[Paper Review] Hadronic resonances in heavy-ion collisions at ALICE (Strangeness in Quark Matter 2013)

A. G. Knospe|arXiv (Cornell University)|Sep 12, 2013

High-Energy Particle Collisions Research16 references7 citations

TL;DR

This study measures K*(892)⁰ and ϕ(1020) resonances in Pb–Pb collisions at √sNN = 2.76 TeV using ALICE detector data to probe the hadronic phase of heavy-ion collisions. By analyzing resonance yields, suppression patterns, and comparisons to theoretical models, it estimates a lower limit of 1.5 fm/c for the hadronic phase lifetime, with no evidence of mass shifts or width broadening for these resonances.

ABSTRACT

Properties of the hadronic phase of high-energy heavy-ion collisions can be studied by measuring the ratios of hadronic resonance yields to the yields of longer-lived particles. These ratios can be used to study the strength of re-scattering effects, the chemical freeze-out temperature, and the lifetime between chemical and kinetic freeze-out. The restoration of chiral symmetry during the early hadronic phase or near the phase transition may lead to shifts in the masses and increases in the widths of hadronic resonances. The ALICE collaboration has measured the spectra, masses, and widths of the K*(892)0 and phi(1020) resonances in Pb-Pb collisions at 2.76 TeV. These results, including RAA and the ratios of the integrated resonance yields to stable hadron yields, are presented and compared to results from other collision systems and to theoretical predictions.

Motivation & Objective

• To investigate the properties of hadronic resonances in the hot, dense medium produced in heavy-ion collisions to probe the hadronic phase.
• To determine whether chiral symmetry restoration near the phase transition leads to observable shifts in resonance masses or widths.
• To quantify re-scattering and regeneration effects in the hadronic phase by measuring resonance-to-stable hadron yield ratios.
• To estimate the lower limit of the hadronic phase lifetime using measured K*(892)⁰/K⁻ ratios and theoretical models.
• To study the pT dependence of resonance suppression via RAA and compare to blast-wave model predictions.

Proposed method

• Reconstruct K*(892)⁰ → π⁻K⁺ and ϕ(1020) → K⁻K⁺ decays using ALICE's ITS, TPC, and VZERO detectors.
• Apply TPC dE/dx particle identification with 2σ cuts to select pion and kaon candidates.
• Extract resonance signals via invariant-mass reconstruction with combinatorial background estimated using event mixing.
• Fit mass distributions with a Breit-Wigner (K*⁰) and Voigtian (ϕ) function, correcting for detector effects and efficiency using GEANT3-simulated Pb–Pb events.
• Correct raw yields for efficiency × acceptance and apply additional PID efficiency correction (91%) for 2σ cuts.
• Extrapolate spectra using Lévy-Tsallis (K*⁰) and Boltzmann-Gibbs blast-wave (ϕ) functions to extend beyond measured pT range.

Experimental results

Research questions

• RQ1 Does the hadronic phase in Pb–Pb collisions at √sNN = 2.76 TeV exhibit measurable re-scattering or regeneration effects on resonance yields?
• RQ2 Are the masses and widths of K*(892)⁰ and ϕ(1020) resonances shifted or broadened compared to vacuum values due to chiral symmetry restoration?
• RQ3 What is the lower limit of the lifetime of the hadronic phase, estimated from the K*(892)⁰/K⁻ yield ratio and theoretical modeling?
• RQ4 How does the suppression of K*(892)⁰ and ϕ(1020) vary with transverse momentum and collision centrality, as quantified by RAA?
• RQ5 Does the RAA of ϕ(1020) follow meson-like or baryon-like suppression patterns across different pT regions?

Key findings

• The invariant mass and width of K*(892)⁰ and ϕ(1020) are consistent with their vacuum values, with no evidence of mass shifts or width broadening.
• The K*(892)⁰/K⁻ yield ratio decreases in more central collisions, indicating enhanced re-scattering effects in dense hadronic matter.
• The ϕ(1020)/K⁻ ratio is independent of collision centrality, suggesting minimal re-scattering effects for this resonance.
• The measured K*(892)⁰/K⁻ ratio of 0.19 ± 0.05, when compared to a theoretical model, implies a lower limit of 1.5 fm/c for the hadronic phase lifetime.
• The K*(892)⁰ spectrum is suppressed by a factor of ≈0.6 across pT < 3 GeV/c relative to blast-wave model predictions assuming no re-scattering.
• The ϕ(1020) RAA is consistent with unity in peripheral collisions and follows the RAA trend of baryons (p, Ξ) at low pT, while lying between mesons and baryons at higher pT.