[Paper Review] Electromagnetic probes of a pure-glue initial state in nucleus-nucleus collisions at LHC

Volodymyr Vovchenko, Iu. Karpenko+5 more|arXiv (Cornell University)|Apr 21, 2016

High-Energy Particle Collisions Research3 citations

TL;DR

This study investigates the electromagnetic signatures of a pure-glue initial state in Pb+Pb collisions at √sNN = 2.76 TeV using (2+1)-dimensional ideal hydrodynamics with a time-dependent (anti)quark fugacity. It finds that a gluon-dominated early stage suppresses dilepton yields significantly but only modestly affects direct photon pT spectra, while enhanced elliptic flows in photons and dileptons emerge as key signatures of a pure-glue initial condition.

ABSTRACT

Partonic matter produced in the early stage of ultrarelativistic nucleus-nucleus collisions is assumed to be composed mainly of gluons, and quarks and antiquarks are produced at later times. To study the implications of such a scenario, the dynamical evolution of the chemically nonequilibrated system is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent (anti)quark fugacity. The equation of state interpolates linearly between the lattice data for the pure gluonic matter and the lattice data for the chemically equilibrated quark-gluon plasma. The spectra and elliptic flows of thermal dileptons and photons are calculated for central Pb+Pb collisions at the LHC energy of $\sqrt{s_{_{ m NN}}} = 2.76$ TeV. We test the sensitivity of the results to the choice of equilibration times, including also the case where the complete chemical equilibrium of partons is reached already at the initial stage. It is shown that a suppression of quarks at early times leads to a significant reduction of the yield of the thermal dileptons, but only to a rather modest suppression of the $p_T$-distribution of direct photons. It is demonstrated that an enhancement of photon and dilepton elliptic flows might serve as a promising signature of the pure-glue initial state.

Motivation & Objective

• To investigate the electromagnetic signatures of a pure-glue initial state in ultrarelativistic nucleus-nucleus collisions at the LHC.
• To assess the impact of delayed chemical equilibration of partons on thermal dilepton and direct photon spectra and flows.
• To determine whether enhanced elliptic flows in photons and dileptons can serve as observable signals of a gluon-dominated early stage.
• To test the sensitivity of observables to different equilibration times, including full initial chemical equilibrium.
• To model the equation of state as a linear interpolation between pure gluonic and chemically equilibrated quark-gluon plasma lattice data.

Proposed method

• Employing (2+1)-dimensional ideal hydrodynamics to simulate the dynamical evolution of a chemically nonequilibrated system in central Pb+Pb collisions.
• Introducing a time-dependent (anti)quark fugacity to model the gradual onset of quark-antiquark production after the initial state.
• Using an equation of state that interpolates linearly between lattice QCD data for pure gluonic matter and chemically equilibrated quark-gluon plasma.
• Calculating thermal dilepton and direct photon spectra and elliptic flow (v2) as functions of transverse momentum (pT) for √sNN = 2.76 TeV.
• Varying the equilibration time to explore scenarios ranging from full initial chemical equilibrium to a pure-glue initial state.
• Comparing the resulting observables to identify signatures sensitive to the initial gluon dominance.

Experimental results

Research questions

• RQ1 How does a pure-glue initial state affect the yield and pT-spectrum of thermal dileptons in central Pb+Pb collisions at √sNN = 2.76 TeV?
• RQ2 To what extent is the direct photon pT-spectrum suppressed when quarks are absent or delayed in the early stage?
• RQ3 Can the elliptic flow (v2) of dileptons and photons serve as a distinctive signature of a gluon-dominated initial state?
• RQ4 How sensitive are the electromagnetic observables to the timing of chemical equilibration of partons?
• RQ5 What is the relative impact of early quark suppression on dileptons versus direct photons in the context of a time-dependent fugacity model?

Key findings

• A pure-glue initial state leads to a significant suppression of thermal dilepton yields due to the absence of early quark-antiquark pairs.
• The pT-distribution of direct photons is only modestly suppressed under early quark suppression, indicating weaker sensitivity to initial partonic composition.
• Enhanced elliptic flows (v2) in both photons and dileptons are observed when the initial state is dominated by gluons, signaling a potential observable signature.
• The magnitude of the enhancement in v2 for dileptons and photons increases with the degree of initial gluon dominance, making it a sensitive probe.
• The results are robust across different equilibration times, with the strongest signatures emerging when chemical equilibrium is delayed or absent initially.
• The linear interpolation of the equation of state between pure-gluon and equilibrated quark-gluon plasma lattice data enables consistent modeling of the transition phase.