Proton-nucleus inclusive reactions and momentum degradation of quarks

The inclusive hadronic reactions p+A→h+X, where h=p and π, are studied in detail at the constituent level. The effect of the nucleus on the projectile quarks is treated in a way that depends on one parameter, which is related to the degradation length of quark momentum in nuclear matter. The quark momentum distribution in the initial proton is treated in the valon model for low-$p_T$ hadronic reactions. Hadronization of the quarks after the collision is treated in the recombination model. The advantage of using nuclear targets over proton targets is discussed. Two unknown parameters in the phenomenological theory are determined by fitting the x and A dependences of the inclusive distributions of p+A→p+X. The theory is then applied to the p+A→π+X reaction without any free parameters. When the resonance contribution to the pion spectrum is carefully taken into account, the predicted inclusive distributions of the pions are in excellent agreement with the data in both the x and A dependences. The momentum-degradation length of the quarks turns out to be 160 fm. The implications of this work are that (1) there exists a unified approach to hadron-nucleus reactions formulated in the quark basis, and (2) quarks are not slowed down significantly in the fragmentation regions of relativistic heavy-ion collisions.