Ratio of Photoproduction Rates of $\jpsi$ and $ψ(2S)$

Abstract

There are different approaches for diffractive photoproduction of charmonia. Recently, a new approach is proposed, in which charm quarks are taken as heavy quarks and the nonperturbative effect related to charmonia can be handled with nonrelativistic QCD. The interaction between the $c\bar c$ pair and the initial hadron is through exchange of soft gluons. The exchange of soft gluons can be studied with heavy quark effective theory and an expansion in the inverse of charm quark mass $m_c$ can be employed. In this approach a simple formula for the S-matrix can be derived by neglecting higher orders in $m_c^{-1}$ and relativistic correction related to charmonia. The S-matrix is related to the usual gluon distribution $g(x)$ at small $x$. This result is different than those from other approaches. Confronting experiment the result is not in agreement with experimental measurement because large errors from higher order in $m_c^{-1}$ and from relativistic corrections. Nevertheless the ratio of cross sections of $\jpsi$ and $ψ(2S)$ can be predicted more precisely than cross-sections. In this letter we show that the ratio predicted in this approach with an estimation of relativistic corrections is in good agreement with the recent measurement at HERA.