Diffractive Photoproduction of Psi(2S) Mesons at HERA

TL;DR

This study presents the first measurement of diffractive diffractive photoproduction of psi(2S) mesons at HERA, examining both the photon-proton energy dependence and the t-dependence at the proton vertex over 40–150 GeV and up to several GeV^2. Using 77 pb^-1 of data from the H1 detector, the analysis separates elastic and proton-dissociative diffractive events and determines the psi(2S) to J/ψ cross-section ratio, finding R ≈ 0.166 with a mild or slightly steeper energy dependence than J/ψ, as anticipated by pQCD models that incorporate the psi(2S) wavefunction node. The t-dependence results show the elastic slope for psi(2S) similar to J/ψ, while the proton-dissociative slope is notably smaller for psi(2S), consistent with node-induced cancellations in the production amplitude. Overall, the findings support perturbative QCD descriptions of heavy-vector-meson diffractive production and highlight the role of the psi(2S) wavefunction in shaping the cross sections and momentum-transfer distributions.

Abstract

Results on diffractive photoproduction of psi(2S) mesons are presented using data collected between 1996 and 2000 with the H1 detector at the HERA ep collider. The data correspond to an integrated luminosity of 77 pb^(-1). The energy dependence of the diffractive psi(2S) cross section is found to be similar to or possibly somewhat steeper than that for J/psi mesons. The dependences of the elastic and proton dissociative psi(2S) photoproduction cross sections on the squared momentum transfer t at the proton vertex are measured. The t-dependence of the elastic channel, parametrised as e^(bt), yields b_(el)^(psi(2S))=(4.31+-0.57+-0.46) GeV^(-2), compatible with that of the J/psi. For the proton dissociative channel the result b_(pd)^(psi(2S))=(0.59+-0.13+-0.12) GeV^(-2) is 2.3 standard deviations smaller than that measured for the J/psi. With proper account of the individual wavefunctions theoretical predictions based on perturbative QCD are found to describe the measurements well.

Figures (6)

• Figure 1: Mass spectrum ($|t|>0.07$ GeV$^2$) for the direct decay channel into leptons ($\mu^+\mu^-$ and $e^+e^-$) after the final selection. The insert shows the mass distribution restricted to the $\psi(2S)$ region. The curve shows the result of a fit combining Gaussian distributions for the $J/\psi$ and $\psi(2S)$ signals, an exponential parametrisation of the radiative tail in the electron decay channel for the $J/\psi$ and a linear background.
• Figure 2: Mass spectra for the selected $\ell\ell\pi\pi$ sample; a) and b) show the $\ell\ell$ and $\ell\ell\pi\pi$ invariant mass distributions; c) shows the mass difference $M_{\ell\ell\pi\pi}-M_{\ell\ell}$ when selecting the $\ell\ell$ pairs reconstructed in the $J/\psi$ mass window.
• Figure 3: Distributions for the decays of $J/\psi$ mesons into muons (a,b) and electrons (c,d). The polar angles (a,c) and the transverse momenta (b,d) of the decay leptons are shown for the data (points) and the DIFFVM Monte Carlo simulation (histograms). The Monte Carlo simulation is normalised to the number of events in the data.
• Figure 4: Distributions for the cascade decays of $\psi(2S)$ mesons: a) the transverse momentum of the decay leptons and b) the decay pions; c) the polar angle of the decay leptons and d) the decay pions; e) the two-pion invariant mass and f) the photon proton center-of-mass energy $W_{\gamma p}$. The points show the data. The histograms show the DIFFVM Monte Carlo simulation normalised to the number of events in the data after event reweighting (see text).
• Figure 5: The ratio $R(W_{\gamma p})=\sigma_{\psi(2S)}/\sigma_{J/\psi}$ for events with $z>0.95$. The inner error bars show the statistical error. The outer error bars show the statistical and systematic errors added in quadrature. An additional normalisation uncertainty of 0.007 due to the errors on the branching ratios is not shown. A fit $R \propto (W_{\gamma p}/90~{\rm GeV})^{\Delta\delta}$ with a value for ${\Delta\delta}$ of 0.24 (solid line) and predictions from Nemchik:1998xb (dashed-dotted line) and Hufner:2000jb (dashed line) are also shown.