Imaging constituent quark shape of proton with exclusive vector meson production at HERA

Abstract

We show within proton hot spot picture that the exclusive vector meson production in electron-proton deeply inelastic scattering is sensitive to the individual width of the constituent quarks of the proton. For comparison, we calculate the exclusive $J/\Psi$ production cross-sections in three cases, $B_u\ge B_d$, $B_u<B_d$ and $B_u\ne B_u^{\prime }\ne B_d$, where the $B_u$, $B_u^{\prime }$ and $B_d$ denote the widths of two up quarks and a down quark. We find that only results calculated with $B_u\ge B_d$ can give a reasonable description of the exclusive $J/\Psi$ production cross-section data at HERA. To test that our results are independent of the details of the model, we retain the average width of the three constituent quarks unchanged and compute the exclusive $J/\Psi$ production cross-sections with contribution weight by setting different proportional coefficients ($W_u$ and $W_d$) for the up and down quarks, respectively. It shows that the results calculated with $W_u\ge W_d$ can well reproduce the exclusive $J/\Psi$ production data at HERA, while the opposite case cannot describe the HERA data. These interesting findings seem to indicate that the up quark has more gluons around it than the down quark at high energy although the spatial distribution of gluons fluctuates event-by-event. To ensure the relevant results independent of the species of the vector meson, we also calculate the ρ production cross-sections with the same group of parameters used in the exclusive $J/\Psi$ production and compare the predictions with the HERA data. It shows that all the results computed in the exclusive ρ productions are consistent with the findings obtained in the exclusive $J/\Psi$ productions.