Abstract:

       Gluon saturation plays the key role in understanding proton and heavy nuclei collisions in the high energy limit, where the gluon momentum fraction x is very small. The Color Glass Condensate (CGC) effective theory is the proper framework to describe such a regime. Because the saturation scale is semi-hard, the coupling constant is typically not small enough. As a consequence, calculations beyond the leading order (LO) are generally required to ensure the convergence of the perturbative results. Recently, tremendous progress has been made in realizing the next-to-leading order (NLO) calculations for the small-x physics, among which single inclusive hadron production in proton-nucleus collisions is one of the most studied processes. The CGC factorization has been confirmed for this observable at the NLO. However, the problem of negative cross section when the hadron transverse momentum becomes large, has not been completely solved, although significant efforts have been devoted. This will be the main focus of this talk. We study this process, and find the long-standing negative cross section at next-to-leading-order (NLO) is driven by the large negative threshold logarithmic contributions. We established a factorization theorem for resumming these logarithms with systematically improvable accuracy within the color glass condensate formalism. We demonstrate how the threshold leading logarithmic accuracy can be realized by a suitable scale choice in the NLO results. The NLO spectrums with the threshold logarithms resummed remain positive and impressive agreements with experimental data are observed.

About the speaker: 

  Liu Hao-Yu obtained Ph.D. under the supervision of Prof. Kuang-Ta Chao from Peking University in 2020, and currently a post-doctoral at Beijing Normal University. His research focus on small-x physics, gluon saturation and CGC factorization theory.