Elastic Electroproduction of rho Mesons at HERA

TL;DR

This study analyzes elastic rho meson electroproduction at HERA using the H1 detector across 1 < Q^2 < 60 GeV^2 and 30 < W < 140 GeV. By measuring the full 15 spin density matrix elements from rho decay angular distributions and analyzing the mass skewing, t-slope, and cross sections, the work probes the soft-to-hard transition in QCD and tests helicity conservation. The results show approximate SCHC with a small helicity-flip contribution (~8% of non-flip amplitudes), a rising longitudinal-to-transverse cross-section ratio R with Q^2, a near-constant phase between longitudinal and transverse amplitudes, and a W-dependence that hints at a harder exchange at large Q^2. The findings support both Generalised Vector Dominance and perturbative QCD approaches, and provide quantitative constraints on polarization models and the gluon-density-driven dynamics in rho production. Collectively, the measurements illuminate the interplay of soft and hard dynamics in diffractive vector-meson electroproduction and its sensitivity to the underlying QCD structure.

Abstract

The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 < Q^2 < 60 GeV^2 and for a hadronic centre of mass energy in the range 30 < W < 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.

Figures (24)

• Figure 1: Uncorrected distributions of the polar angle of the scattered electron, the polar angle of the $\rho$ meson, the azimuthal angle of the $\rho$ meson in the laboratory frame, and the transverse momenta of the two pion candidates, for the 1996 data sample (points) and for the Monte Carlo simulation (histograms), after all selection cuts.
• Figure 2: Acceptance corrected $m_{\pi\pi}$ mass distributions for the 1996 data sample, after statistical subtraction of the remaining $\omega$ and $\phi$ background contributions, divided into five bins in $Q^2$. The superimposed curves are the result of fits to skewed relativistic Breit-Wigner distributions using the Ross-Stodolsky parameterisation of eq. (\ref{['eq:ross']}), with the $\rho$ mass and width fixed at the PDG values and assuming no non-resonant background. The solid curves are the results of the fits, the dashed curves correspond to the non-skewed Breit-Wigner contributions. The errors on the data are statistical only.
• Figure 3: Same data as in Fig. \ref{['fig:massRS']}, but compared to the Söding parameterisation of eq. (\ref{['eq:sod']}). The solid curves are the results of the fits, the dashed curves correspond to the non-skewed Breit-Wigner contributions, and the dotted curves correspond to the interferences between the resonant and the non-resonant amplitudes. The errors on the data are statistical only.
• Figure 4: $Q^2$ dependence of the skewing parameters for elastic $\rho$ production: n, for the Ross-Stodolsky parameterisation of eq. (\ref{['eq:ross']}), and $f_I/f_{\hbox{$\rho$}}$, for the Söding parameterisation of eq. (\ref{['eq:sod']}). For the present measurements (full circles), the inner error bars are statistical, and the full error bars include the systematic errors added in quadrature. The other measurements are from H1 h1_phot and ZEUS zeus_photzeus_phottt in photoproduction, and from H1 H1_rho_94, ZEUS rho_zeus and E665 E665 in electroproduction.
• Figure 5: Angle definition for the helicity system in elastic $\rho$ meson production.