The analysis focuses on particles produced at midrapidity, within |y| < 0.5, in three of the highest-multiplicity event classes ever selected in proton-proton collisions. Using a dedicated high-multiplicity trigger that selects roughly the top 0.1 percent of events based on the signal in the V0 scintillator arrays, ALICE pushed the average charged-particle multiplicity density to 35.8 particles per unit of pseudorapidity, about a factor of five higher than in average inelastic proton-proton collisions. That density is comparable to what is seen in peripheral lead-lead collisions at 2.76 TeV per nucleon pair, dramatically narrowing the multiplicity gap between small and large collision systems and providing a crucial bridge for comparing physics across system sizes.

The experimental technique combines two independent particle-identification approaches. At low transverse momentum, between roughly 0.1 and 0.75 GeV/c depending on the particle species, the Inner Tracking System alone identifies hadrons through their specific energy loss in silicon. At higher momenta, from about 0.6 up to 4.0 GeV/c, a two-dimensional fit exploits the correlated information from the Time Projection Chamber and the Time-Of-Flight detector, simultaneously modeling the energy-loss and time-of-flight responses for pions, kaons, and protons while accounting for background from mismatched TOF signals. The data sample, collected in 2016 during LHC Run 2 with an integrated luminosity of 13 inverse picobarns for the high-multiplicity trigger, yielded about 80 million events for the inner-tracking analysis and 8 million for the TPC-TOF analysis.

After correcting the raw yields for tracking efficiency, detector acceptance, and contamination from secondary particles produced in weak decays or interactions with detector material, the collaboration obtained transverse-momentum spectra for each particle species in each multiplicity class. The most striking feature is a hardening of the spectra with increasing multiplicity, and that hardening is mass-dependent: the heavier the particle, the more its spectrum shifts toward higher momenta. This mass ordering is precisely the signature associated with radial flow in heavy-ion collisions, where the hot, dense medium expands outward hydrodynamically and gives heavier particles a bigger boost. Quantitatively, the kaon-to-pion ratio rises by a factor of about six, and the proton-to-pion ratio by a factor of about fifteen, as the transverse momentum increases from roughly 0.3 to 3.0 GeV/c.

The proton-to-pion ratio itself tells a compelling story. At low transverse momentum, below about 1 GeV/c, the ratio is suppressed relative to expectations, while at intermediate momenta around 3 GeV/c it shows a clear enhancement. This baryon-to-meson enhancement at intermediate transverse momentum has long been a hallmark of heavy-ion collisions, where it is attributed either to collective flow or to quark recombination, a process in which thermal quarks from the medium combine to form hadrons. Observing a similar pattern in proton-proton collisions, where no large medium was expected to form, reinforces the accumulating evidence that collective-like phenomena are not exclusive to nucleus-nucleus systems.

Perhaps the most consequential result emerges when the transverse-momentum-integrated kaon-to-pion and proton-to-pion ratios are plotted against the charged-particle multiplicity and compared across proton-proton, proton-lead, and lead-lead collisions at various energies. The measurements from all systems and energies fall on smooth, continuous trends, suggesting that particle production is governed primarily by the final-state charged-particle multiplicity rather than by the collision energy or the size of the colliding system. The kaon-to-pion ratio increases by a factor of 1.39 plus or minus 0.19 from low-multiplicity proton-proton collisions to the highest multiplicities in lead-lead collisions, a hint of strangeness enhancement, while the proton-to-pion ratio decreases by a factor of 1.23 plus or minus 0.15, consistent with antibaryon-baryon annihilation in the hadronic phase of heavy-ion collisions. The new high-multiplicity proton-proton points follow both trends seamlessly.

To probe the underlying mechanism, the collaboration compared its results with several theoretical models. PYTHIA 8, the workhorse Monte Carlo generator based on the Lund string model, was tested with four distinct tunes: the standard Monash 2013 configuration, a Ropes tune in which overlapping strings form higher-tension color ropes that boost baryon and strangeness production, a Shoving tune in which strings repel each other and generate a microscopic transverse pressure, and a CLR-BLC Mode 3 tune that introduces string-junction topologies during color reconnection to enhance baryon production. EPOS4, the latest version of the EPOS generator, takes a different approach, dynamically separating each event into a dense core that undergoes collective expansion and a dilute corona that fragments independently.

The comparison reveals that no single model consistently captures all the observed features. The Monash and Ropes tunes reproduce the pion spectra qualitatively within about 20 percent over the full momentum range, while EPOS4 underestimates them. All model calculations underestimate the kaon-to-pion ratio above 0.5 GeV/c, and none of the PYTHIA 8 tunes captures the low-momentum depletion seen in the highest-multiplicity data. EPOS4 provides a better description of the momentum-dependent kaon-to-pion and proton-to-pion ratios than the PYTHIA 8 tunes, and it reproduces the multiplicity trend of the pion mean transverse momentum across the full range, though it falls short for protons in the highest multiplicity classes. The Shoving tune, notably, gives a quantitatively good description of the pion mean transverse momentum, while all PYTHIA 8 tunes underestimate the kaon and proton values.

The mean transverse momentum itself adds another layer of evidence. For all three particle species, it rises continuously with multiplicity, and the rise is steeper for heavier hadrons, mirroring the pattern seen in heavy-ion collisions and consistent with radial flow. Interestingly, at similar multiplicity the mean transverse momentum in proton-proton collisions is higher than in lead-lead collisions, a difference that models will need to explain. The integrated yields were obtained by fitting the measured spectra with Levy-Tsallis functions, with only 6 to 9 percent of the yield extrapolated from unmeasured momentum regions, and the associated systematic uncertainties on the extrapolations range from 0.9 to 1.8 percent.

These measurements do not claim that a quark-gluon plasma forms in proton-proton collisions, and the nuclear modification factor in small systems remains consistent with unity at high momentum, in contrast to the strong jet suppression seen in lead-lead collisions. But by extending light-flavor hadron measurements to charged-particle densities previously unreachable in proton-proton events, ALICE has delivered exactly the kind of high-precision input that theorists need to constrain and improve their models of collective behavior in small systems. Whether the mass-dependent spectral hardening, the intermediate-momentum baryon enhancement, and the multiplicity-driven scaling of particle ratios ultimately trace back to a tiny droplet of deconfined matter, to dense overlapping strings, or to some other mechanism entirely, the new data make clear that the smallest collisions at the LHC are far from trivial, and that the physics of the very large and the very small is more deeply connected than anyone expected.

Subject of Research: Pion, kaon, and proton production in high-multiplicity proton-proton collisions at 13 TeV measured by ALICE

Article Title: (\pi ), K, and p production in high-multiplicity pp collisions at (\mathbf {\sqrt{ s} = 13}) TeV

Article References: ALICE Collaboration, Abdallah, D. A. H., Abualrob, I. J., Acharya, S., Rinella, G. A., Aglietta, L., Agrawal, N., Ahammed, Z., Ahmad, S., Ahuja, I., Akbar, Z., Akishina, V., Al-Turany, M., Alessandro, B., Molina, R. A., Ali, B., Alici, A., Alme, J., Alocco, G., … Zurlo, N. (2026). $$\pi $$, K, and p production in high-multiplicity pp collisions at $$\mathbf {\sqrt{ s} = 13}$$ TeV. The European Physical Journal C, 86(9), Article 1127. https://doi.org/10.1140/epjc/s10052-026-16235-5

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16235-5

Keywords: ALICE, LHC, proton-proton collisions, pions, kaons, protons, high multiplicity, radial flow, strangeness enhancement, PYTHIA 8, EPOS4, quark-gluon plasma