Measurement of the cross section for hard exclusive $\ddot{I}{€}^0$ leptoproduction

Abstract

We report on a measurement of hard exclusive $\ddot{I}{€}^0$ muoproduction on the proton by COMPASS using 160 GeV/$c$ polarised $\hat{I}{¼}^{+}$ and $\hat{I}{¼}^{-}$ beams of the CERN SPS impinging on a liquid hydrogen target. From the average of the measured $\hat{I}{¼}^{+}$ and $\hat{I}{¼}^{-}$ cross sections, the virtual-photon proton cross section is determined as a function of the squared four-momentum transfer between initial and final proton in the range $0.08(\text{GeV/}c{)}^2<|t|<0.64(\text{GeV/}c{)}^2$. The average kinematics of the measurement are $⟨Q^2⟩=2.0(\text{GeV}/c{)}^2$, $⟨\hat{I}½⟩=12.8\text{GeV}$, $⟨x_{Bj}⟩=0.093$ and $⟨-t⟩=0.256(\text{GeV}/c{)}^2$. Fitting the azimuthal dependence reveals a combined contribution by transversely and longitudinally polarised photons of $(8.1\pm 0.9_{\text{stat}}{}_{-1.0}^{+1.1}{|}_{\text{sys}})\text{nb}/(\text{GeV}/c{)}^2$, as well as transverse-transverse and longitudinal-transverse interference contributions of $(-6.0\pm 1.3_{\text{stat}}{}_{-0.7}^{+0.7}{|}_{\text{sys}})\text{nb}/(\text{GeV}/c{)}^2$ and $(1.4\pm 0.5_{\text{stat}}{}_{-0.2}^{+0.3}{|}_{\text{sys}})\text{nb}/(\text{GeV}/c{)}^2$, respectively. Our results provide important input for modelling Generalised Parton Distributions. In the context of the phenomenological Goloskokov-Kroll model, the statistically significant transverse-transverse interference contribution constitutes clear experimental evidence for the chiral-odd GPD ${\overline{E}}_T$.

9 pages, 5 figures