Elastic Electroproduction of $\rh0$ and $J/Ψ$ Mesons at large $Q^2$ at HERA

TL;DR

This study presents the elastic electroproduction of ρ^0 and J/ψ mesons at large Q^2 and high W at HERA, extracting cross sections, t-slope parameters, and polarization observables with the H1 detector. The rho data reveal a transition region with both soft and hard contributions, while the J/ψ data show hard-dominated dynamics already at moderate Q^2, evidenced by a strong W-dependence and compact meson wave function effects. The results include a rising longitudinal fraction for ρ with Q^2 and SU(4) symmetry in the J/ψ to ρ cross-section ratio, providing important constraints on diffraction models—including soft pomeron and perturbative two-gluon exchange approaches—and on the gluon structure of the proton at high Q^2. Collectively, the measurements illuminate the interplay between non-perturbative and perturbative QCD in exclusive vector-meson production and help map the transition from soft to hard diffraction.

Abstract

The total cross sections for the elastic electroproduction of $\rh0$ and $J/Ψ$ mesons for $Q^2$ $>$ 8 GeV$^2$ and $<W > \simeq 90$ GeV/c$^2$ are measured at HERA with the H1 detector. The measurements are for an integrated electron$-$proton luminosity of $\simeq$~3~pb$^{-1}$. The dependences of the total virtual photon$-$proton ($γ^* p$) cross sections on $Q^2$, $W$ and the momentum transfer squared to the proton ($t$), and, for the $ρ$, the dependence on the polar decay angle ($\cos θ^*$), are presented. The $J/Ψ$ : $\rh0$ cross section ratio is determined. The results are discussed in the light of theoretical models and of the interplay of hard and soft physics processes.

Figures (7)

• Figure 1: Diffractive vector meson production: a) elastic; b) proton dissociation.
• Figure 2: a) $m_{\pi^+\pi^-}$ mass distribution for the events with $Q^2$$>$ 8 ${\rm GeV}^2$, $40 < \hbox{$W$} < 140$${\rm GeV/c^2}$, $|t|$$<$ 0.5 ${\rm GeV}^2$, $E_{max}$$<$ 1 GeV and no proton dissociation signal in the forward detectors; the superimposed curve is the result of a fit to a relativistic Breit-Wigner distribution over the background (\ref{['eq:ps_bg']}), which is described by the dashed curve. In the insert: same distribution without the $E_{max}$ and $t$ cuts; b) $m_{l^+l^-}$ mass distribution for the events with $Q^2$$>$ 8 ${\rm GeV}^2$, $30 < \hbox{$W$} < 150$${\rm GeV/c^2}$ and no proton dissociation signal in the forward detectors; the superimposed curve is the result of a fit to a Gaussian distribution over an exponential background in the range 2.4 $<$$m_{l^+l^-}$$<$ 4.5 ${\rm GeV/c^2}$.
• Figure 3: $|t|$ distributions for a) the $\varrho$ sample; the data points are not corrected for background; the dashed line is the result of a fit taking the background into account, as described in the text; b) the $J/\psi$ sample, corrected for the presence of background. Both distributions are corrected for acceptance, losses and smearing effects. The solid lines correspond to the elastic exponential slopes.
• Figure 4: $t$ slope for elastic $\varrho$ production in fixed target and HERA experiments a) as a function of $Q^2$; b) as a function of $W$ for photoproduction and for $Q^2$$\simeq$ 10 ${\rm GeV}^2$.
• Figure 5: $Q^2$ dependence of the $\gamma ^{(*)}p \rightarrow V p$ cross section a) for $\varrho$ production, the ZEUS points being given for the restricted $x$ range $0.0014 < x < 0.004$; b) for $J/\psi$ production$^6$, the curves being the result of the fits described in the text.