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$\Lambda\rm{K}$ femtoscopy in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV

The collaboration
Phys.Rev.C 103 (2021) 055201, 2021.

Abstract (data abstract)
The first measurements of the scattering parameters of $\Lambda$K pairs in all three charge combinations ($\Lambda$K$^{+}$, $\Lambda$K$^{-}$, and $\Lambda\mathrm{K^{0}_{S}}$) are presented. The results are achieved through a femtoscopic analysis of $\Lambda$K correlations in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV recorded by ALICE at the LHC. The femtoscopic correlations result from strong final-state interactions, and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the non-femtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the $\Lambda\mathrm{K^{+}}$ interaction and attractive in the $\Lambda\mathrm{K^{-}}$ and $\Lambda\mathrm{K^{0}_{S}}$ interactions. The results suggest an effect arising either from different quark--antiquark interactions between the pairs ($\rm s\overline{s}$ in $\Lambda$K$^{+}$ and $\rm u\overline{u}$ in $\Lambda$K$^{-}$) or from different net strangeness for each system (S=0 for $\Lambda$K$^{+}$, and S=$-2$ for $\Lambda$K$^{-}$). Finally, the $\Lambda$K systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space--time of the single-particle $\Lambda$ and K source distributions.

• #### Figure 1a

Data from Figure 1a

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Invariant mass distributions in the 0--10\% centrality interval of (a) p$\uppi^{-}$ pairs showing the $\Lambda$ peak for V$^{0}$ candidates.

• #### Figure 1b

Data from Figure 1b

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Invariant mass distributions in the 0--10\% centrality interval of $\uppi^{+}\uppi^{-}$ pairs showing the $\mathrm{K^{0}_{S}}$ peak for V$^{0}$ candidates.

• #### Figure 2a ALICE

ALICE data from Figure 2a

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 0--10\% centrality interval.

• #### Figure 2a THERM

THERMINATOR 2 simulation data from Figure 2a

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 0--10\% centrality interval.

• #### Figure 2b ALICE

ALICE data from Figure 2b

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 0--10\% centrality interval.

• #### Figure 2b THERM

THERMINATOR 2 simulation data from Figure 2b

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 0--10\% centrality interval.

• #### Figure 2c ALICE

ALICE data from Figure 2c

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 0--10\% centrality interval.

• #### Figure 2c THERM

THERMINATOR 2 simulation data from Figure 2c

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 0--10\% centrality interval.

• #### Figure 2d ALICE

ALICE data from Figure 2d

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 10--30\% centrality interval.

• #### Figure 2d THERM

THERMINATOR 2 simulation data from Figure 2d

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 10--30\% centrality interval.

• #### Figure 2e ALICE

ALICE data from Figure 2e

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 10--30\% centrality interval.

• #### Figure 2e THERM

THERMINATOR 2 simulation data from Figure 2e

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 10--30\% centrality interval.

• #### Figure 2f ALICE

ALICE data from Figure 2f

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 10--30\% centrality interval.

• #### Figure 2f THERM

THERMINATOR 2 simulation data from Figure 2f

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 10--30\% centrality interval.

• #### Figure 2g ALICE

ALICE data from Figure 2g

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 30--50\% centrality interval.

• #### Figure 2g THERM

THERMINATOR 2 simulation data from Figure 2g

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 30--50\% centrality interval.

• #### Figure 2h ALICE

ALICE data from Figure 2h

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 30--50\% centrality interval.

• #### Figure 2h THERM

THERMINATOR 2 simulation data from Figure 2h

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 30--50\% centrality interval.

• #### Figure 2i ALICE

ALICE data from Figure 2i

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 30--50\% centrality interval.

• #### Figure 2i THERM

THERMINATOR 2 simulation data from Figure 2i

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THERMINATOR 2 simulation correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 30--50\% centrality interval.

• #### Figure 3a

Data from Figure 3a

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 0--10\% centrality interval.

• #### Figure 3b

Data from Figure 3b

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 0--10\% centrality interval.

• #### Figure 3c

Data from Figure 3c

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 0--10\% centrality interval.

• #### Figure 3d

Data from Figure 3d

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 10--30\% centrality interval.

• #### Figure 3e

Data from Figure 3e

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 10--30\% centrality interval.

• #### Figure 3f

Data from Figure 3f

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 10--30\% centrality interval.

• #### Figure 3g

Data from Figure 3g

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Measured correlation function for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system in the 30--50\% centrality interval.

• #### Figure 3h

Data from Figure 3h

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Measured correlation function for the $\Lambda\mathrm{K^{-}}\oplus\overline{\Lambda}\mathrm{K^{+}}$ system in the 30--50\% centrality interval.

• #### Figure 3i

Data from Figure 3i

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Measured correlation function for the $\Lambda\mathrm{K^{0}_{S}}\oplus\overline{\Lambda}\mathrm{K^{0}_{S}}$ system in the 30--50\% centrality interval.

• #### Figure 4a left LamKs

LamKs results from the left panel of Figure 4a

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Extracted imaginary and real components of the complex scattering length, $\Im f_{0}$ and $\Re f_{0}$, for the $\Lambda\mathrm{K^{0}_{S}}$ system.

• #### Figure 4a left LamKchM

LamKchM results from the left panel of Figure 4a

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Extracted imaginary and real components of the complex scattering length, $\Im f_{0}$ and $\Re f_{0}$, for the $\Lambda\mathrm{K^{-}}$ system.

• #### Figure 4a left LamKchP

LamKchP results from the left panel of Figure 4a

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Extracted imaginary and real components of the complex scattering length, $\Im f_{0}$ and $\Re f_{0}$, for the $\Lambda\mathrm{K^{+}}$ system.

• #### Figure 4a right LamKs

LamKs results from the right panel of Figure 4a

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Extracted effective range of the interaction, $d_{0}$, for the $\Lambda\mathrm{K^{0}_{S}}$ system.

• #### Figure 4a right LamKchM

LamKchM results from the right panel of Figure 4a

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Extracted effective range of the interaction, $d_{0}$, for the $\Lambda\mathrm{K^{-}}$ system.

• #### Figure 4a right LamKchP

LamKchP results from the right panel of Figure 4a

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Extracted effective range of the interaction, $d_{0}$, for the $\Lambda\mathrm{K^{+}}$ system.

• #### Figure 4b 0010

Results for 0--10\% centrality from Figure 4b

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The extracted $\lambda_{\mathrm{Fit}}$ and radius parameter for the $\Lambda$K system in the 0--10\% centrality interval.

• #### Figure 4b 1030

Results for 10--30\% centrality from Figure 4b

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The extracted $\lambda_{\mathrm{Fit}}$ and radius parameter for the $\Lambda$K system in the 10--30\% centrality interval.

• #### Figure 4b 3050

Results for 30--50\% centrality from Figure 4b

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The extracted $\lambda_{\mathrm{Fit}}$ and radius parameter for the $\Lambda$K system in the 30--50\% centrality interval.

• #### Figure 5 0010

Results for 0--10\% centrality from Figure 5

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Extracted fit $R_{\mathrm{inv}}$ parameters as a function of pair transverse mass ($m_{\mathrm{T}}$) for the $\Lambda$K system in the 0--10\% centrality...

• #### Figure 5 1030

Results for 10--30\% centrality from Figure 5

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Extracted fit $R_{\mathrm{inv}}$ parameters as a function of pair transverse mass ($m_{\mathrm{T}}$) for the $\Lambda$K system in the 10--30\% centrality...

• #### Figure 5 3050

Results for 30--50\% centrality from Figure 5

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Extracted fit $R_{\mathrm{inv}}$ parameters as a function of pair transverse mass ($m_{\mathrm{T}}$) for the $\Lambda$K system in the 30--50\% centrality...

• #### Figure 6a

Data from Figure 6a

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Spherical harmonics component $C_{00}$ of the $\Lambda\mathrm{K^{+}}$ correlation function for the 0--10\% centrality interval.

• #### Figure 6b

Data from Figure 6b

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Spherical harmonics component $\Re C_{11}$ of the $\Lambda\mathrm{K^{+}}$ correlation function for the 0--10\% centrality interval.

• #### Figure 7a Normal

Data from Figure 7a (normal)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the normal mixed-event reference distribution for the 0--10\% centrality interval.

• #### Figure 7a Stavinskiy

Data from Figure 7a (Stavinskiy)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the Stavinskiy method for the 0--10\% centrality interval.

• #### Figure 7b Normal

Data from Figure 7b (normal)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the normal mixed-event reference distribution for the 10--30\% centrality interval.

• #### Figure 7b Stavinskiy

Data from Figure 7b (Stavinskiy)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the Stavinskiy method for the 10--30\% centrality interval.

• #### Figure 7c Normal

Data from Figure 7c (normal)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the normal mixed-event reference distribution for the 30--50\% centrality interval.

• #### Figure 7c Stavinskiy

Data from Figure 7c (Stavinskiy)

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Correlation functions for the $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ system built using the Stavinskiy method for the 30--50\% centrality interval.

• #### Figure 8a ALICE

ALICE data from Figure 8a

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One-dimensional $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ correlation function for the experimental data in the 0--10\% centrality interval.

• #### Figure 8a THERM

THERMINATOR 2 simulation data from Figure 8a

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One-dimensional $\Lambda\mathrm{K^{+}}\oplus\overline{\Lambda}\mathrm{K^{-}}$ correlation function for the THERMINATOR 2 simulation with impact parameter b = 2 fm.

• #### Figure 8b ALICE

ALICE data from Figure 8b

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$\Re C_{11}$ component of a spherical harmonic decomposition for the experimental data in the 0--10\% centrality interval.

• #### Figure 8b THERM

THERMINATOR 2 simulation data from Figure 8b

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$\Re C_{11}$ component of a spherical harmonic decomposition for the THERMINATOR 2 simulation with impact parameter b = 2 fm.

• #### Figure 8c

Data from Figure 8c

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The pair source distribution from the THERMINATOR 2 simulation in the out direction.

• #### Figure 8d

Data from Figure 8d

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The pair source distribution from the THERMINATOR 2 simulation in the side direction.

• #### Figure 8e

Data from Figure 8e

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The pair source distribution from the THERMINATOR 2 simulation in the long direction.

• #### Figure 8f

Data from Figure 8f

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The temporal characteristics of the pair source distribution from the THERMINATOR 2 simulation.

• #### Figure 9a

Data from Figure 9a

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Probing the effect of varying the source shift in the outward direction, $\mu_{\mathrm{out}}$, within the THERMINATOR 2 framework. To achieve...

• #### Figure 9b

Data from Figure 9b

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Probing the effect of varying the source shift in the outward direction, $\mu_{\mathrm{out}}$, within the THERMINATOR 2 framework. To achieve...

• #### Figure 9c

Data from Figure 9c

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Probing the effect of varying the source shift in the outward direction, $\mu_{\mathrm{out}}$, within the THERMINATOR 2 framework. To achieve...

• #### Figure 9d

Data from Figure 9d

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Probing the effect of varying the source shift in the outward direction, $\mu_{\mathrm{out}}$, within the THERMINATOR 2 framework. To achieve...

• #### Figure 10a ALICE

ALICE data from Figure 10a

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Spherical harmonics component $C_{00}$ of the $\Lambda\mathrm{K^{+}}$ correlation function for the 0--10\% centrality interval.

• #### Figure 10a THERM muOut0

THERMINATOR 2 simulation with a shift $\mu_{\mathrm{out}} = 0$ fm from Figure 10a

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• #### Figure 10a THERM muOut3

THERMINATOR 2 simulation with a shift $\mu_{\mathrm{out}} = 3$ fm from Figure 10a

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• #### Figure 10b ALICE

ALICE data from Figure 10b

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Spherical harmonics component $\Re C_{11}$ of the $\Lambda\mathrm{K^{+}}$ correlation function for the 0--10\% centrality interval.

• #### Figure 10b THERM muOut0

THERMINATOR 2 simulation with a shift $\mu_{\mathrm{out}} = 0$ fm from Figure 10b

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• #### Figure 10b THERM muOut3

THERMINATOR 2 simulation with a shift $\mu_{\mathrm{out}} = 3$ fm from Figure 10b

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