Pion-Kaon femtoscopy as a probe of the space-time emission anisotropies due to interactions at the hadronic stage of matter evolution in relativistic heavy-ion collisions

TL;DR

This work probes how the hadronic stage shapes pion-kaon femtoscopy in heavy-ion collisions by contrasting two models at 5.02 TeV: iHKM, which includes a hadronic cascade, and LQTH, which assumes sudden hadronization without rescattering. The authors extract radii and pair-emission asymmetries from non-identical particle correlation functions, focusing on the out direction and their dependence on multiplicity and pair velocity. They find that iHKM reproduces ALICE trends without additional delays, whereas LQTH requires a kaon emission delay to match the data, highlighting the impact of hadronic interactions. The emission asymmetry scales with multiplicity and shows a nontrivial beta_T dependence, with the ratio mu_out/R_out displaying approximate multiplicity independence, underscoring universal space-time emission characteristics shaped by the hadronic phase.

Abstract

Emission asymmetries between pions and kaons reflect the role of the hadronic phase in the cooling of a droplet of deconfined strongly-interacting matter. This study compares results from two models at the same collision energy of $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV to investigate how interactions in the hadronic phase affect particle emission. The first model, iHKM, provides a complete description of all stages of the evolution; from the initial scattering and thermalization to the final hadronic state, while the second model, LQTH (LHYQUID+THERMINATOR2), assumes a sudden conversion into hadrons, neglecting further interactions. To increase the sensitivity to hadronic interactions, the analysis was performed as a function of the pair transverse velocity for pairs with nearly equal velocity vectors. The obtained predictions are compared with previously measured ALICE data at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV as a function of the cube root of the average particle multiplicity density at midrapidity, showing that both radii and emission asymmetries scale with particle multiplicity, regardless of the collision energy. The iHKM model reproduces the measured trends both qualitatively and quantitatively, whereas the LQTH model requires additional parameters-in particular, a time delay in the emission of kaons-to achieve quantitative agreement. The comparative analysis also indicates a possible non-monotonic behavior of the asymmetry as a function of transverse velocity, and a constant scaling of the ratio between the emission asymmetry and femtoscopic radii with particle multiplicity. These results highlight the importance of including interactions in the hadronic stage for a complete description of the emission function.

Figures (9)

• Figure 1: (Color online) Spherical Harmonic components ($C^0_0$ and $\mathop{\mathrm{Re}}\nolimits C^1_1$) of the femtoscopic correlation function of charged pion-kaon pairs calculated in iHKM for 0-5% central Pb–Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with the respective fit lines estimated in the $CorrFit$ package.
• Figure 2: The $r_{\rm inv}$ distribution with long-range tails from the resonance decays (blue) and the corresponding Gaussian fit function (red).
• Figure 3: Pair $\beta_{\mathrm{T}}$ distribution as a function of $k_{\mathrm{T}}$.
• Figure 4: (Color online) $R_{\rm out}$ (upper panel) and $\mu_{\rm out}$ (lower panel) as functions of $\langle \mathrm{d}N_{\rm ch}/\mathrm{d}\eta\rangle^{\frac{1}{3}}$ for $\sqrt{s_{\rm NN}}=5.02$ TeV Pb$-$Pb collisions simulated using LQTH and iHKM. Different colors in LQTH results correspond to calculations with different values of additional delays in kaon emission.
• Figure 5: (Color online) $R_{\rm out}$ (upper panel) and $\mu_{\rm out}$ (lower panel) as functions of $\langle\beta_{\rm T}\rangle$ in different centrality classes for $\sqrt{s_{\rm NN}}=5.02$ TeV Pb$-$Pb collisions simulated using LQTH and iHKM. The points are shifted around each $\langle\beta_{\rm T}\rangle$ bin center for better visibility.