Energy Loss in Small Collision Systems

We present leading hadron suppression predictions in $Pb+Pb$ and $p+Pb$ collisions from a convolved radiative and collisional energy loss model in which partons propagate through a realistic background, and in which the radiative energy loss receives a short pathlength correction. We find that the short pathlength correction is small for $D$ meson $R_{AA}(p_T)$ in both $Pb+Pb$ and $p+Pb$ collisions. However the short pathlength correction leads to a surprisingly large reduction in suppression for $\pi$ mesons in $p+Pb$ and even $Pb+Pb$ collisions, providing a qualitative explanation for the rapid rise in $\pi$ meson $R_{AA}(p_T)$ at the LHC. We find that the size of the short pathlength correction to $R_{AA}(p_T)$ is acutely sensitive to the chosen distribution of scattering centers in the plasma. Furthermore we find that conventional elastic energy loss models, which apply the central limit theorem to the number of scatterings, dramatically overpredict suppression in $\mathrm{p}+\mathrm{A}$ collisions, calling for short pathlength corrections to elastic energy loss.