The production of $\eta$ and ${\eta }^{\prime }$ mesons is studied in proton-proton and proton-lead collisions collected with the LHCb detector. Proton-proton collisions are studied at center-of-mass energies of $5.02$ and $13\mathrm{TeV}$ and proton-lead collisions are studied at a center-of-mass energy per nucleon of $8.16\mathrm{TeV}$. The studies are performed in center-of-mass (c.m.) rapidity regions $2.5<y_{\mathrm{c}.\mathrm{m}.}<3.5$ (forward rapidity) and $-4.0<y_{\mathrm{c}.\mathrm{m}.}<-3.0$ (backward rapidity) defined relative to the proton beam direction. The $\eta$ and ${\eta }^{\prime }$ production cross sections are measured differentially as a function of transverse momentum for $1.5<p_{\mathrm{T}}<10\mathrm{GeV}$ and $3<p_{\mathrm{T}}<10\mathrm{GeV}$, respectively. The differential cross sections are used to calculate nuclear modification factors. The nuclear modification factors for $\eta$ and ${\eta }^{\prime }$ mesons agree at both forward and backward rapidity, showing no significant evidence of mass dependence. The differential cross sections of $\eta$ mesons are also used to calculate $\eta /{\pi }^0$ cross-section ratios, which show evidence of a deviation from the world average. These studies offer new constraints on mass-dependent nuclear effects in heavy-ion collisions, as well as $\eta$ and ${\eta }^{\prime }$ meson fragmentation.

• Received 27 October 2023
• Accepted 22 January 2024

DOI:https://doi.org/10.1103/PhysRevC.109.024907

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