Quenching of Light Hadron Spectra in $p$-A Collisions from Fully Coherent Energy Loss

We single out the role of fully coherent induced gluon radiation on light hadron production in $p$-A collisions. The effect has an interesting color structure, as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in $p$-Pb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in $p$-A collisions.

Figure 1

Contribution to light hadron production in $p$-A collisions from the partonic process $ab\to cd$, as viewed in the nucleus rest frame, followed by the fragmentation $c\to h$ (represented here) or $d\to h$. In addition to the incoming parton $b$, the nucleus provides the nuclear broadening ${\ell }_{\perp A}$ arising from parton multiple scattering.

Figure 2

Rapidity dependence of $R_{p\text{−}\mathrm{A}}^R$, Eq. (7), at $p_{\perp }=2\mathrm{GeV}$, in the $gg\to (gg{)}_R$ channel for $R=\mathbf{1}$ (green line), $R=\mathbf{8}$ (red line), and $R=\mathbf{27}$ (blue line). The color-averaged nuclear modification factor $R_{p\text{−}\mathrm{A}}^h$, Eq. (6), is shown for $\overline{\xi }=1/2$ (dashed black line).

Figure 3

FCEL effects on $R_{p\text{−}\mathrm{A}}^h$ for different channels, as a function of $y$ at $p_{\perp }=2\mathrm{GeV}$ (top) and as a function of $p_{\perp }$ at $y=0$ and $y=5$ (bottom), in $p$-Pb collisions at $\sqrt{s}=8.16\mathrm{TeV}$.

Figure 4

FCEL effects on $R_{p\text{−}\mathrm{A}}^h$ as a function of $p_{\perp }$ in $p$-Pb collisions at $\sqrt{s}=5.02\mathrm{TeV}$ (red band) in comparison to ALICE charged pion data [47].