Double-diffractive chi meson production at the hadron colliders

TL;DR

Khoze, Martin, Ryskin, and Stirling analyze double-diffractive χ meson production at hadron colliders by combining Regge theory and perturbative QCD. They predict that exclusive central production pp → p + χ + p dominates χ_c and χ_b channels at Tevatron and LHC due to the low scale and gap survival effects, with inclusive dissociation contributing at a smaller level. The work provides detailed cross-section estimates for χ_c and χ_b, explores the role of nonperturbative mechanisms, and shows how large transverse energy flows can enable perturbative analyses and parity determination via azimuthal correlations. They also discuss leveraging exclusive χ_c production to probe the X(3872) state, proposing parity tests and spin assignments through energy-flow observables in high-E_perp events, while acknowledging substantial theoretical uncertainties from low-scale QCD dynamics.

Abstract

The double-diffractive production of chi_c and chi_b mesons, with a rapidity gap on either side, is studied, using both the Regge formalism and the perturbative QCD approach. Due to the rather low scale, the exclusive double-diffractive process pp \to p + chi + p is predicted to dominate, whereas the probability that the incoming protons dissociate is expected to be relatively small. We evaluate the corresponding chi production cross sections at the Tevatron and LHC energies. For the double-diffractive process with proton dissociation, it is possible to select events with large transverse momenta transferred through the rapidity gaps, by measuring the transverse energy, E_\perp, flows in the proton fragmentation regions. Then the large E_\perp provides a scale to justify the use of perturbative QCD, and to allow a spin-parity analysis of the centrally produced system to be performed, by studying the azimuthal angular correlations between the directions of the forward and backward E_\perp flows. The central production of the new X(3872) charmonium state is considered.

Figures (3)

• Figure 1: (a) The QCD diagram for double-diffractive exclusive production of a $\chi (0^+)$ meson, $pp\to p + \chi + p$, where the gluons of the hard subprocess $gg\to \chi$ are colour screened by the second $t$-channel gluon. (b) The rescattering or absorptive corrections to $pp\to p + \chi + p$, where the shaded region represents the soft $pp$ rescattering corrections, leading to the suppression factor $\hat{S}^2$.
• Figure 2: (a) The Pomeron-Pomeron fusion diagram for double-diffractive exclusive production of a $\chi (0^+)$ meson, $pp\to p + \chi + p$; see (\ref{['eq:PPfusion']}). (b) The rescattering or absorptive corrections to diagram (a), where the shaded region represents the soft $pp$ rescattering corrections, leading to the suppression factor $\hat{S}^2$. (c) The amplitude (squared) for inclusive double-diffractive $\chi$ production; the final diagram shows how high-mass diffraction is given in terms of the triple-Pomeron interaction; see (\ref{['eq:PPP']}).
• Figure 3: The diagram used to calculate the non-perturbative contribution to the exclusive process $pp \to p+\chi+p$. The amplitude, $A^{PP}$, describing the Pomeron-Pomeron $\to \chi$ subprocess, is controlled by the charm quark loop. It is evaluated using (\ref{['eq:APP2']}).