Small-X Physics in PHENIX

Abstract

Measurements of particle production at large rapidities allow to probe parton distributions at small-x. Early measurements of d+Au collisions at RHIC have shown that the inclusive yield of hadrons in the forward rapidity (deuteron) direction are suppressed relative to p+p collisions. Theoretical interpretations of these results have attributed the suppression as due to suppression of small-x partons in the Au nucleus due to nuclear shadowing or gluon saturation, as well as due to initial-state parton energy loss. Kinematically constrained measurements of di-hadron pairs for selected combinations of transverse momentum and rapidity can provide restricted selections on the parton momentum fractions allowing to probe the x-dependence of the suppression in greater detail. Two new electromagnetic calorimeters with full azimuthal coverage over the pseudo-rapidity intervals of 3.1 < |$\eta$| < 3.8 have been incorporated into the PHENIX experiment at RHIC. These calorimeters have been utilized to study forward di-hadron correlations in d+Au collisions at $\sqrt{s_{NN}} = 200$, allowing to probe parton momentum fractions down to x ~10^{-3} in the gold nucleus. Recent results from PHENIX on the small-x suppression will be presented.