A measurement of the t dependence of the helicity structure of diffractive ρ meson electroproduction at HERA

The helicity structure of the diffractive electroproduction of rho mesons, e + p --> e + rho + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 <t' <3 GeV2, where t' = \t\ - \t\ min. The data used are collected with the HI detector at HERA in the kinematic domain 2.5 <Q(2) <60 GeV2, 40 <W <120 GeV No t dependence of the r(00)(04) spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the rho meson is observed for the spin density matrix element combinations r(00)(5) + 2r(11)(5) and r(00)(1) + 2r(11)(1). These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons. (C) 2002 Elsevier Science B.V. All rights reserved.