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Spin density matrix elements in exclusive $$\rho ^0$$ meson muoproduction

Journal Article · · European Physical Journal. C, Particles and Fields (Online)
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  1. Brookhaven National Laboratory (BNL), Upton, NY (United States)
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We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\rho ^0$$ meson muoproduction at COMPASS using 160 GeV/c polarised μ+ and μ– beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/c2 < W < 17.0 GeV/c2, 1.0 (GeV/c)2 < $$\mathrm{Q}$$2 < 10.0 (GeV/c)2 and 0.01 (GeV/c)2 < $$p_{\textrm{T}}^2$$ < 0.5 (GeV/c)2. Here, W denotes the mass of the final hadronic system, $$\mathrm{Q}$$2 the virtuality of the exchanged photon, and $$p_{\textrm{T}}$$ the transverse momentum of the $$\rho ^0$$ 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 ($$\gamma ^*_T \rightarrow V^{ }_L$$) 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 may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive $$\rho ^0$$ production.
Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Organization:
USDOE Office of Science (SC), High Energy Physics (HEP)
Contributing Organization:
The COMPASS Collaboration
Grant/Contract Number:
SC0012704
OSTI ID:
2336583
Report Number(s):
BNL--225486-2024-JAAM
Journal Information:
European Physical Journal. C, Particles and Fields (Online), Journal Name: European Physical Journal. C, Particles and Fields (Online) Journal Issue: 10 Vol. 83; ISSN 1434-6052
Publisher:
Springer NatureCopyright Statement
Country of Publication:
United States
Language:
English

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Figures / Tables (22)

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