A rapidly expanding quark-gluon plasma, as it is formed in a relativistic heavy ion collision, is characterized by an anisotropic momentum distribution of partons. An anisotropic quark-gluon plasma exhibits peculiar properties, such as soft-mode instabilities, which have attracted much interest in recent years. An important practical question is how the anisotropy reveals itself in observable phenomena, which may allow us to probe how anisotropic the matter is at early times in a heavy ion reaction. Promising effects in which the anisotropy can show up are explored in two recent articles:
- Photon Production from an Anisotropic Quark-Gluon Plasma
- The Heavy Quark Potential in an Anisotropic (Viscous) Plasma.
In the first article, the authors show that the yield of thermal photons from an anisotropic plasma is strongly dependent on the emission angle with respect to the beam axis. In the second article it is shown that the momentum anisotropy of the plasma induced an anisotropy of the screened potential of a heavy quark-antiquark pair. These results suggest that the phenomenology of quark-gluon plasma signatures needs to be re-investigated broadly to fully explore the implications of the unavoidable plasma anisotropies at early times in the fireball evolution. Note that even the minimal shear viscosity-to-entropy density ratio of $1/4\pi$ implies a value $\xi \sim 1$ for the anisotropy parameter at early times ($T\tau \sim 1$).