Speaker
Kazuhiro Watanabe
(ODU/JLab)
Description
Heavy quark pair production in high energy proton-nucleus (pA) collisions provides valuable information of the gluon saturation dynamics in a heavy nucleus at small value of Bjorken x. Nowadays, large amounts of data accumulated by RHIC and LHC enable us to examine calculations in small-x saturation formalism or Color Glass Condensate (CGC). Essentially, heavy quark pair production has been studied in the saturation/CGC framework at leading order (LO) with use of the running coupling Balitsky-Kovchegov equation (rcBK) which includes a subset of next-to-leading order (NLO) corrections. Although there are model dependence concerning a description of bound state formation at long distance, the recent comparisons of theoretical computations with data have clarified significant issues of the previous saturation/CGC framework. Some of the important topics are listed below.
The previous saturation/CGC framework can roughly describe $J/\psi$ production in pp/pA collisions, whereas $\Upsilon$ production cannot be described by the previous saturation/CGC framework. In fact, the mean transverse momentum of produced $\Upsilon$ is much larger than the typical saturation scale of target hadron. We found that Sudakov effect on top of the saturation effect provides strong $p_\perp$-broadening for $\Upsilon$ production. This fact indicates higher order calculations must be important to understand further quarkonium production in the saturation/CGC framework.
In addition to $\Upsilon$ production, $\psi(2S)$ production in pA collisions is also important. RHIC and LHC experiments reported that $\psi(2S)$ suppression in pA collision compared to pp collision is stronger than that of $J/\psi$. The previous saturation/CGC framework can not describe the data because the initial state interaction should not interfere with bound state formation in later stage. Currently, the description of bound state formation is being improved in the saturation/CGC framework. The data can be naturally interpreted by taking into account final state interaction in pA collision.
In this talk, I will firstly present a brief review of the saturation/CGC framework and then illustrate the above topical issues.
Primary author
Kazuhiro Watanabe
(ODU/JLab)
