Multiplicity dependence of thermal parameters in pp collisions at $\sqrt{s}=7$ TeV from statistical hadronization fits

Abstract

We perform a systematic thermal analysis of identified hadron yields measured by the ALICE Collaboration in proton-proton collisions at $\sqrt{s}=7$ TeV across charged-particle multiplicity classes within the statistical hadronization model using the Thermal-FIST framework. Global fits are used to extract the chemical freeze-out temperature $T$, system volume $V$, and strangeness saturation parameter $γ_S$. The extracted temperature remains approximately constant at $T \simeq 155$-$165$ MeV across multiplicity, while the effective volume exhibits an approximately linear increase with event activity. In contrast, $γ_S$ shows a clear rise with multiplicity, indicating a progressive reduction of strangeness suppression. Derived thermodynamic quantities obtained within the model show that the energy density increases with multiplicity, while the average energy per particle increases from $\sim 0.85$ GeV to $\sim 0.99$ GeV, remaining close to $1$ GeV. Particle-to-pion ratios reproduce the strangeness-dependent hierarchy observed by ALICE. A systematic comparison of fits constrained by hidden- and open-strangeness hadrons reveals a persistent offset in $γ_S$, corresponding to an approximately $4σ$ separation between the $ϕ$- and $Ω$-constrained results within the present quadrature-summed uncertainty treatment. These results indicate that, although high-multiplicity proton-proton collisions exhibit several features compatible with an approximate thermal description, a single global freeze-out parameterization does not fully capture the strange sector.