期刊
EUROPEAN PHYSICAL JOURNAL C
卷 81, 期 2, 页码 -出版社
SPRINGER
DOI: 10.1140/epjc/s10052-020-08740-y
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- SCOAP<SUP>3
The study reports on the measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive omega meson muoproduction on the proton at COMPASS, revealing a violation of s-channel helicity conservation and a significant contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing energy. These results provide important insights for modeling Generalised Parton Distributions (GPDs) and assessing the role of parton helicity-flip GPDs in exclusive omega production.
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive omega meson muoproduction on the proton at COMPASS using 160 GeV/c polarised mu(+) and mu(-) beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/c(2) < W < 17.0 GeV/c(2), with the average kinematics < Q(2)>= 2.1 (GeV/c)(2), < W >=7.6 GeV/c(2), and < p(T)(2)>=0.16 (GeV/c)(2). Here, Q(2) denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and p(T) the transverse momentum of the omega meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (gamma(*)(T) -> V-L) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive omega production.
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