论文标题
独家$ω$ MESON MUODUCTUCT $^*$中的自旋密度矩阵元素
Spin Density Matrix Elements in Exclusive $ω$ Meson Muoproduction $^*$
论文作者
论文摘要
我们使用160 GEV/$ C $极化$μ^{+μ^{+} $和$μ^{ - } $ beams在指南针上对质子的旋转密度矩阵元素(SDME)进行了测量。该尺寸涵盖了5.0 GEV/$ C^2 $ $ <w <$ 17.0 GEV/$ C^2 $,平均KineMatics $ \ langle Q^{2} \ rangle = $ 2.1(gev/$ c $) p^{2} _ {\ rm t} \ rangle = 0.16 $(gev/$ c $)$^2 $。在这里,$ q^2 $表示交换的光子的虚拟性,$ w $ the Final Hadronic System的质量和$ p_t $相对于虚拟 - photon方向,$ω$梅森的横向动量。测量的非零SDME用于横向极化的虚拟光子向纵向极化的媒介介子($γ^*_ t \ to v_l $)的过渡,表明违反了$ S $ -CHANNEL HELICITY CORTIONARTION。此外,我们观察到随着$ w $的增加而减少的非自然交换(UPE)过渡的巨大贡献。结果为建模广义Parton分布(GPD)提供了重要的输入。特别是,他们可以允许以模型依赖性的方式评估UPE过渡的贡献,并评估Parton Helicity-Flip GPD在独家$ω$生产中的作用。
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive $ω$ meson muoproduction on the proton at COMPASS using 160 GeV/$c$ polarised $ μ^{+}$ and $ μ^{-}$ 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 $ω$ 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 ($γ^*_T \to V_L$) 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 $ω$ production.