Hard exclusive meson production in muon scattering at COMPASS

¹ National Institute for Nuclear Research, Division of High Energy Physics, ul. Pasteura 7 Warsaw, PL 02-093, Poland

^* Work supported by the Polish NCN Grant 2020/37/B/ST2/01547.
^** e-mail: andrzej.sandacz@ncbj.gov.pl

Published online: 17 November 2022

Abstract

We report on measurements of hard exclusive muoproduction of ρ⁰, ω and π⁰ mesons on the proton at COMPASS using 160 GeV/c polarized μ⁺ and μ⁻ beams scattering off a liquid hydrogen target. The extracted Spin Density Matrix Elements for vector mesons indicate a violation of s-channel helicity conservation for transitions of transversely polarized virtual photons to longitudinally polarized vector mesons, γ_T → V_L, where V denotes ρ⁰ or ω meson. Additionally, for ρ⁰ production we observe a dominant contribution of natural-parity-exchange transitions and a small contribution of unnaturalparity-exchange transitions. On the contrary, the contribution of unnaturalparity-exchange for ω production is significant and it decreases with increasing W, being still non-negligible at the largest W values accessible at COMPASS. Further, we present also the absolute value of the differential cross section for exclusive π⁰ production and its dependence on squared four-momentum transfer t and azimuthal angle ø between the scattering plane and π⁰ production plane. The results provide an important input for modeling Generalized Parton Distribution (GPDs), and in particular to constrain chiral-odd (transversity) GPDs.