Pion-kaon femtoscopy and the lifetime of the hadronic phase in Pb$-$Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV

In this paper, the first femtoscopic analysis of pion-kaon correlations at the LHC is reported. The analysis was performed on the Pb-Pb collision data at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV recorded with the ALICE detector. The non-identical particle correlations probe the spatio-temporal separation between sources of different particle species as well as the average source size of the emitting system. The sizes of the pion and kaon sources increase with centrality, and pions are emitted closer to the centre of the system and/or later than kaons. This is naturally expected in a system with strong radial flow and is qualitatively reproduced by hydrodynamic models. ALICE data on pion-kaon emission asymmetry are consistent with (3+1)-dimensional viscous hydrodynamics coupled to a statistical hadronization model, resonance propagation, and decay code THERMINATOR 2 calculation, with an additional time delay between 1 and 2 fm$/c$ for kaons. The delay can be interpreted as evidence for a significant hadronic rescattering phase in heavy-ion collisions at the LHC.


PLB 813 (2021) 136030
HEP Data
e-Print: arXiv:2007.08315 | PDF | inSPIRE

Figure 1

The $C^{0}_{0}$ (top panel), $\Re C^{1}_{1}$ (middle panel), and $\Im C^{1}_{1}$ (bottom panel) SH components of the charged pion--kaon femtoscopic correlation functions for Pb--Pb collisions at \sqs = 2.76 TeV in the 5--10\% centrality class, positive field polarity. The different charge combinations of pions and kaons are shown with different colours and markers. The statistical and systematic uncertainties are shown as vertical bars and boxes, respectively.

Figure 2

The $C^{0}_{0}$ (top panel) and $\Re C^{1}_{1}$ (botton panel) components of the pion--kaon correlation functions in the 5--10\% centrality class showing the non-femtoscopic background in the spherical-harmonic representation, positive field polarity. The background fit corresponds to a 6$^{\text{th}}$ order polynomial function common for all charge combinations. The two structures visible in the correlation function at 0.11 GeV$/c$ and at 0.29 GeV$/c$ correspond to the remaining effect from track merging and the K$^{*}$ resonance, respectively. The statistical and systematic uncertainties are shown as vertical bars and boxes, respectively.

Figure 3

The $C_0^0(k^*)$ and $\Re C_1^1(k^*)$ parts of the correlation function for (left) $\rm \uppi^{-}K^{-}$ and (right) $\rm \uppi^{-}K^{+}$ pairs, shown as markers for the 5--10\% centrality, with the corresponding fits calculated using the CorrFit package shown as dashed lines. Only half of the statistics is used, corresponding to one magnetic field (positive field polarity). The statistical and systematic uncertainties are shown as vertical lines and boxes, respectively.

Figure 4

Pion--kaon correlation functions in the Cartesian representation for all charge combinations. The $C_-$ is on the negative side of the $k^*$ axes while $C_+$ is on the positive. The femtoscopic fits are shown as a solid black line and were computed using the CorrFit package. The statistical and systematic uncertainties are smaller than the markers.

Figure 5

Pion--kaon source size (upper panel) and emission asymmetry (lower panel) for Pb--Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV as a function of $\langle$d$N_{\rm ch}/$d$\eta\rangle^{1/3}$. The solid lines show predictions from calculation of source size and emission asymmetry using the THERMINATOR 2 model with default and selected values of additional delay with a mean time of $\Delta\uptau$ and width $\upsigma_{\text t}$ for kaons~. The statistical and systematic uncertainties are combined and shown as square brackets. The uncertainty related to the fraction of primary pairs is reported separately as a correlated model-dependent systematic uncertainty of $\pm$15\% (20\%).