The photon PDF in events with rapidity gaps

TL;DR

This work develops a formalism to include experimental rapidity-gap vetoes in photon-initiated processes by constructing a modified photon PDF with a gap-aware evolution and a Sudakov factor. It combines this with a soft survival model to account for additional hadronic interactions, yielding a realistic γγ luminosity and cross sections for lepton and W-boson pair production at the LHC. The authors provide detailed predictions at 13 TeV, analyze the relative importance of coherent, incoherent, and evolution components, and validate the approach against CMS semi-exclusive μ⁺μ⁻ data, showing that a predominantly coherent input is favored. The framework demonstrates substantial suppression due to gap vetoes and survival, offers testable predictions for future measurements, and suggests that gap-sensitive observables can constrain the photon PDF and soft QCD dynamics.

Abstract

We consider photon-initiated events with large rapidity gaps in proton-proton collisions, where one or both protons may break up. We formulate a modified photon PDF that accounts for the specific experimental rapidity gap veto, and demonstrate how the soft survival probability for these gaps may be implemented consistently. Finally, we present some phenomenological results for the two-photon induced production of lepton and $W$ boson pairs.

Figures (7)

• Figure 1: Schematic diagram corresponding to the diffractive topology described in text, where a quark of rapidity $y_q$ is emitted beyond the edge of a LRG region.
• Figure 2: The photon PDF $x\gamma(x,\mu^2)$ subject to the rapidity gap constraint (\ref{['dif']}), for different values of $\delta$ and for $\mu^2=200, \, 10^4\,{\rm GeV}^2$, with the usual inclusive PDF shown for comparison.
• Figure 3: As in Fig. \ref{['fig:split']}, but with the NNPDF2.3QED Ball:2013hta set taken for the input PDF at $Q_0^2=2\,{\rm GeV}^2$. The 68% confidence error bands are shown in the inclusive case.
• Figure 4: Feynman diagrams for (a) bare and (b) screened amplitudes for coherent photon--induced lepton pair production
• Figure 5: Schematic Feynman diagrams indicating how screening effects may be included in the case of semi--exclusive photon--induced lepton pair production. The vertical dashed lines in the lower plots indicate the amplitude and the complex conjugate on the left and right hand side, respectively, while the remaining two diagrams with the pomeron exchange included in the conjugate amplitude and in both amplitude and the conjugate are not shown.