Multi-strange baryon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

ALICE Collaboration

The production of ${\rm\Xi}^-$ and ${\rm\Omega}^-$ baryons and their anti-particles in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity ($|y| < 0.5$) for charged $\rm\Xi$ and $\rm\Omega$ hyperons have been studied in the range $0.6 < p_{\rm T} < 8.0$ GeV/$c$ and $1.2 < p_{\rm T} < 7.0$ GeV/$c$, respectively, and in several centrality intervals (from the most central 0-10% to the most peripheral 60-80% collisions). These spectra have been compared with the predictions of recent hydrodynamic models. In particular, the Krak${\'o}$w and EPOS models give a satisfactory description of the data, with the latter covering a wider $p_{\rm T}$ range. Mid-rapidity yields, integrated over $p_{\rm T}$, have been determined. The hyperon-to-pion ratios are similar to those at RHIC: they rise smoothly with centrality up to $\langle N_{\rm part}\rangle$~150 and saturate thereafter. The enhancements (yields per participant nucleon relative to pp collisions) increase both with the strangeness content of the baryon and with centrality, but are less pronounced than at lower energies.

Figures

Figure 1

Invariant mass distributions for $\rm\Xi$ (top) and $\rm\Omega$ (bottom) selected candidates from 0-80% most central Pb-Pb collisions at $\snn=2.76$ TeV. The plots are for candidates in the rapidity interval $|y|< 0.5$ at $\pt>0.6$ and 1.2 GeV/$c$ for $\rm\Xi$ and $\rm\Omega$ candidates, respectively. The arrows point to the PDG mass values.
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Figure 2

Acceptance $\times$ efficiency factors for $\rm\Xi^{-}$ (circles) and $\rm\Omega^{-}$ (squares) at mid-rapidity as a function of $\pt$, both for the most central 0-10% (full symbols) and the most peripheral 60-80% (open symbols) Pb-Pb collisions. The points already take into account the branching ratios of the corresponding measured decay channels. Those for the $\rm\Omega^{-}$ are also scaled by a factor of 0.75, to avoid overlap with the $\rm\Xi^{-}$ at high $\pt$.
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Figure 3

Transverse momentum spectra for $\rm\Xi^{-}$ and $\rm\Omega^{-}$ (a,b) and their anti-particles (c,d) in five different centrality classes, from the most central (0-10%) to the most peripheral (60-80%) Pb-Pb collisions at $\snn=2.76$ TeV, for $|y|< 0.5$ at $\pt>1.8$ GeV/$c$ and $|y|< 0.3$ at $\pt< 1.8$ GeV/$c$. The statistical error bars are smaller than the symbols for most data points, while the systematic uncertainties are represented by the open boxes.
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Figure 4

Transverse momentum spectra for $\rm\Xi$ (top) and $\rm\Omega$ (bottom) hyperons (average of particle and anti-particle) in five different centrality classes, compared to hydrodynamic models. Ratios of models to data are also shown.
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Figure 5

Top: Enhancements in the rapidity range $|y|< 0.5$ as a function of the mean number of participants $\avNpart$, showing LHC (ALICE, full symbols), RHIC and SPS (open symbols) data. The LHC data use interpolated pp values (see text). Boxes on the dashed line at unity indicate statistical and systematic uncertainties on the pp or p-Be reference. Error bars on the data points represent the corresponding uncertainties for all the heavy-ion measurements and those for p-Pb at the SPS.
Bottom: Hyperon-to-pion ratios as a function of $\avNpart$, for A-A and pp collisions at LHC and RHIC energies. The lines mark thermal model predictions.
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