[Paper Review] Measuring QGP thermalization time with dileptons

Mauricio Martínez, Michael Strickland|arXiv (Cornell University)|Sep 22, 2007

High-Energy Particle Collisions Research13 citations

TL;DR

This paper proposes that high-energy dilepton production in a quark-gluon plasma (QGP) is sensitive to the plasma's isotropization time, tau_iso, and early-time momentum-space anisotropies. Using a phenomenological model for time-dependent hard momentum scale p_hard(tau) and anisotropy parameter xi(tau), it demonstrates that dilepton yields can experimentally probe the onset of hydrodynamic behavior and the magnitude of initial anisotropies in QGP formation.

ABSTRACT

We calculate the medium dilepton yield from a quark-gluon plasma which has a time-dependent local momentum-space anisotropy. A phenomenological model for the hard momentum scale, p_hard(tau), and plasma anisotropy parameter, xi(tau), is constructed which interpolates between free streaming behavior at early times (tau > tau_iso). We show that high-energy dilepton production is sensitive to the assumed plasma isotropization time, tau_iso, and can therefore be used to experimentally determine the time of onset for hydrodynamic expansion of a quark-gluon plasma and the magnitude of expected early-time momentum-space anisotropies.

Motivation & Objective

• To investigate how dilepton yields in a quark-gluon plasma depend on the time of isotropization, tau_iso.
• To model the time evolution of the hard momentum scale p_hard(tau) and anisotropy parameter xi(tau) in a non-equilibrium QGP.
• To determine whether dilepton production can serve as a measurable probe of early-time QGP dynamics and hydrodynamic onset.
• To constrain the magnitude of initial momentum-space anisotropies through experimental dilepton data.

Proposed method

• A phenomenological model is constructed for p_hard(tau) and xi(tau) that interpolates between free streaming at early times (tau > tau_iso) and hydrodynamic behavior at later times.
• The model assumes a time-dependent local momentum-space anisotropy in the QGP, parameterized by xi(tau), which affects the medium dilepton yield.
• The dilepton yield is calculated within a transport framework that accounts for the evolving anisotropy and hard scale in the plasma.
• The time evolution of the plasma is constrained by requiring consistency with free-streaming behavior before tau_iso and hydrodynamic behavior after.
• The model is calibrated to reproduce expected QGP evolution, with tau_iso as a key adjustable parameter.
• The sensitivity of dilepton spectra to tau_iso and xi(tau) is analyzed to assess their detectability in experiments.

Experimental results

Research questions

• RQ1 How does the isotropization time tau_iso influence the yield of high-energy dileptons in a quark-gluon plasma?
• RQ2 To what extent is dilepton production sensitive to early-time momentum-space anisotropies in the QGP?
• RQ3 Can dilepton spectra be used to experimentally determine the onset time of hydrodynamic expansion in heavy-ion collisions?
• RQ4 What is the quantitative relationship between the anisotropy parameter xi(tau) and the resulting dilepton yield?

Key findings

• High-energy dilepton production is highly sensitive to the assumed isotropization time tau_iso, making it a viable probe for the onset of hydrodynamic behavior in the QGP.
• The model shows that momentum-space anisotropies at early times significantly alter the dilepton yield, especially at high invariant masses.
• The time-dependent behavior of p_hard(tau) and xi(tau) leads to measurable deviations in dilepton spectra compared to isotropic or equilibrium models.
• The sensitivity of dilepton spectra to tau_iso suggests that experimental measurements could constrain the early evolution of the QGP.
• The model predicts that dilepton yields are most sensitive to anisotropy during the transition phase from free streaming to hydrodynamic expansion.
• The results indicate that dilepton production offers a unique, experimentally accessible channel to probe non-equilibrium dynamics in the early QGP.