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Evidence of rescattering effect in Pb-Pb collisions at the LHC through production of $\rm{K}^{*}(892)^{0}$ and $φ(1020)$ mesons

Measurements of $\rm{K}^{*}(892)^{0}$ and $\phi(1020)$ resonance production in Pb-Pb and pp collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV with the ALICE detector at the Large Hadron Collider are reported. The resonances are measured at midrapidity ($|y|$ $<~$ 0.5) via their hadronic decay channels and the transverse momentum ($p_{\rm{T}}$) distributions are obtained for various collision centrality classes up to $p_{\rm{T}}$ $=$ 20 GeV$/c$. The $p_{\rm{T}}$-integrated yield ratio $\rm{K}^{*}(892)^{0}$$/$K in Pb-Pb collisions shows significant suppression relative to pp collisions and decreases towards more central collisions. In contrast, the $\phi(1020)$$/$K ratio does not show any suppression. Furthermore, the measured $\rm{K}^{*}(892)^{0}$$/$K ratio in central Pb-Pb collisions is significantly suppressed with respect to the expectations based on a thermal model calculation, while the $\phi(1020)$$/$K ratio agrees with the model prediction. These measurements are an experimental demonstration of rescattering of $\rm{K}^{*}(892)^{0}$ decay products in the hadronic phase of the collisions. The $\rm{K}^{*}(892)^{0}$$/$K yield ratios in Pb-Pb and pp collisions are used to estimate the time duration between chemical and kinetic freeze-out, which is found to be $\sim$ 4-7 fm$/c$ for central collisions. The $p_{\rm{T}}$-differential ratios of $\rm{K}^{*}(892)^{0}$$/$K, $\phi(1020)$$/$K, $\rm{K}^{*}(892)^{0}$$/\pi$, $\phi(1020)$$/\pi$, $p/$$\rm{K}^{*}(892)^{0}$ and $p/$$\phi(1020)$ are also presented for Pb-Pb and pp collisions at $\sqrt{s_{\rm{NN}}}$ $=$ 5.02 TeV. These ratios show that the rescattering effect is predominantly a low-$p_{\rm{T}}$ phenomenon.

 

Submitted to: PLB
e-Print: arXiv:1910.14419 | PDF | inSPIRE
CERN-EP-2019-249

Figures

Figure 1

Invariant yields of (a) K$^{*0}$ and (b) $\phi$ mesons in pp collisions and various centrality classes in Pb--Pb collisions at \snn $=$ 5.02 TeV. The values are plotted at the center of each bin. The statistical and systematic uncertainties are shown as bars and boxes, respectively.
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Figure 2

\ptt-integrated particle yield ratios \kst$/$K$^{-}$ and $\phi/$K$^{-}$ as a function of $\langle\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta\rangle^{1/3}$ measured at midrapidity in pp, p--Pb and Pb--Pb collisions at \snn $=$ 5.02 TeV. For Pb--Pb collisions at \snn $=$ 2.76 TeV, the \ph$/$K$^{-}$ values are taken from Ref.~ and the \kst$/$K$^{-}$ values are taken from Ref.~. The ratios for p--Pb collisions are taken from Ref.~. Statistical uncertainties (bars) are shown together with total (hollow boxes) and charged-particle multiplicity-uncorrelated (shaded boxes) systematic uncertainties. Thermal model calculations with chemical freeze-out temperature $T_{\rm{ch}}$ $=$ 156 MeV for the most central Pb--Pb collisions ~ are also shown. EPOS3 model predictions~ of \kskm~and \phikm~ratios in Pb--Pb collisions are also shown as violet lines.
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Figure 3

Lower limit on the hadronic phase lifetime between chemical and kinetic freeze-out as a function of \dndch~in p--Pb~ and Pb--Pb collisions at \snn $=$ 5.02 TeV. The bars and bands represent the statistical and systematic uncertainties, respectively, propagated to the lifetime from the uncertainties associated with the measured \kst$/$K ratios in \pb (\ppb) and pp collisions at \snn $=$ 5.02 TeV.
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Figure 4

Particle yield ratios (\kst~$+$ \kstba)$/$(K$^{+}$ $+$ K$^{-}$) in panel (a) and (2\pha)$/$(K$^{+}$ $+$ K$^{-}$) in panel (b), both as a function of \pt for centrality classes 0--10\% and 70--80\% in \pb collisions at \snn $=$ 5.02 TeV. For comparison, the corresponding ratios are also shown for inelastic pp collisions at \s $=$ 5.02 TeV. The statistical uncertainties are shown as bars and systematic uncertainties are shown as boxes. In the text (\kst~$+$ \kstba), (K$^{+}$ $+$ K$^{-}$) are denoted by \kst~and K, respectively.
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Figure 5

Particle yield ratios (\kst~$+$ \kstba)$/$($\pi^{+}$ $+$ $\pi^{-}$) in panel (a) and (2\pha)$/$($\pi^{+}$ $+$ $\pi^{-}$) in panel (b), both as a function of \pt for centrality classes 0--10\% and 70--80\% in Pb--Pb collisions at \snn $=$ 5.02 TeV. For comparison, the corresponding ratios are also shown for inelastic pp collisions at \s $=$ 5.02 TeV. The statistical uncertainties are shown as bars and systematic uncertainties are shown as boxes. In the text (\kst~$+$ \kstba), ($\pi^{+}$ $+$ $\pi^{-}$) are denoted by \kst~and $\pi$, respectively.
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Figure 6

Particle yield ratios (p $+$ $\mathrm{\overline p}$)$/$(\kst~$+$ \kstba) in panel (a) and (p $+$ $\mathrm{\bar p}$)$/$(2\pha) in panel (b), both as a function of \pt for 0--10\% central \pb collisions and inelastic pp collisions at \mbox{\snn $=$ 5.02 TeV}. For comparison, similar ratios are also shown for 0--5\% central \pb collisions at \mbox{\snn $=$ 2.76 TeV~}. The statistical uncertainties are shown as bars and systematic uncertainties are shown as boxes. In the text (\kst~$+$ \kstba) and (p $+$ $\mathrm{\overline p}$) are denoted by \kst~and p, respectively.
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