Measurement of the Spin-Density Matrix Elements in Exclusive Electroproduction of rho^0 Mesons at HERA

TL;DR

The paper reports a detailed extraction of 15 spin-density-matrix elements from exclusive rho^0 electroproduction at two Q^2 ranges using ZEUS at HERA, testing SCHC and natural parity exchange. By comparing angular distributions to a full helicity-amplitude framework, it finds small SCHC violations, most evident at low Q^2, and shows high-Q^2 results in agreement with perturbative QCD expectations involving two-gluon exchange. The ratio R = σ_L/σ_T is determined with minimal dependence on SCHC assumptions, and the data support a dominant natural parity exchange mechanism. Overall, the study provides precise constraints on helicity-flip amplitudes and confirms the applicability of pQCD to rho^0 production at high Q^2 and W.

Abstract

Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the azimuthal angle between the positron scattering plane and the rho^0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) rho^0 mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.

Figures (8)

• Figure 1: Schematic diagram of the reaction $ep \rightarrow e \rho^0 p$, indicating the kinematic variables used in this analysis.
• Figure 2: Schematic diagrams of (a) the process $ep \rightarrow e \rho^0 p$ in the $\gamma^* p$ centre-of-mass system, and (b) the decay of the $\rho^0$ in its rest frame. Three angles suffice to describe the reaction: the azimuthal angle between the scattering plane and the production plane, $\Phi_h$; and the two $\rho^0$ decay angles, $\phi_h$, the azimuthal angle between the production and decay planes, defined in either the $\gamma^* p$ system or in the $\rho^0$ rest frame; and $\theta_h$, which is the polar angle of the positively-charged decay product defined with respect to the direction of the $\rho^0$ momentum vector in the $\gamma^* p$ system, or, equivalently, the direction opposite to the momentum-vector of the final-state proton in the rest frame of the $\rho^0$ meson. This choice of the spin-quantisation axis defines the helicity frame.
• Figure 3: Acceptance as a function of $\cos{\theta_h}$, $\phi_h$ and $\Phi_h$, for the BPC and the DIS data samples. The bars indicate the statistical uncertainties.
• Figure 4: Observed distributions for $\cos{\theta_h}$ (a), $\phi_h$ (b), $\Phi_h$ (c) and $\psi_h= \phi_h -\Phi_h$ (d, e, f) of the reconstructed BPC data (points) and the reconstructed Monte Carlo events (histograms). The solid histograms correspond to the Monte Carlo data reweighted with Eq. (\ref{['full_equation']}), in which the results of the present analysis were used for the 15 combinations of matrix elements $r^{04}_{ik}$, $r^{\alpha}_{ik}$. The dashed histograms correspond to the SCHC hypothesis. The distributions are not corrected for acceptance. The error bars indicate the statistical uncertainty. The statistical uncertainty of the simulated distributions is negligible.
• Figure 5: Observed distributions for $\cos{\theta_h}$ (a), $\phi_h$ (b), $\Phi_h$ (c) and $\psi_h= \phi_h -\Phi_h$ (d, e, f) of the reconstructed DIS data (points) and the reconstructed Monte Carlo events (histograms). The solid histograms correspond to the Monte Carlo data reweighted with Eq. (\ref{['full_equation']}), in which the results of the present analysis were used for the 15 combinations of matrix elements $r^{04}_{ik}$, $r^{\alpha}_{ik}$. The dashed histograms correspond to the SCHC hypothesis. The distributions are not corrected for acceptance. The error bars indicate the statistical uncertainty. The statistical uncertainty of the simulated distributions is negligible.