Abstract
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive rho(0) meson muo-production at COMPASS using 160 GeV/c polarised mu(+) and mu-beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/c(2) V-L) indicate a violation of s-channel helicity conservation.
Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs).
In particular, they mayallow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive rho(0) production.