[Paper Review] Probing the QCD phase transition with chiral mixing in dilepton production

Azumi Sakai, Masayasu Harada+4 more|arXiv (Cornell University)|Aug 7, 2023

High-Energy Particle Collisions ResearchPhysics and Astronomy3 citations

TL;DR

This study investigates chiral symmetry restoration (CSR) in hot QCD matter via dilepton production, using chiral mixing between ρ and a₁ mesons to probe CSR signatures. By simulating relativistic heavy-ion collisions with realistic hydrodynamics and in-medium spectral functions, it shows that naive extrapolation of low-temperature chiral mixing theorems leads to a spurious 25% overestimate of dilepton yield at M = 1.2 GeV, while the correct CSR scenario produces a moderate but distinct enhancement in 1.1 < M < 1.4 GeV due to in-medium a₁ effects.

ABSTRACT

We perform a systematic study of dilepton emission in a hot QCD medium based on three different scenarios of chiral mixing, each of which yields a characteristic structure in the vector spectral function. The in-medium spectral functions are accommodated into the state-of-the-art hydrodynamic simulations for a relativistic viscous fluid to calculate the dilepton production rate, fully accounting for the space-time evolution of a created fireball in relativistic heavy-ion collisions. We demonstrate that the low-temperature theorem of chiral mixing extrapolated toward a chiral crossover, often used in the literature, leads to critical shortcomings: the inadequacy of width broadening, and a substantial overestimate of the dilepton yield maximized around the invariant mass of $M = 1.2$ GeV. The proper prescription offers a milder yet sizable increase in the window of $1.1 &lt; M &lt; 1.4$ GeV as the direct signature of chiral symmetry restoration.

Motivation & Objective

• To assess the impact of chiral symmetry restoration (CSR) on dilepton production in relativistic heavy-ion collisions.
• To compare two distinct scenarios of chiral mixing: one based on proper CSR with degenerate ρ and a₁ mesons, and another based on naive extrapolation of the low-temperature theorem.
• To determine whether the resulting dilepton spectra can distinguish between these scenarios, particularly in the invariant mass window 1.1–1.4 GeV.
• To evaluate the reliability of current theoretical models in predicting dilepton yields under LHC conditions, especially regarding spectral function broadening and medium effects.
• To clarify the role of chiral mixing as a measurable signature of CSR, avoiding false signals from incorrect theoretical assumptions.

Proposed method

• Employed three different scenarios of chiral mixing to model in-medium spectral functions of vector mesons, incorporating chiral dynamics at finite temperature.
• Used state-of-the-art relativistic viscous hydrodynamic simulations to model the space-time evolution of the QCD fireball in heavy-ion collisions.
• Calculated dilepton production rates by integrating the in-medium spectral functions over the evolving medium, accounting for temperature, density, and lifetime profiles.
• Applied the low-temperature theorem for chiral mixing and compared it with a physically consistent CSR scenario where ρ and a₁ become degenerate.
• Utilized profile functions to estimate contributions from the hadronic phase, finding 70% of thermal dileptons originate from this phase.
• Validated results against experimental data trends from NA60, focusing on the M > 0.8 GeV region where baryonic rescattering effects are minimal.

Experimental results

Research questions

• RQ1 How does the naive extrapolation of the low-temperature chiral mixing theorem affect dilepton yield predictions at high temperatures?
• RQ2 What distinguishes the dilepton spectrum produced under a physically consistent chiral symmetry restoration scenario from one based on incorrect extrapolation?
• RQ3 In which invariant mass window is the signal of chiral symmetry restoration most clearly observable in dilepton production?
• RQ4 To what extent does the absence of proper width broadening in the false CSR model distort the predicted dilepton yield?
• RQ5 Can chiral mixing effects be reliably extracted from dilepton spectra in a hot, dilute medium relevant to LHC conditions?

Key findings

• The false CSR scenario, based on naive extrapolation of the low-temperature theorem, leads to a 25% overestimate of dilepton yield at M = 1.2 GeV compared to the proper CSR scenario.
• The false CSR model fails to reproduce width broadening effects, resulting in an unphysical, sharp peak at M = 1.2 GeV, which corresponds to the vacuum mass of the a₁ meson.
• The correct CSR scenario produces a moderate but significant enhancement in the 1.1 < M < 1.4 GeV window due to the in-medium a₁ meson filling the region separated by the a₁ρπ threshold onset.
• This enhancement compensates for a reduction at the ρ peak, indicating a redistribution of spectral strength rather than a simple peak shift.
• The spectral function in the false CSR model does not resolve the 0.47 GeV mass difference between ρ and a₁, leading to unphysical superposition and incorrect predictions.
• The study concludes that widely used low-temperature theorems should be applied with caution in high-temperature regimes, as they generate misleading signatures of CSR.