Elastic electroproduction of $\rho$ mesons at HERA

Abstract.

The elastic electroproduction of $\rho$ mesons is studied at HERA with the H1 detector for a photon virtuality in the range $1 < Q^2 < 60 {\rm GeV^2}$ and for a hadronic centre of mass energy in the range $30 < W < 140$ GeV. The shape of the ($\pi \pi$) 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 $8 \pm 3\%$ 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.