[Paper Review] Recent experimental results from the relativistic heavy-ion collisions at LHC and RHIC

I. Selyuzhenkov|arXiv (Cornell University)|Sep 8, 2011

High-Energy Particle Collisions Research9 references9 citations

TL;DR

This paper reviews recent experimental results from relativistic heavy-ion collisions at the LHC and RHIC, focusing on the properties of the quark-gluon plasma (QGP) formed in Pb–Pb and Au–Au collisions. Using multiplicity, flow, HBT correlations, and hard probes, it demonstrates that the QGP at LHC energies exhibits extreme energy densities (~2 TeV/rapidity), strong radial flow (up to 60% of c), and significant particle suppression, while beam energy scan data hint at a possible QCD critical point near 7–20 GeV/nucleon.

ABSTRACT

A new era has started in the field of relativistic heavy-ion physics with lead beams delivered by the Large Hadron Collider (LHC) in November 2010. In this proceedings I highlight the main results from experimental measurements with Pb-Pb collisions at the incident energy of 2.76 TeV/nucleon recorded by the LHC experiments. Recent experimental developments from the Relativistic Heavy Ion Collider (RHIC) at the GeV incident energy scale are also discussed. All together LHC and RHIC measurements provide new insights on the properties and features of the new hot and dense form of matter created in the course of the relativistic heavy-ion collision.

Motivation & Objective

• To summarize key experimental results from the first LHC heavy-ion run at 2.76 TeV/nucleon and recent RHIC data.
• To investigate the properties of the quark-gluon plasma (QGP) formed in relativistic heavy-ion collisions, including its energy density, flow, and thermalization.
• To examine evidence for collectivity, partonic medium effects via jet quenching and anisotropic flow, and possible signatures of the QCD critical point via higher-order moments of net proton number.
• To explore probes of local parity violation in strong interactions through charge-dependent azimuthal correlations.
• To compare results across energies and experiments to constrain the QGP equation of state and the QCD phase diagram.

Proposed method

• Analysis of charged particle multiplicity and spectra from ALICE, ATLAS, and CMS for Pb–Pb collisions at √sNN = 2.76 TeV.
• Use of Hanbury-Brown-Twiss (HBT) correlations of identical pions to extract freeze-out volume and decoupling time from two-particle momentum correlations.
• Fitting identified particle spectra with blast wave models to extract kinetic freeze-out temperature (Tfo) and radial flow velocity (⟨βt⟩).
• Measurement of nuclear modification factor (RAA) for high-pT particles to probe jet quenching and medium-induced energy loss.
• Analysis of higher-order cumulants (skewness S, kurtosis κ) of net proton number distributions to probe proximity to the QCD critical point.
• Investigation of charge-dependent azimuthal correlations relative to the reaction plane to search for signatures of local parity violation via chiral magnetic effects.

Experimental results

Research questions

• RQ1 What are the key features of the quark-gluon plasma formed in Pb–Pb collisions at 2.76 TeV/nucleon, as revealed by multiplicity, flow, and HBT measurements?
• RQ2 How does the energy density and particle production in the QGP scale with collision energy, and what does this imply for the initial state and thermalization?
• RQ3 What evidence exists for the QCD critical point in the beam energy scan program at RHIC, based on net proton number fluctuations?
• RQ4 To what extent do charge-dependent azimuthal correlations at RHIC and LHC indicate local parity violation in strong interactions?
• RQ5 How do the collective behavior and partonic medium effects in the QGP differ between RHIC and LHC energies?

Key findings

• The charged particle multiplicity density in central Pb–Pb collisions at 2.76 TeV/nucleon reaches dNch/dη ≈ 1600, a factor of 2.15 higher than at top RHIC energy.
• The energy density in central Pb–Pb collisions at LHC is estimated at ~2 TeV per unit of rapidity, representing a 2.5–3 fold increase over RHIC.
• Radial flow velocity in the QGP reaches ~60% of the speed of light, with kinetic freeze-out temperature decreasing to 80 MeV, indicating strong collectivity.
• The HBT homogeneity volume increases with collision energy, and the decoupling time is found to be ~10 fm/c, consistent with a long-lived fireball.
• Higher-order moments of net proton number show deviations from the Hadron Gas Resonance model below 39 GeV/nucleon, suggesting possible proximity to the QCD critical point.
• Charge-dependent azimuthal correlations show a signal consistent with local parity violation at high energies (200 GeV), but the signal diminishes at 7.7–11.5 GeV, with significant background contributions complicating interpretation.