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 ρ mesons, e+p-->e+ρ+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 H1 detector at HERA in the kinematic domain 2.5<Q²<60 GeV², 40<W<120 GeV. No t dependence of the r⁰⁴₀₀ spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the ρ meson is observed for the spin density matrix element combinations r⁵₀₀+2r⁵₁₁ and r¹₀₀+2r¹₁₁. These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons.