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 $\gamma_S$. The extracted temperature remains approximately constant at $T \simeq 155$-$165$ MeV across multiplicity, while the volume exhibits an approximately linear increase with event activity. In contrast, $\gamma_S$ 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 exhibit a hierarchy with strangeness content consistent with ALICE measurements. A systematic comparison of fits constrained by hidden- and open-strangeness hadrons reveals a persistent offset in $\gamma_S$ at the $\sim 4\sigma$ level, indicating a tension between $\phi$- and $\Omega$-constrained fits. These results suggest that while high-multiplicity proton-proton collisions approach thermal-like behavior, a single global freeze-out description may not fully capture the strange sector.