$\pi$, K, and p production in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV

TL;DR

This study measures the production of pions, kaons, and protons in high-multiplicity proton-proton collisions at 13 TeV, revealing features similar to heavy-ion collisions and exploring particle production scaling with multiplicity.

Contribution

First detailed measurement of identified particle spectra in high-multiplicity pp collisions at 13 TeV, extending multiplicity range and comparing with models and other systems.

Findings

Mass- and multiplicity-dependent spectral hardening observed.

Enhanced p/π ratio at intermediate pT similar to heavy-ion collisions.

Particle ratios scale with multiplicity, not collision energy or system size.

Abstract

This paper presents the measurement of $\pi^{\pm}$, K$^{\pm}$, and p($\bar{\text{p}}$) production in high-multiplicity proton-proton collisions at $\sqrt{s} = 13$ TeV at midrapidity ($|y| < 0.5$) using the ALICE detector at the LHC. The transverse-momentum ($p_{\rm T}$) spectra of these particles are reported for three high-multiplicity classes. The results show a mass- and multiplicity-dependent hardening of the $p_{\rm T}$ spectra and an enhancement of the p/$\pi$ ratio at intermediate $p_{\rm T}$. These features are similar to those observed in heavy-ion collisions, where quark-gluon plasma formation is expected. The new measurements have extended the highest average charged-particle multiplicity density per unit of pseudorapidity achieved in pp collisions, roughly a factor five higher than that in average inelastic pp collisions, thereby reducing the multiplicity gap between small and large collision systems. In addition, the results are further compared with previously published measurements and with model calculations obtained using distinct tunes of the PYTHIA 8 Monte Carlo generator, as well as with predictions from the EPOS4. The comparison of the $p_{\rm T}$-integrated K/$\pi$ and p/$\pi$ ratios across different collision systems and energies suggests that particle production scales with charged-particle multiplicity, rather than with collision energy or system size. While the PYTHIA 8 tunes and the EPOS4 model are able to reproduce some of these measurements, either quantitatively or qualitatively, none of them consistently describes all observed features of the data.