Probing high energy QCD via 2-particle correlations

Abstract

Recent measurements of the forward rapidity di-hadron azimuthal angular correlations in proton (deuteron)-nucleus (pA) collisions at RHIC [E. Braidot [STAR Collaboration], Nucl. Phys. A854 , 168-174 (2011); A. Adare et al. [PHENIX Collaboration], Phys. Rev. Lett. 107 , 172301 (2011).] and the saturation-based fits to the data [C. Marquet, Nucl. Phys. A 796 , 41 (2007).; K. Tuchin, Nucl. Phys. A 846 , 83 (2010); J. L. Albacete and C. Marquet, Phys. Rev. Lett. 105 , 162301 (2010); D. Kharzeev, E. Levin and L. McLerran, Nucl. Phys. A 748 , 627 (2005); A. Kovner, M. Lublinsky, Phys. Rev. D83 , 034017 (2011); Phys. Rev. D 84 , 094011 (2011); A. Stasto, B.-W. Xiao, F. Yuan, Phys. Lett. B 716 , 430 (2012).] have generated much work on understanding the properties of multi-gluon correlators in the high energy limit. Whereas forward rapidity single inclusive particle production in pA collisions probes dipoles (two-point function of Wilson lines, path ordered exponentials of gluon fields, which satisfies the BK-JIMWLK equations), di-hadron production probes quadrupoles, four-point functions of Wilson lines, which are not well-understood. We show that the evolution equation for the quadrupole derived in the Color Glass Condensate (CGC) formalism reduces, in the dilute regime, to the previously known BJKP equation for the energy dependence of four Reggeized gluons. We outline how one may establish a direct connection between the CGC formalism and Reggeized-gluon exchange to high energy processes.