Light neutral-meson production in pp collisions at = 13 TeV

The momentum-differential invariant cross sections of $\textit{π}$ and $\textit{η}$ mesons are reported for pp collisions at = 13 TeV at midrapidity ($\textit{|y| <}$ 0$\textit{.}$8). The measurement is performed in a broad transverse-momentum range of 0$\textit{.}$2 $\textit{< p}$ $\textit{<}$ 200 GeV/$\textit{c}$ and 0$\textit{.}$4 $\textit{< p}$ $\textit{<}$ 60 GeV/$\textit{c}$ for the $\textit{π}$ and $\textit{η}$, respectively, extending the $\textit{p}$ coverage of previous measurements. Transverse-mass-scaling violation of up to 60% at low transverse momentum has been observed, agreeing with measurements at lower collision energies. Transverse Bjorken $\textit{x}$ ($\textit{x}$ ) scaling of the $\textit{π}$ cross sections at LHC energies is fulfilled with a power-law exponent of $\textit{n}$ = 5$\textit{.}$01 ± 0$\textit{.}$05, consistent with values obtained for charged pions at similar collision energies. The data are compared to predictions from next-to-leading order perturbative QCD calculations, where the $\textit{π}$ spectrum is best described using the CT18 parton distribution function and the NNFF1.0 or BDSS fragmentation function. Expectations from PYTHIA8 and EPOS LHC overestimate the spectrum for the $\textit{π}$ and are not able to describe the shape and magnitude of the $\textit{η}$ spectrum. The charged-particle multiplicity dependent $\textit{π}$ and $\textit{η p}$ spectra show the expected change of the spectral shape, characterized by a flatter slope with increasing multiplicity. This is demonstrated across a broad transverse-momentum range and up to events with a charged-particle multiplicity exceeding five times the mean value in minimum bias collisions. The $\textit{η/π}$ ratio depends on the charged-particle multiplicity for $\textit{p}$ $\textit{<}$ 4 GeV/$\textit{c}$. PYTHIA8 and EPOS LHC qualitatively explain this behavior with an increasing contribution from the feed-down of heavier particles to the $\textit{π}$ spectrum.