  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.
