Resolve the proton-to-pion ratio puzzle in LHC heavy-ion collisions

Determine the physical mechanism that resolves the discrepancy in the proton-to-pion yield ratio observed in LHC heavy-ion collisions relative to statistical/thermal model predictions, reconciling the tension between proton yields and strange-baryon yields. Specifically, assess whether proposed explanations—such as additional hadronic resonances, multi-particle interactions, S-matrix scattering effects, or two distinct chemical freeze-out temperatures—can quantitatively account for the observed proton-to-pion ratio while remaining consistent with other identified-particle yields.

Background

The paper reviews identified-particle yields at the LHC and notes a longstanding tension between proton yields and strange-baryon yields when compared to thermal/statistical models, often highlighted by poor fits to the proton-to-pion ratio. A variety of solutions have been proposed in the literature, including expanding the hadronic resonance spectrum, accounting for multi-particle interactions, using an S-matrix formalism, or employing multiple freeze-out temperatures.

Despite these proposals, the authors explicitly state that the resolution of the proton-to-pion ratio puzzle remains an open question, underscoring the need for a definitive mechanism that reconciles the full set of identified-particle yields.