Central exclusive χ c meson production at the Tevatron revisited

Abstract

Motivated by the recent experimental observation of exclusive χ c events at the Tevatron, we revisit earlier studies of central exclusive scalar χ c0 meson production, before generalising the existing formalism to include χ c1 and χ c2 mesons. Although χ c0 production was previously assumed to be dominant, we find that the χ c1 and χ c2 rates for the experimentally considered χ c → J/ψγ → µ + µ − γ decay process are in fact comparable to the χ c0 rate. We have developed a new Monte Carlo event generator, SuperCHIC, which models the central exclusive production of the three χ 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 χ c1 and χ c2 production has already been observed at the Tevatron.2 In the χc case the agreement becomes especially striking after taking into account the revised value of the total χc0 width which has been reduced by a factor 1.4 [37] as compared to the value in the Review of Particle Properties (2002) used in [5].3 CHIC is a publicly available Monte Carlo implementation of the χc0 analysis of Ref. [5]. 4 This applies not only to χc(1P ) states, but also to possible higher excitations χc(nP ). 5 S enh is largely independent of the χ spin assignment. Note also that the relative number of events where the forward protons dissociate is larger for χc1 and χc2 than for χc0.

Exclusive physics at the LHC with SuperChic 2

“…For this reason the authors have previously produced the publicly available SuperChic MC [5,6], for the CEP of lighter Standard Model (SM) objects within the Durham model. While first considering χ c,b and η c,b quarkonia, this has subsequently been extended to include γ γ and light meson pair (ππ, η( )η( ), …) production, as well as the photoproduction of C-odd vector mesons.…”

“…By extending to |η| < 5 we may increase the event sample by a factor of ∼1. 5, and in addition it should be emphasised that the only necessary requirement of the production mechanism for these radiation zeros to be present is that the initial-state gluons be in a colour-singlet state. That is, the effect is independent of the particle polarisations, and of any additional kinematic effects specific to the pure CEP process, such as the J PC z = 0 ++ selection rule.…”

“…While photon exchange generally prefers smaller values of the proton p ⊥ and so will have a smaller impact on b el (28), this contribution cannot necessarily be neglected completely, in particular in the forward region where the slope of the Pomeron form factor in (26) can be quite high, and the average photon virtuality is larger. Again, we will expect screening corrections to have some influence on this value: in particular, as the expected suppression is larger at higher proton p ⊥ , we will expect these to increase b el compared to the bare case, see [5] for additional discussion. Such an effect, although fairly small, is clearly seen in Fig.…”

“…The CEP of χ c,b (and η c,b ) quarkonia states has been considered in [5,6,78]. These processes are implemented in the new MC, and in this section we present updated predictions for a short selection of χ c,b cross sections.…”

Central Exclusiveπ+π−Production inpp̄Collisions at √s= 0.9 and 1.96 TeV at the Tevatron

“…The data presented in this paper, from pp collisions at √ s = 0.9 and 1.96 TeV, extend to above the charmonium threshold where exclusive g + g → χ c production involves perturbative procedures [2,3]. With two large rapidity gaps and central hadrons, the process is expected to be dominated by double pomeron exchange.…”