Photo-production of psi' Mesons at HERA

TL;DR

This paper reports the first detailed measurement of quasi-elastic ψ(2S) photo-production at HERA in the range $40<W_{cp}<160$ GeV, with quasi-elastic events defined by $z>0.95$ and ψ(2S) reconstructed through $J/c( ightarrow \ell^+\ell^-)\pi^+\pi^-$ or $\ell^+\ell^-$. Cross-sections in the photoproduction limit are extracted at $W_{cp}=80$ GeV, yielding $\sigma(\gamma p→ψ(2S)Y)\approx 18$ nb, and the ψ(2S)/J/ψ cross-section ratio is measured as ~$0.15$, consistent with fixed-target results and theoretical expectations. A colour-dipole QCD framework, treating the $c\bar{c}$ pair as a dipole with a node in the ψ(2S) wavefunction, naturally explains the observed suppression of ψ(2S) relative to J/ψ and predicts a forward cross-section ratio near 0.17, which agrees with the data. These results support the colour-dipole picture of high-energy vector-meson production and extend the phenomenology of diffractive charmonium production to the ψ(2S) state.

Abstract

Quasi-elastic (z >0.95) photo-production of psi' mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The psi' mesons were identified through their decays to l+l- and to J/psi pi+ pi-, where the J/psi subsequently decays to l+l-, the lepton l being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be [18.0 +- 2.8 (stat) +- 3.0(syst)] nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the psi' to J/psi quasi-elastic cross-sections is 0.150 +- 0.027 (stat) +- 0.022 (syst).

Figures (4)

• Figure 1: Elastic production of charmonium
• Figure 2: The kinematic variables describing $\psi(2S)$ production at HERA. The incoming positron and proton are denoted $e$ and $p$ respectively, the exchanged photon by $\gamma$, the outgoing positron by $e^{\prime}$, and the $\psi(2S)$ meson produced by $\psi(2S)$. The remainder of the final state is denoted $Y$. The associated four-momenta are indicated in brackets in the diagram.
• Figure 3: The effective mass plots for the four track sample: (a) the four track effective mass, (b) the di-lepton effective mass, and (c) the $\Delta M$ distribution where $\Delta M = M_{(l^{+}l^{-}\pi^{+}\pi^{-})} - M_{(l^{+}l^{-})}$. (d) Effective mass distributions for the two track sample: the di-lepton effective mass with a fit to two gaussians, convoluted with an exponential tail to account for di-electron decays in which a decay electron has lost energy through radiation before entering the detector, plus a linear background.
• Figure 4: The ratio of the cross-sections for $\psi(2S)$ and $J/\psi$ photo-production on nucleons as a function of photon-proton centre-of-mass energy, $W_{\gamma p}$. The results from the present analysis are shown together with those from earlier fixed target experiments ref:SLAC_primeref:NA14_primeref:E401_primeref:EMC_primeref:NMC_prime. The ratios from the fixed target experiments have been corrected using the latest decay branching ratios. The error bars shown on the low energy measurements are statistical only. The combined statistical and systematic errors are shown for the H1 result. Horizontal bars indicate the ranges of $W_{\gamma p}$ over which the measurements were made, where they are specified. The common uncertainty due to the imprecisely known branching ratios is not shown.