The effects of parton energy loss in nuclear matter on the Drell-Yan process in pA and πA collisions at fixed-target energies are investigated. Calculations are based on the Baier-Dokshitzer-Mueller-Peigné-Schiff (BDMPS) framework embedded in a next-to-leading order calculation, using the transport coefficient extracted from J/ψ measurements. Model calculations prove in good agreement with preliminary measurements by the E906 experiment, despite a slightly different magnitude, supporting a consistent picture between Drell-Yan and J/ψ data. Predictions for the COMPASS future measurements in πA collisions at √ s = 18.9 GeV are also performed. At higher collision energy ( √ s = 38.7 GeV), Drell-Yan measurements are only slightly affected by energy loss effects. On the contrary, the E906 results turn out in clear disagreement with nuclear PDF effects alone. The comparison of E772, E866, and E906 measurements indicates for the first time a clear violation of QCD factorization in Drell-Yan production in pA collisions.
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.
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