Proton-proton and deuteron-gold collisions at RHIC

We try to understand recent data on proton-proton and deuteron-gold collisions at RHIC, employing a modified parton model approach. 1 Interesting data from RHIC The nuclear modification factor R = (dNAA/d pt) / (Ncoll dNpp/d pt) shows interesting features: For AuAu, it is much smaller than one for central collisions, whereas for d-Au, it is bigger than one for central collisions. With decreasing centrality, the modification factor for d-Au approaches one and goes even below one. Also interesting is the fact that R decreases with increasing pseudo-rapidity (see fig 1, data from [1,2]). Figure 1: Left: Centrality dependence of the nuclear modification factor. Top to bottom: 0-20%, 20-40%, 40-60%, 60-88%. Right: Rapidity dependence of the nuclear modification factor. Top to bottom: eta = 0, 1, 2.2, 3.2. Invited talk, given at the XXth Winter Workshop on Nuclear Dynamics, Trelawny Beach, Jamaica, March 2004. 1 2 High parton densities In this chapter we want to discuss theoretical implications of high parton densities. Let us first consider parton-parton scattering. A parton from the projectile, after emitting several (initial state) partons, interacts with a corresponding parton from the target, see figure 2(left). To simplify further discussions, we use a symbolic parton ladder for this diagram, as shown in figure 2(right).