Nuclear modification factor of neutral pions in the forward and backward regions in $p$-Pb collisions

Abstract

The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of $8.16~{\rm TeV}$ with the LHCb detector. The $\pi^0$ production cross section is measured differentially in transverse momentum ($p_{\rm T}$) for $1.5<p_{\rm T}<10.0~{\rm GeV}$ and in center-of-mass pseudorapidity ($\eta_{\rm c.m.}$) regions $2.5<\eta_{\rm c.m.}<3.5$ (forward) and $-4.0<\eta_{\rm c.m.}<-3.0$ (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of $\pi^0$ production, while the backward measurement shows the first evidence of $\pi^0$ enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.