Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spin Density Matrix Elements in Exclusive $\omega$ Meson Muoproduction
  $^*$   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 $\langle Q^{2} \rangle=$ 2.1
(GeV/$c$)$^2$, $\langle W \rangle= 7.6$ GeV/$c^2$, and $\langle p^{2}_{\rm T}
\rangle = 0.16$ (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
($\gamma^*_T \to V_L$) 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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The categories of this paper are: patt-sol
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The categories of this paper are: acc-phys
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The categories of this paper are: atom-ph
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The categories of this paper are: q-alg
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The categories of this paper are: math
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The categories of this paper are: alg-geom
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The categories of this paper are: q-bio
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The categories of this paper are: solv-int
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The categories of this paper are: supr-con
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The categories of this paper are: mtrl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: eess
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The categories of this paper are: chem-ph
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The categories of this paper are: econ
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The categories of this paper are: astro-ph