Spin density matrix elements in exclusive $$\omega $$ meson muoproduction

Abstract

We 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 for Exclusive $\rho^{0}$ Meson Muoproduction at COMPASS

“…The kinematic dependences of u 1 , u 2 and u 3 are shown in Fig, 5 (right). We do not observe any significant UPE contribution in contrast to an large contribution fo exclusive omega muoproduction [12].…”

Exclusive measurements at COMPASS

“…In addition, the SDMEs allow one to study the contributions of the unnatural-parity exchange (UPE) processes and the natural-parity exchange (NPE) ones. While a significant contribution from UPE transitions was measured in the production [6], the NPE dominance is observed in the 0 case. These results, together with the kinematic dependence of the SDMEs, will be helpful for constraining the GPD models.…”

“…COMPASS is capable of the hard exclusive production of a variety of mesons. With the 2012 data, COMPASS has measured the cross-section of exclusive 0 production [5] and the Spin Density Matrix Elements (SDMEs) of the exclusive production of [6]. Following this last result, COMPASS has performed the study on the SDMEs of the exclusively produced 0 using the 2012 data.…”

Probing nucleon spin structure in deep-inelastic scattering, proton-proton collisions and Drell-Yan processes

“…Using unpolarized nuclear targets, HERMES [89] and COMPASS [90] determined spin density matrix elements (SDMEs), which describe how the spin components of the virtual photon are transferred to the created vector meson. The SDMEs parameterize the function W(W, Q 2 , t, Φ, φ, cos Θ) appearing in the cross section for vector-meson production.…”

“…Top right: ratio R (longitudinal to transverse vector mesons) at HERMES [89], in comparison to calculations with (red solid) and without (green dashed) pion pole (Goloskokov-Kroll model [91]). Bottom: COMPASS SDMEs [90] describing the spin transfer from the virtual photon γ * to the vector meson (here: ω), both of which can be either longitudinally (L) or transversely (T) polarized. The SDMEs are grouped in five categories A through E. [97] based on [98]) and with GPDs in transverse position space (right [64]).…”