Light neutral-meson production in pp collisions at $\mathbf{\sqrt{s}}$ = 13 TeV
ALICE Collaboration
TL;DR
This study measures the production cross sections of $\pi^{0}$ and $\eta$ mesons in inelastic $pp$ collisions at $\sqrt{s}=13$ TeV with ALICE, at midrapidity, across a broad $p_{\rm T}$ range to test scaling laws and constrain fragmentation functions. By combining three photon-reconstruction methods (EMCal, PHOS, PCM) and invariant-mass techniques, the analysis provides detailed cross sections and multiplicity-dependent spectra, revealing a $m_{\rm T}$-scaling violation at low $p_{\rm T}$ and validating $x_{\rm T}$ scaling for $\pi^{0}$ with $n=5.01 \pm 0.05$. NLO pQCD calculations with CT18 PDFs and NNFF1.0/BDSS FFs describe the data best for $\pi^{0}$, while PYTHIA8 and EPOS LHC overpredict the yields and struggle with the $\eta$ spectrum. The multiplicity-dependent results show spectral shape hardening with increasing $\text{d}N_{\rm ch}/\text{d}\eta$ and a multiplicity-dependent $\eta/\pi^{0}$ ratio at low $p_{\rm T}$, providing stringent constraints for hadronization models and MC tuning, including feed-down effects from heavier states.
Abstract
The momentum-differential invariant cross sections of ${π^{0}}$ and $η$ mesons are reported for pp collisions at $\sqrt{s}$ = 13 TeV at midrapidity ($|y|<0.8$). The measurement is performed in a broad transverse-momentum range of $0.2<p_{\rm T}<200$ GeV/$c$ and $0.4 < p_{\rm T} < 60$ GeV/$c$ for the ${π^{0}}$ and $η$, respectively, extending the $p_{\rm T}$ 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 $x$ ($x_{\rm T}$) scaling of the ${π^{0}}$ cross sections at LHC energies is fulfilled with a power-law exponent of $n = 5.01 \pm 0.05$, 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 ${π^{0}}$ 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 ${π^{0}}$ and are not able to describe the shape and magnitude of the $η$ spectrum. The charged-particle multiplicity dependent ${π^{0}}$ and $η$ $p_{\rm T}$ 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 $η/π^{0}$ ratio depends on the charged-particle multiplicity for $ p_{\rm T} < 4$ GeV/$c$. PYTHIA8 and EPOS LHC qualitatively explain this behavior with an increasing contribution from the feed-down of heavier particles to the ${π^{0}}$ spectrum.