions of soft-photon-approximations, SPA1 and SPA2, wherethe radiative amplitudes contain only the leading terms proportional to$\omega^{-1}$. The SPA1, which does not have the correct energy-momentumrelations, performs surprisingly well in the kinematic range considered. Wediscuss also azimuthal correlations between outgoing particles. The azimuthaldistributions are not isotropic and are different for our standard model andSPAs. We discuss also the possibility of a measurement oftwo-photon-bremsstrahlung in the $pp \to pp \gamma \gamma$ reaction. In ourcalculations we impose a cut on the relative energy loss ($0.02 < \xi_{i} <0.1$, $i = 1,2$) of the protons where measurements by the ATLAS Forward Proton(AFP) detectors are possible. The AFP requirement for both diffractivelyscattered protons and one forward photon (measured at LHCf) reduces the crosssection for $p p \to p p \gamma$ almost to zero. On the other hand, much lesscross-section reduction occurs for $pp \to pp \gamma \gamma$ when photons areemitted in opposite sides of the ATLAS interaction point and can be measured bytwo different arms of LHCf. For the SPA1 ansatz we find $\sigma(pp \to pp\gamma \gamma) \simeq 0.03$ nb at $\sqrt{s} = 13$ TeV and with the cuts $0.02 <\xi_{i} < 0.1$, $8.5 < y_{3} < 9$, $-9 < y_{4} < -8.5$. Our predictions can beverified by ATLAS and LHCf combined experiments.

physics letters b

Exclusive diffractive bremsstrahlung of one and two photons at forward  rapidities: Possibilities for experimental studies in $pp$ collisions at the  LHC