Spin Density Matrix Elements in exclusive vector meson muoproduction at COMPASS

Abstract

We present results on Spin Density Matrix Elements (SDMEs) measured in hard exclusive muoproduction of [Formula: see text] and [Formula: see text] mesons on the proton at COMPASS using 160 GeV/[Formula: see text] polarized [Formula: see text] and [Formula: see text] beams scattering off a liquid hydrogen target. The measurements cover the range 5 GeV/[Formula: see text] [Formula: see text] 17 GeV/[Formula: see text], 1.0 (GeV/[Formula: see text])2 [Formula: see text] 10.0 (GeV/[Formula: see text])2 and 0.01 (GeV/[Formula: see text])2 [Formula: see text] 0.5 (GeV/[Formula: see text])2. Here, [Formula: see text] denotes the virtuality of the exchanged photon, [Formula: see text] the mass of the final hadronic system and [Formula: see text] the transverse momentum of the vector meson with respect to the virtual-photon direction. The measured non-zero SDMEs for transitions of transversely polarized virtual photons to longitudinally polarized vector mesons ([Formula: see text]) indicate a violation of [Formula: see text]-channel helicity conservation. Additionally, for [Formula: see text] production we observe a dominant contribution of natural-parity-exchange transitions and a small contribution of unnatural-parity-exchange transitions. On the contrary, the contribution of unnatural-parity-exchange for [Formula: see text] production is significant and it decreases with increasing [Formula: see text], being still non-negligible at the largest [Formula: see text] values accessible at COMPASS. The results provide an important input for modeling Generalized Parton Distribution (GPDs).