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

The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of $8.16$ TeV with the LHCb detector. The $\pi^0$ production cross section is measured differentially in transverse momentum ($p_{T}$) for $1.5< p_{T} <10.0$ GeV and in center-of-mass pseudorapidity ($\eta_{CM}$) regions $2.5<\eta_{CM}<3.5$ (forward) and $-4.0<\eta_{CM}<-3.0$ (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.