One- and three-dimensional identical charged-kaon femtoscopic correlations in Pb--Pb collisions at $\mathbf{ \sqrt{s_\mathrm{NN}}=5.02}$ TeV

TL;DR

This study uses identical charged-kaon (K$^{\pm}$K$^{\pm}$) femtoscopy in Pb$-$Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV to extract 1D and 3D emission radii and the time of maximal kaon emission. The analysis reveals that radii shrink with increasing $k_T$ and with decreasing multiplicity, signaling strong collective flow and smaller emission regions in peripheral collisions. The 3D radii follow a power-law $R_i(m_T)=a\,m_T^b$, with $a$ decreasing for more peripheral events, and exhibit overall good agreement with integrated hydrokinetic model (iHKM) predictions, though $R_{\rm out}$ is underpredicted in central collisions by ~1 fm. The time of maximal kaon emission, $\tau_K$, decreases with multiplicity and aligns with iHKM expectations, suggesting earlier kaon emission in smaller, faster-evolving systems. Together, these results constrain hydrodynamic descriptions and the role of hadronic rescattering in heavy-ion collisions.

Abstract

The identical charged-kaon correlations induced by quantum-statistics effects and final-state interactions are measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The results of one- (1D) and three-dimensional (3D) analyses show that the obtained system-size parameters (radii) are smaller for more peripheral collisions and decrease with increasing pair transverse momentum $k_{\rm T}$. The 1D parameters agree within uncertainties with those obtained in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The observed power-law dependence of the extracted 3D radii as a function of the pair transverse momentum is a signature of the collective flow in the particle-emitting system created in Pb$-$Pb collisions. This dependence is well reproduced by the integrated hydrokinetic model calculations except for the outward projection of the radius (measured in the longitudinally co-moving system) for the most central collisions. The time of maximal emission for kaons is extracted from the 3D analysis in a wide collision centrality range from 0 to 90%. Its reduction with decreasing charged-particle multiplicity is well reproduced by the hydrokinetic model predictions, and means that kaons are emitted earlier in more peripheral events.

Figures (11)

• Figure 1: 1D experimental CFs before (empty markers) and after (solid markers) the momentum resolution (MR) correction given by Eq. (\ref{['MRcorr']}) for the 0--5% (top), 30--40% (middle), and 70--90% (bottom) centrality classes. Only CFs in the $k_\mathrm{T}$ [0.2, 0.4] GeV$/c$ (top and middle panels) and [0.2, 0.5] GeV$/c$ (bottom panel) ranges are presented.
• Figure 2: 1D K$^{\pm}$K$^{\pm}$ correlation functions (black markers) for four $k_\mathrm{\rm T}$ ranges and for the 0--5% centrality class, fitted with Eq. (\ref{['eq1']}) (red bands). The CFs are normalized to unity in $0.1<q_\mathrm{inv}<0.15$ GeV$/c$. Statistical uncertainties are shown as bars and systematic uncertainties are shown as boxes. The width of the red bands represents the uncertainties of the Eq. (\ref{['eq1']}) fit.
• Figure 3: 3D K$^{\pm}$K$^{\pm}$ correlation functions (black markers) in projections onto the $q_\mathrm{out}$ (left column), $q_\mathrm{side}$ (middle column), and $q_\mathrm{long}$ (right column) axes for four $k_\mathrm{\rm T}$ ranges fitted with Eq. (\ref{['eq3']}) (red lines). To project onto $q_\mathrm{i}$ (i$=$out, side, long) component, the others are integrated over the range $|q_\mathrm{i}| < 0.04$ GeV$/c$ to illustrate the femtoscopic effect for the corresponding axis. Statistical uncertainties are shown by bars, and systematic uncertainties are smaller than the markers.
• Figure 4: 1D radii (left) and $\lambda$ parameters (right) as a function of the pair transverse momentum $k_\mathrm{T}$. Statistical uncertainties are shown as bars and systematic uncertainties are depicted as boxes. Points for the 5--10%, 10--20%, 20--30%, 30--40% (50--70%, 70--90%) centrality classes are slightly shifted with respect to the 0--5% (40--50%) centrality class in the $x$ axis direction for clarity.
• Figure 5: 1D radii (left) and $\lambda$ parameters (right) as a function of the cube root of the charged-particle multiplicity density $\langle {\rm d}N_{\rm ch}/{\rm d}\eta \rangle^{1/3}$ in the low $k_\mathrm{T}$ range compared with results obtained in Pb--Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV ALICE:2015hvw. Statistical uncertainties are shown as bars, and systematic uncertainties are shown as boxes.