Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet

In a central nucleus-nucleus collision at high-energies, medium partons kicked by a near-side jet acquire a momentum along the jet direction and subsequently materialize as the observed ridge particles. They carry direct information on the early parton momentum distribution which can be extracted by using the ridge data for central AuAu collisions at √ sNN = 200 GeV. The extracted parton momentum distribution has a thermal-like transverse momentum distribution but a non-Gaussian, relatively flat rapidity distribution at mid-rapidity with sharp kinematic boundaries at large rapidities that depend on the transverse momentum. In central high-energy heavy-ion collisions, the state of the parton medium during the early stage of a nucleus-nucleus collision is an important physical quantity. It furnishes information for the investigation of the mechanism of parton production in the early stages of the collision of two heavy nuclei. It also provides the initial data for the evolution of the system toward the state of quark-gluon plasma. Not much is know about the early state of the partons from direct experimental measurements. Recently, the STAR Collaboration observed a ∆φ-∆η correlation of particles associated with a near-side jet in central AuAu collisions at √ sNN = 200 GeV at RHIC, where ∆φ and ∆η are the azimuthal angle and pseudorapidity differences relative to a high-pt trigger particle [1, 2, 3, 4]. The near-side correlations can be decomposed into a “jet” component with fragmentation and radiation products at (∆φ,∆η)∼(0,0), and a “ridge” component at ∆φ∼0 with a ridge structure in ∆η. While many theoretical models have been proposed [5, 6, 7], a momentum kick model was presented to describe the ridge phenomenon [6, 7]. The model assumes that a near-side jet occurs near the surface and it kicks medium partons, loses energy along its way, and fragments into the trigger particle and other fragmentation products in the “jet” component. The kicked medium partons, each of which acquires a momentum kick q from the near-side jet, materialize by parton-hadron duality as ridge particles. The ridge particle momentum distribution is related to the initial momentum distribution by a momentum shift. We can therefore extract the initial parton momentum distribution from the ridge data for central AuAu collisions at √ sNN = 200 GeV obtained by the STAR Collaboration [1, 2, 3]. Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet2 We parametrize the normalized initial parton momentum distribution as dF ptdptdydφ = Aridge(1− x) e √ m2+p t /T