AbstractWe report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $$\omega $$
ω
meson muoproduction on the proton at COMPASS using 160 GeV/c polarised $$ \mu ^{+}$$
μ
+
and $$ \mu ^{-}$$
μ
-
beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/$$c^2$$
c
2
$$< W<$$
<
W
<
17.0 GeV/$$c^2$$
c
2
, with the average kinematics $$\langle Q^{2} \rangle =$$
⟨
Q
2
⟩
=
2.1 (GeV/c)$$^2$$
2
, $$\langle W \rangle = 7.6$$
⟨
W
⟩
=
7.6
GeV/$$c^2$$
c
2
, and $$\langle p^{2}_{\mathrm{T}} \rangle = 0.16$$
⟨
p
T
2
⟩
=
0.16
(GeV/c)$$^2$$
2
. Here, $$Q^2$$
Q
2
denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and $$p_T$$
p
T
the transverse momentum of the $$\omega $$
ω
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 ($$\gamma ^*_T \rightarrow V_L$$
γ
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 $$\omega $$
ω
production.
Spin Density Matrix Elements in Exclusive Production of Omega Mesons at HERMES
