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

Abstract: 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.

• The paper reports detailed measurements of Ξ and Ω baryon yields and transverse momentum spectra, finding statistically similar particle and anti-particle yields.
• The analysis employs invariant mass fitting with Monte Carlo-based efficiency corrections across various centrality classes to isolate the baryon signals.
• The paper finds pronounced strangeness enhancement and hyperon-to-pion ratios that align with thermal model predictions, advancing our understanding of quark-gluon plasma properties.

Multi-strange Baryon Production at Mid-rapidity in Pb-Pb Collisions at √sNN = 2.76 TeV

The study of multi-strange baryon production in heavy-ion collisions provides a unique probe to explore the properties of the strongly interacting matter formed during such events. This paper by the ALICE Collaboration at CERN presents a detailed investigation of the production yields and transverse momentum (pT) spectra of Ξ and Ω baryons and their anti-particles in Pb-Pb collisions at √sNN = 2.76 TeV, using data collected by the ALICE detector.

Experimental Setup and Analysis

The ALICE experiment, designed specifically for heavy-ion collision studies, provides comprehensive tracking and particle identification capabilities. Multi-strange baryons are reconstructed through their characteristic decay patterns into final states containing only charged particles. The analysis uses a sample from the 2010 Pb-Pb data-taking period, focusing on events that span five centrality classes from the most central (0-10%) to peripheral (60-80%) collisions.

For the extraction of raw yields, invariant mass distributions are analyzed, and signal extraction involves fitting the distributions to isolate the particle signals from background noise. The acceptance and efficiency corrections are based on Monte Carlo simulations, ensuring that the results accurately reflect the experimental conditions.

Results

The paper reports that particle and anti-particle yields are statistically compatible within the errors, indicative of the almost vanishing net-baryon number at LHC energies. The spectral distributions of Ξ and Ω baryons are compared with several hydrodynamic models, providing critical insights into the dynamics of baryon production.

Hydrodynamic models such as VISH2+1, HKM, Kraków, and EPOS are tested against the experimental data. Notably, the EPOS model shows a commendable ability to describe both the yields and spectral shapes across different centrality classes, with Kraków model also providing a reasonable match, particularly in central collisions.

Strangeness Enhancement

A significant aspect of this study is the quantification of strangeness enhancement, defined as the yield of strange particles in heavy-ion collisions normalized to the number of participating nucleons, relative to pp references. The analysis reveals a pronounced enhancement of strange baryon yields which increase with their strangeness content and with centrality. However, this enhancement appears less pronounced at LHC energies compared to findings at lower energies, indicating possible differences in the medium's properties or the dynamics at different energies.

The hyperon-to-pion ratios, another critical metric, align well with thermal model predictions, implying that the system produced in these collisions can be described by a nearly thermalized state.

Conclusions and Implications

The study enhances our understanding of baryon production and the properties of the quark-gluon plasma, a state of matter believed to have existed in the early universe. The comparison between data and hydrodynamic models improves the constraints on the collective behavior of the system, suggesting the role of various processes, including hadronic rescattering, which may be essential to accurately describe heavy-ion collisions.

The findings stimulate further exploration into the interplay between hard scatterings and the hydrodynamic evolution of the system, and the role of initial conditions vs. final state effects in the production of strange baryons. As data from ongoing and future LHC runs become available, these insights will prove invaluable for refining theoretical models of heavy-ion collisions and the properties of strongly interacting matter.

Continued investigations in this domain are expected to shed light on the transition between hadronic and partonic phases, advancing our understanding not only of QCD under extreme conditions but also offering analogs for other relativistic systems in cosmology and astrophysics.