[Paper Review] Non-perturbative Heavy-Flavor Transport at RHIC and LHC

M. He, Rainer J. Fries+1 more|arXiv (Cornell University)|Aug 1, 2012

High-Energy Particle Collisions Research3 references4 citations

TL;DR

This paper presents a non-perturbative framework for heavy-flavor transport in relativistic heavy-ion collisions at RHIC and LHC, combining strong-coupling T-matrix calculations for heavy quark diffusion in the Quark-Gluon Plasma and effective hadronic theory for meson diffusion in the hadronic phase. It successfully predicts flavor-separated non-photonic electron spectra and D/B-meson observables, showing that coalescence and hadronic diffusion significantly enhance elliptic flow and modify suppression patterns, especially for charm quarks near Tc.

ABSTRACT

We calculate open heavy-flavor (HF) transport in relativistic heavy-ion collisions by applying a strong-coupling treatment in both macro- and microscopic dynamics (hydrodynamics and non-perturbative diffusion interactions). The hydrodynamic medium evolution is quantitatively constrained by bulk and multi-strange hadron spectra and elliptic flow. The heavy quark (HQ) transport coefficient is evaluated from a non-perturbative $T$-matrix approach in the Quark-Gluon Plasma (QGP) which, close to the critical temperature, leads to resonance formation and feeds into the recombination of heavy quarks on a hydrodynamic hypersurface. In the hadronic phase, the diffusion of HF mesons is obtained from effective hadronic theory. We compute observables at RHIC and LHC for non-photonic electrons and HF mesons, respectively.

Motivation & Objective

• To develop a non-perturbative treatment of heavy-flavor transport in the Quark-Gluon Plasma (QGP) and hadronic medium, addressing limitations of perturbative QCD in describing heavy quark diffusion.
• To explain the large suppression and substantial elliptic flow of non-photonic electrons at RHIC, which cannot be accounted for by leading-order pQCD.
• To predict flavor-separated spectra of non-photonic electrons at RHIC and D/B-meson observables at LHC, enabling comparison with upcoming experimental data.
• To incorporate both macroscopic hydrodynamic evolution and microscopic non-perturbative scattering dynamics consistently, ensuring thermal equilibrium limits are respected in recombination.

Proposed method

• Employing a relativistic Langevin approach with transport coefficients derived from a non-perturbative T-matrix formalism for heavy quark–light quark and heavy quark–gluon scattering in the QGP.
• Using in-medium quark and meson masses at Tc = 170 MeV, with resonance formation near threshold enhancing relaxation rates by factors of 3–5 over pQCD.
• Implementing hadronization via resonance recombination with thermal light quarks on the hydrodynamic hypersurface, with coalescence probability tied to the T-matrix scattering rate.
• Continuing Langevin simulations for D and B mesons in the hadronic phase using effective hadronic scattering amplitudes, with scaling for bottom quarks.
• Constraining the hydrodynamic medium evolution with bulk and multi-strange hadron spectra and elliptic flow data, using a hybrid EoS from lattice QCD and chemical equilibrium models.
• Applying FONLL pQCD spectra and EPS09 shadowing for initial HQ spectra in p+p collisions, then evolving them through the medium using the full transport framework.

Experimental results

Research questions

• RQ1 How does non-perturbative heavy quark diffusion in the QGP affect the suppression and elliptic flow of non-photonic electrons at RHIC?
• RQ2 To what extent do coalescence with thermal light quarks and diffusion in the hadronic phase contribute to the final D-meson v2 and RAA at LHC?
• RQ3 Why is the observed D-meson RAA at LHC higher than predicted by pQCD-based models, and can this be explained by momentum-dependent relaxation rates?
• RQ4 How does the larger mass of bottom quarks affect their thermalization and resulting suppression compared to charm quarks?
• RQ5 Can the non-prompt J/ψ suppression at LHC be consistently explained by the transport and hadronization of B-mesons within this non-perturbative framework?

Key findings

• The charm-quark RAA exhibits a flow bump at pt ≈ 1.5 GeV due to combined QGP diffusion and recombination effects, with v2 reaching ∼8% at pt ≈ 2 GeV.
• Bottom electrons show less suppression than charm electrons below pt ≈ 5 GeV, and their v2 saturates at higher pt due to the larger mb/mq ratio.
• D-meson RAA is systematically slightly too high compared to ALICE data but reproduces the pT-shape well, primarily due to the falling momentum dependence of the non-perturbative relaxation rate.
• D-meson v2 is enhanced by up to 20% due to diffusion in the hadronic phase, with coalescence and QGP diffusion each contributing roughly half of the final v2.
• B-meson RAA reaches ∼0.4 at pt > 10 GeV, and v2 reaches up to 5%, indicating less collectivity than for D mesons, consistent with CMS non-prompt J/ψ suppression of ∼0.37 at pt ≈ 9.3 GeV.
• The model predicts that both D- and B-meson observables are significantly influenced by non-perturbative effects, with hadronic phase diffusion playing a non-negligible role in shaping final spectra.