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 $1.5<p_T<10.0\mathrm{GeV}$ and in center-of-mass pseudorapidity (${\eta }_{\mathrm{c}.\mathrm{m}.}$) regions $2.5<{\eta }_{\mathrm{c}.\mathrm{m}.}<3.5$ (forward) and $-4.0<{\eta }_{\mathrm{c}.\mathrm{m}.}<-3.0$ (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.

• Received 25 April 2022
• Accepted 10 June 2022

DOI:https://doi.org/10.1103/PhysRevLett.131.042302

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