Heavy-flavour spectra in high energy nucleus-nucleus collisions

Abstract: The propagation of the heavy quarks produced in relativistic nucleus-nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R_AA and the elliptic flow coefficients v_2 of electrons from heavy-flavour decay.

Heavy-flavour Spectra in High Energy Nucleus-Nucleus Collisions

The paper "Heavy-flavour spectra in high energy nucleus-nucleus collisions" addresses the dynamics of heavy quarks in relativistic heavy-ion collisions, notably those occurring at RHIC and LHC. Discussed within are the complex interplay between heavy quark propagation and the deconfined medium resulting from such collisions, elucidated using stochastic methods and theoretical hydrodynamics models.

Theoretical Framework and Methods

The study primarily utilizes the Langevin dynamics framework to model heavy quark propagation through a deconfined medium, described by hydrodynamic equations under two scenarios: ideal and viscous hydrodynamics. The transport coefficients pertinent for Langevin equations are determined via a hybrid approach combining hard-thermal-loop (HTL) techniques for soft scatterings and perturbative QCD for hard scatterings. This meticulous calculation enables a comprehensive description of the energy loss mechanisms influencing charm and bottom quarks in the quark-gluon plasma (QGP).

Key Observables: Nuclear Modification Factor and Elliptic Flow

Heavy-flavour quark dynamics are analyzed through observables like the nuclear modification factor (R_{AA}) and the elliptic flow (v_2). The asymptotic value of (R_{AA}\approx 0.2) at high (p_T) indicates substantial opacity and parton energy loss in the medium. Meanwhile, the elliptic flow (v_2) provides insight into the azimuthal anisotropy of emitted particles, hinting at the hydrodynamic behavior and small thermalization times inherent to the medium.

Results and Implications

The calculated heavy-quark transport coefficients exhibit mild sensitivity to the intermediate cutoff ( |t|^* ), with significant contributions arising from hard scatterings, as expected from perturbative QCD predictions. Furthermore, the results suggest a larger anisotropy parameter (v_2) and stronger quenching for charm quarks at LHC energies compared to RHIC, elucidating the expected increase in medium effects at higher energies.

The findings align well with experimental data, particularly concerning high (p_T) values, while highlighting discrepancies at lower (p_T), possibly due to the omitting coalescence mechanism during hadronization—a crucial aspect given the role of hadronization in altering particle spectra.

Future Directions

While the current methodology is rooted firmly in perturbative QCD and HTL treatments, the results—especially concerning discrepancies at low (p_T)—suggest exploration into non-perturbative or hybrid scenarios, potentially including coalescence models. The impending data from LHC at varied energies will further validate and refine theoretical models, highlighting any divergences from perturbative predictions, thus steering future research towards refining transport calculations and medium characterizations.

In summary, this research provides valuable insights into the quark-gluon plasma dynamics and strong interaction physics in high-energy nuclear collisions, setting the stage for advanced studies and validations against forthcoming experimental data.