QCD Matter and Multistage Collision Inference

Heavy-ion observations connect to QCD through a sequence of dynamical models. Thermodynamics, initial conditions, transport, hadronization, and detector-level observables must be combined before a fitted parameter can be interpreted as a property of matter.

Enter this chapter

Begin with Thermodynamics and the QCD Equation of State to relate pressure, energy density, and conserved-charge response.

Helpful background. Thermodynamics and the QCD Equation of State and Initial Conditions and Pre-Equilibrium Evolution develop the definitions used by the chapter’s opening pages.

Topics

1. Thermodynamics and the QCD Equation of State — What do current continuum-controlled calculations establish about QCD pressure, energy, entropy, trace anomaly, sound speed, and susceptibilities across temperature?
2. QCD Phase Structure and Crossover Thermodynamics — What is established about crossover thermodynamics, chiral and deconfinement diagnostics, pseudo-critical temperatures, scaling, and phase boundaries at small chemical potentials?
3. Conserved-Charge Fluctuations and Criticality — How do conserved-charge cumulants, correlations, finite-size/time effects, and dynamic critical behavior connect thermal QCD calculations to critical-point searches?
4. Dense QCD Regimes, Access, and Evidence Boundaries — Which dense-QCD regimes are controlled by perturbation theory, EFT, sign-problem-limited lattice methods, functional methods, compact-star inference, or model assumptions, and what remains unresolved?
5. Initial Conditions and Pre-Equilibrium Evolution — How are nuclear geometry, small-x saturation, CGC fields, Glasma evolution, fluctuations, and deposited conserved quantities converted into a versioned switching-surface state?
6. Hydrodynamization and the Kinetic-to-Hydrodynamic Map — How is a versioned far-from-equilibrium switching-surface state evolved and matched to hydrodynamics while keeping hydrodynamization, isotropization, kinetic equilibration, and thermalization distinct?
7. Viscous Hydrodynamics, Particlization, Afterburners, and Bulk–Flow Observables — How do viscous hydrodynamics, particlization, hadronic afterburners, and detector-level bulk and flow observables form one uncertainty-aware collision stage?
8. Hard Probes and Jet–Medium Tomography — Given a factorized, IRC-safe hard-process and jet baseline, which medium modifications and responses can a reconstructed observable constrain with controlled uncertainty?
9. Electromagnetic Probes and Thermal Radiation — How do electromagnetic-current emission rates become photon and dilepton observables that distinguish thermal radiation from prompt, pre-equilibrium, and hadronic backgrounds?
10. Open Heavy-Flavor Transport and Hadronization — Which heavy-quark transport kernel and hadronization model are identifiable from open-heavy-flavor suppression, flow, correlations, and species-resolved observables?
11. Quarkonium In-Medium Dynamics and Regeneration — How do in-medium quarkonium spectra, dissociation, transport, feed-down, and regeneration combine into a hierarchy- and evidence-controlled observable prediction?
12. Multistage Bayesian Global Inference and Evidence — How can a multistage heavy-ion model be calibrated globally while preserving priors, emulator uncertainty, correlated systematics, model discrepancy, identifiability, posterior prediction, and evidence cutoffs?

How the ideas fit together

The equation of state closes hydrodynamic evolution, but final particle spectra also depend on the initial state, pre-equilibrium dynamics, viscous corrections, and hadronic rescattering. Electromagnetic and heavy-flavor probes sample different parts of that history, providing complementary constraints rather than independent direct measurements of one temperature or transport coefficient.

A global Bayesian analysis therefore infers parameters conditional on a multistage model and its discrepancy assumptions Everett et al. 2021, §§ II–III. A narrow posterior within one model does not remove uncertainty from alternative stage descriptions, unresolved dense-QCD physics, or imperfect observable matching.

Where to go next

Continue to Many-Body QFT and Quantum Matter to compare these state, response, and inference methods with quantum gases and condensed-matter systems.

References

• Everett, D., et al. (JETSCAPE Collaboration). “Multisystem Bayesian Constraints on the Transport Coefficients of QCD Matter.” Physical Review C 103, no. 5 (2021): 054904. DOI.