Physics with tagged forward protons at the LHC

TL;DR

This paper shows that tagging forward protons in exclusive double-diffractive pp → p+M+p processes at the LHC provides a clean environment to study QCD diffractive dynamics and search for New Physics. It develops a perturbative two-gluon (Pomeron) framework where the bare amplitude uses skewed unintegrated gluon densities with Sudakov suppression, and rescattering is treated with a two-channel eikonal model to predict the survival factor $\langle S^2\rangle$ and azimuthal correlations. The authors find that soft rescattering generates rich $p_{t}$ and $\phi$-dependent structures, highly sensitive to the form of the QCD Pomeron–proton vertex $\beta(t)$, and they apply the formalism to double-diffractive meson production, arguing that Pomeron–Pomeron fusion preferentially produces non-$q\bar{q}$ states (e.g., glueballs) with distinctive $P_T$ and $\phi$ patterns that qualitatively align with WA102 data. The work provides a concrete, testable framework for probing proton opacity and soft rescattering and for filtering exotic resonances through forward proton tagging at the LHC.

Abstract

We emphasize the importance of tagging the outgoing forward protons to sharpen the predictions for New Physics at the LHC (such as the diffractive production of a light Higgs boson). The rescattering effects lead to a rich distinctive structure of the cross section as a function of the transverse momenta of the protons. We show that a study of the correlations between the proton transverse momenta for double-diffractive production of central dijets will provide a detailed check of the whole diffractive formalism. Adopting a perturbative two-gluon structure of the Pomeron, we emphasize that 2++ quarkonium production, via Pomeron-Pomeron fusion, is strongly suppressed. This offers a favourable production mechanism for non-qqbar states, such as glueballs.

Figures (3)

• Figure 1: The bare amplitude $A^{(a)}$ and the rescattering correction $A^{(b)}$ for the double-diffractive process $pp \rightarrow p+M+p$.
• Figure 2: The factor $F(\vec{p}_{1t},\vec{p}_{2t})$ of (\ref{['eq:a22']}) and (\ref{['eq:a24']}), which specifies the forward going proton transverse momentum dependence of the $pp \rightarrow p+M+p$ cross section, for typical values of $p_{1t},p_{2t}$ and the azimuthal angle $\phi$. The first three plots correspond to $p_{2t}=0.2,0.4,0.7\:{\rm GeV}$ respectively, and show the results obtained using the KMR two-channel eikonal model of Ref. KMRsoft to calculate the soft rescattering, as described in Section 4. The dashed curves show the sensitivity to the form of the QCD Pomeron-proton vertex $\beta(t)$, by replacing the dipole form (\ref{['eq:a20']}) by the exponential form (\ref{['eq:a21']}). The dotted curves in Fig.(d) correspond to the use of a naı̈ve single-channel eikonal model (with $A^{(b)}$ computed from (\ref{['eq:a8']}) and (\ref{['eq:a9']})) as compared to that obtained with the 'realistic' two-channel eikonal model of Ref. KMRsoft; in both cases the exponential form factor was used, so the dashed curves are the same in plots (d) and (b).
• Figure 3: The dependence of the survival probability, $S^2$, of the rapidity gaps on the azimuthal angle $\phi$ between the transverse momenta $\vec{p}_{it}$ of the forward going protons in the process $pp \rightarrow p+M+p$, for typical values of $p_{1t}$ and $p_{2t}$.