Central exclusive meson pair production in the perturbative regime at hadron colliders

TL;DR

This work develops and applies a perturbative QCD framework for central exclusive meson-pair production at hadron colliders, focusing on the gg -> M Mbar subprocess under the $J_z=0$ selection rule. By computing both flavour-nonsinglet and flavour-singlet amplitudes (including MHV-based checks) and incorporating CEP survival factors, the authors predict strong suppression for non-singlet final states and enhanced rates for singlet channels like $\eta'\eta'$, with explicit predictions for angular distributions and cross sections. They also compare perturbative results to non-perturbative double-Pomeron exchange contributions, discuss symmetric vs skewed CEP mechanisms, and embed the results in the SuperCHIC Monte Carlo, highlighting potential experimental tests and the sensitivity to parton distributions and meson mixing. The findings have practical implications for background estimates to resonant CEP (e.g., $\chi_{c0}$) and offer a pathway to probe the gluonic content of mesons and the perturbative structure of CEP at the LHC and beyond.

Abstract

The central exclusive production (CEP) of heavy resonance states that subsequently decay into meson pairs, MMbar, is an important signature for such processes at hadron colliders. However there is a potentially important background from the direct QCD production of meson pairs, as mediated for example by the exclusive gg --> MMbar hard scattering subprocess. This is in fact an interesting process in its own right, testing novel aspects of perturbative QCD technology. We explicitly calculate the gg --> MMbar helicity amplitudes for different meson states within the hard exclusive formalism, and comment on the application of MHV techniques to the calculation. Using these results, we describe how meson pair CEP can be calculated in the perturbative regime, and present some sample numerical predictions for a variety of final states. We also briefly consider the dominant non-perturbative contributions, which are expected to be important when the meson transverse momentum is small.

Figures (15)

• Figure 1: The perturbative mechanism for the exclusive process $pp \to p\,+\, X \, +\, p$, with the eikonal and enhanced survival factors shown symbolically.
• Figure 2: A typical diagram for the $\gamma\gamma\to M\overline{M}$ process.
• Figure 3: Basic Feynman diagrams for the $gg\to M\overline{M}$ process, grouped into individually gauge invariant subsets $T_1$ (upper) and $T_2$ (lower), and with the relevant colour factors shown schematically. The inclusion of all permutations of these diagrams is implicit.
• Figure 4: Differential cross section ${\rm d}\sigma/{\rm d}|\cos \theta|$ at $\sqrt{\hat{s}}=5$ GeV, for the $gg\to M\overline{M}$ process for non-favour singlet scalar mesons. For comparison, the distribution for three choices of meson wavefunction are shown, the asymptotic form $\phi(x)\propto x(1-x)$, the form (\ref{['CZ']}) proposed in Chernyak81, and a $\delta$-function $\phi(x)\propto\delta(x-\frac{1}{2})$.
• Figure 5: Representative tree diagrams contributing to the $g(k_1)g(k_2)\to q\overline{q}q\overline{q}$ process with $|J_z|=2$ incoming gluons. Quark labels follow the same notation as (\ref{['PT']}) and $\pm$ signs represent particle helicity, with all momenta defined as incoming. All contributing amplitudes are of these three types.