The rapidity dependence of the average transverse momentum in p + Pb collisions at the LHC: The Color Glass Condensate versus hydrodynamics

We show that in proton–lead (p + Pb) collisions at the LHC, the Color Glass Condensate (CGC) and hydrodynamics lead to qualitatively different behavior of the average transverse momentum, 〈p⊥〉〈p⊥〉, with the particle rapidity. In hydrodynamics, the 〈p⊥〉〈p⊥〉 decreases as one goes from zero rapidity, y=0y=0, to the proton fragmentation region since the number of particles decreases. In contrast, in the CGC the saturation momentum increases as one goes from y=0y=0 to the proton fragmentation region, and so the 〈p⊥〉〈p⊥〉 increases. At the LHC, the difference between the two models may be large enough to be tested experimentally.