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 𝜋0 production cross section is measured differentially in transverse momentum (𝑝𝑇) for 1.5<𝑝𝑇<10.0  GeV and in center-of-mass pseudorapidity (𝜂c.m.) regions 2.5<𝜂c.m.<3.5 (forward) and −4.0<𝜂c.m.<−3.0 (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of 𝜋0 production, while the backward measurement shows the first evidence of 𝜋0 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.