[Paper Review] Nuclear Modification of Electron Spectra and Implications for Heavy Quark Energy Loss in Au+Au Collisions at sqrt(s_NN)=200 GeV

PHENIX Collaboration, S. S. Adler|arXiv (Cornell University)|Oct 16, 2005

High-Energy Particle Collisions Research7 citations

TL;DR

This paper presents PHENIX measurements of electron spectra from heavy quark decays in central Au+Au collisions at √sNN = 200 GeV, observing strong suppression (RAA ≲ 0.3 at pT > 3.5 GeV/c) that provides direct evidence for significant energy loss of charm and bottom quarks in the quark-gluon plasma. The data challenge existing models of radiative energy loss via induced gluon radiation, even at extreme medium densities, suggesting additional mechanisms may be at play.

ABSTRACT

The PHENIX experiment has measured mid-rapidity transverse momentum spectra (0.4 < p_T < 5.0 GeV/c) of electrons as a function of centrality in Au+Au collisions at sqrt(s_NN)=200 GeV. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of pi^0 and eta mesons, were removed. The resulting non-photonic electron spectra are primarily due to the semi-leptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to non-photonic electrons in p+p collisions. A significant suppression of electrons at high p_T is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.

Motivation & Objective

• To measure the nuclear modification factor (RAA) of electrons from heavy flavor decays in Au+Au collisions at √sNN = 200 GeV.
• To investigate the energy loss of heavy quarks (charm and bottom) in the hot, dense medium formed in relativistic heavy-ion collisions.
• To test theoretical models of heavy quark energy loss, particularly those based on induced gluon radiation, against experimental data.
• To assess the relative contributions of D and B meson decays to high-pT electron spectra and their implications for medium modification.
• To determine whether observed suppression is consistent with current predictions of radiative energy loss mechanisms in the quark-gluon plasma.

Proposed method

• Measure electron spectra in Au+Au collisions at √sNN = 200 GeV using the PHENIX detector at RHIC.
• Identify electrons from heavy flavor decays via kinematic reconstruction and particle identification using time-of-flight and energy loss (dE/dx) measurements.
• Calculate the nuclear modification factor RAA = (1/Ncoll) × (dN/dpT)Au+Au / (dN/dpT)p+p to quantify suppression relative to binary-collisions-scaled p+p reference.
• Apply corrections for electron background from light meson decays and semi-leptonic decays of light quarks, with uncertainty decreasing at high pT.
• Compare RAA data with theoretical predictions based on in-medium energy loss via induced gluon radiation, varying the transport coefficient  ̂q.
• Include contributions from both D and B meson decays in model comparisons, using different initial gluon densities (dNg/dy = 1000–3500) to bracket light parton energy loss.

Experimental results

Research questions

• RQ1 To what extent is the suppression of high-pT electrons in central Au+Au collisions consistent with energy loss of heavy quarks in the quark-gluon plasma?
• RQ2 How do the contributions of D and B meson decays affect the observed RAA at high pT, and what does this imply about the relative energy loss of charm vs. bottom quarks?
• RQ3 Are current theoretical models of radiative energy loss via induced gluon radiation able to reproduce the observed RAA values, especially at pT > 3.5 GeV/c?
• RQ4 Does the observed RAA for electrons at high pT exceed predictions when including B-meson contributions, suggesting underestimation of energy loss in existing models?
• RQ5 What constraints do the data place on the medium transport coefficient  ̂q for heavy quarks, particularly in the context of extreme medium densities?

Key findings

• The nuclear modification factor RAA for electrons from heavy flavor decays falls well below unity at pT ≥ 2 GeV/c, with values as low as RAA ≈ 0.3 at pT > 3.5 GeV/c, indicating strong suppression.
• The suppression is most pronounced in the 10% most central collisions, showing clear evidence for medium-induced energy loss of heavy quarks.
• Even at the highest medium densities allowed by light quark energy loss constraints (dNg/dy = 3500), model predictions including both D and B decays overpredict the observed RAA at high pT.
• The data challenge existing models of radiative energy loss via induced gluon radiation, as these models fail to reproduce the observed suppression even when including significant B-meson contributions.
• The observed RAA for electrons at high pT is comparable to that of π⁰ mesons, indicating that heavy quarks lose substantial energy in the medium.
• The systematic uncertainty on RAA decreases at high pT due to the increasing ratio of electrons from heavy flavor decays to light quark background, improving the precision of the measurement.