Light Messengers From Heavy Quarks: What Electrons Reveal About the Medium Produced at RHIC
by Ralf Averbeck
High energy collisions of gold nuclei at RHIC produce a hot and dense phase of matter that resembles the state of the universe a few microseconds after the big bang. Remarkably, this primordial substance made of strongly interacting constituents was found to be characterized best as an almost "perfect" fluid, i.e. a medium with almost vanishing viscosity. Key observations that led to this conclusion include a strong suppression of high energy pions and other light particles, commonly interpreted in terms of energy loss of light quarks and gluons while these propagate through the medium. Furthermore, a prominent azimuthal anisotropy of particle emission was observed to scale with the number of constituent quarks present in the different particle species, which provides evidence that collective behavior characteristic for a low viscosity fluid develops already in a very early stage of the collision when quarks and gluons are the relevant degrees of freedom. However, in nature all fluids have non-zero viscosity. This might reflect a conjectured quantum mechanical lower limit, which was derived using a relation between string theories and gravitational physics. One big question concerning the medium produced at RHIC is how close it comes to this limit.
An investigation of the production and propagation of heavy flavor, i.e. particles carrying heavy quarks (charm or bottom), allows us to better understand and quantify the properties of the produced medium. Due to their large mass these particles are produced almost exclusively in the first collisions of the beam nucleons such that they can be used to probe the whole evolution of the medium.
One approach to study heavy-flavor production is to measure electrons from the decay of such particles. The PHENIX experiment has been optimized for electron measurements. Recently, PHENIX has published two papers on electrons from heavy-flavor decays in proton+proton and gold+gold collisions.
The electron spectrum from heavy-flavor decays measured in proton+proton collisions  over a momentum range in which both charm decays (at low momenta) and bottom decays (at high momenta) should be relevant can be described reasonably well by a state-of-the-art model calculation in the framework of quantum chromodynamics, the established theory of the strong interaction.
In gold+gold collisions, the total yield of such electrons is consistent with the yield measured in proton+proton collisions multiplied by a simple geometrical factor to account for the fact that one gold+gold collision includes many nucleon+nucleon collisions . This geometrical scaling simply indicates that the heavy quarks initially produced in nucleon+nucleon collisions do not disappear in a gold+gold collision, and that further mechanisms that could lead to additional in-medium production are not important at RHIC.
The momentum distributions of electrons from heavy-flavor decays, however, carry an imprint of the strong interaction of heavy flavor with the medium as is shown in Fig. 1 . Going from peripheral to central gold+gold collisions these electrons are more and more suppressed at large momenta. This exciting observation indicates that the medium is so dense that even heavy flavor suffers from significant energy loss. In the most central collisions the suppression is almost as strong as observed for pions as the upper panel of Fig. 2 demonstrates, thus severely constraining models describing the underlying energy loss mechanism.
Heavy flavor does not only lose energy in the medium, it even participates in the collective flow which is commonly seen as an indication for early thermalization at RHIC. The most prominent flow component is the second Fourier harmonic of particle emission relative to the orientation of the reaction plane. The parameter v_2, which is a measure for the strength of this so-called elliptic flow, is shown for electrons from heavy-flavor decays in the lower panel of Fig. 2, indicating the pronounced flow of heavy flavor.
Energy loss and elliptic flow are related to the propagation of heavy flavor through the produced medium. Microscopic models can help to estimate the transport properties of the bulk medium. Models that simultaneously describe the suppression and elliptic flow of electrons from heavy-flavor decays as shown in Fig. 2 suggest that the medium's viscosity is within a factor of 1.5 to 3 to the conjectured lower limit, making it the most perfect fluid ever observed.