Elastic electroproduction of ρ mesons at HERA

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 GeV2 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 +/- 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.