Central exclusive chi_c meson production at the Tevatron revisited

TL;DR

The paper addresses the problem of exclusive central production of $\chi_c$ mesons at the Tevatron, including all three $J^P$ states, in light of the observed exclusive $\chi_c$ signal. It extends the perturbative CEP framework to $\chi_{c0,1,2}$ via a new Monte Carlo generator, SuperCHIC, and accounts for both perturbative and enhanced absorptive corrections. The findings show that $\chi_{c0}$, $\chi_{c1}$, and $\chi_{c2}$ can all contribute to the observed decay chain $\chi_c \to J/\psi \gamma \to \mu^+\mu^-\gamma$, with the total cross section at Tevatron energies around $65$ nb, consistent with CDF measurements after updates; discrimination among the three states with current cuts is challenging, though forward proton tagging and alternative decay channels could help. The work broadens CEP studies of quarkonia, tests the underlying diffractive formalism, and informs future measurements at the LHC and in other decay channels.

Abstract

Motivated by the recent experimental observation of exclusive chi_c events at the Tevatron, we revisit earlier studies of central exclusive scalar chi_c0 meson production, before generalising the existing formalism to include chi_c1 and chi_c2 mesons. Although chi_c0 production was previously assumed to be dominant, we find that the chi_c1 and chi_c2 rates for the experimentally considered chi_c -> J/psi gamma -> mu+ mu- gamma decay process are in fact comparable to the chi_c0 rate. We have developed a new Monte Carlo event generator, SuperCHIC, which models the central exclusive production of the three chi_c states via this decay chain, and have explored possible ways of distinguishing them, given that their mass differences are not resolvable within the current experimental set-up. Although we find that the severity of current experimental cuts appears to preclude this, the acceptance does not change crucially between the three states and so our conclusions regarding the overall rates remain unchanged. This therefore raises the interesting possibility that exclusive chi_c1 and chi_c2 production has already been observed at the Tevatron.

Figures (8)

• Figure 1: Schematic diagrams for CEP of a system $A$ within the approach of Refs. Khoze00KMRprosp and soft - nns2. The integration over the loop momentum $Q_\perp$ in diagram (a) results in a $J_z=0$ selection rule Khoze00a, where $J_z$ is the projection of the total angular momentum along the proton beam axis. It is also necessary to compute the probability, ${\hat{S}}^2$, that the rapidity gaps survive soft (softnns1) and semi-hard (JHEP - bbkm) rescattering; these two possible types of unitarity (or absorptive) corrections are exemplified in diagrams (c) and (d) respectively, where the dashed lines represent Pomeron exchanges (as in version (b) of diagram (a)).
• Figure 2: Impact of the absorptive corrections on the distribution (in arbitrary units) of the difference in azimuthal angle of the outgoing protons for the CEP of various $J^P$$\chi_c$ states at the LHC, using the two channel eikonal model of Ref. soft. The solid (dashed) lines are the distributions including (excluding) the survival factor. For completeness we also show the result for pseudoscalar $\eta_c$ production.
• Figure 3: Dependence of the survival factor $S^2_{\rm eik}$ on the transverse momentum of centrally produced $\chi_c$ mesons at the Tevatron, using the two channel eikonal model of Ref. soft. Also shown (dashed line) is the survival factor corresponding to $\eta_c$ production.
• Figure 4: The perturbative mechanism for the exclusive process $pp \to p+\chi+p$, with the eikonal and enhanced survival factors shown symbolically.
• Figure 5: LO and NLO PDFs at $Q^2=1.5\,$ GeV$^2$, plotted as function of $x$. Four representative PDF sets are displayed, and a large uncertainty at small $x$ is clear.