Parton energy loss in high energy hard forward processes in proton-nucleus collisions

We calculate the spectrum of energy loss differences due to gluon radiation in high energy hard forward processes in proton-nucleus collisions as compared to proton-proton collisions. We find that the nuclear induced energy loss scales linearly with the beam energy. We evaluate the spectrum and “typical” energy losses in a logarithmic and large $N_c$ approximation. The energy losses found appear large enough to be phenomenologically important.

Figure 1

$\text{Gluon}\to \text{quark}\text{−}\text{antiquark}$ production in proton-nucleus collisions. The circles represent the nuclei. The numbers above the Feynman diagrams label the various places that a gluon can be emitted. (a) is a purely initial state interactions; (c) is a purely final state interaction; (b) is a mixture of initial and final state interactions.

Figure 2

Two examples for the graphs in Fig. 1. (a) and (b) are typical diagrams from class C and B, respectively.

Figure 3

Two-jet production with small-$x$ evolution. (a) is a cross diagram without a gluon radiation in the dipole form. (b) is the energy loss diagram.

Figure 4

$\text{Quark}\to \text{quark}\text{−}\text{gluon}$ production in proton-nucleus collisions. The circles represent the nuclei. The numbers above the Feynman diagrams label the various places that a gluon can be emitted. (a), (b) and (c) represent all possible places that the interactions can happen.

Figure 5

A quark splits into a quark and gluon in the large $N_c$ limit.

Figure 6

A heavy quark-antiquark pair propagates through a nucleus without energy loss. The circles denote nucleons in the nucleus. The last inelastic scattering happens at a longitudinal position $\xi$. Interactions before or after the last inelastic interaction are not shown.

Figure 7

One typical diagram coming from $C_{23}^{(s)}$ in the color singlet model.

Figure 8

One graph from $A_{31}$. The vertical dashed lines indicate the intermediate states.