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

Alexeev, G.D.; Alexeev, M.G.; Alice, C.; Amoroso, A.; Andrieux, V.; Anosov, V.; Augsten, K.; Augustyniak, W.; Azevedo, C.D. R.; Badelek, B.

doi:10.1140/epjc/s10052-023-11359-4

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

COMPASS

Collaboration

Oct 30, 2022

32 pages

Published in:

Eur.Phys.J.C 83 (2023) 10, 924

Published: Oct 13, 2023

e-Print:

2210.16932 [hep-ex]

DOI:

10.1140/epjc/s10052-023-11359-4

Report number:

CERN-EP-2022-231

Experiments:

CERN-SPS-NA58

View in:

reference search7 citations

Citations per year

Abstract: (Springer)

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

\mu ^{+}

and

\mu ^{-}

beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/

c^2< W<

17.0 GeV/

c^2

, 1.0 (GeV/c)

^2< 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,

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.

muon: secondary beam
muon+: polarized beam
muon-: polarized beam
spin: density matrix
helicity: conservation law
hydrogen: liquid: target
photon: virtual
photon: polarization
polarization: transverse
parton: distribution function

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Spin density matrix elements in exclusive ρ0\rho ^0ρ0 meson muoproduction

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