In a framework that makes close contact with modern effective field theories for nonrelativistic bound states at zero temperature, we study the real-time evolution of a static quark-antiquark pair in a medium of gluons and light quarks at finite temperature. For temperatures ranging from values larger to smaller than the inverse distance of the quark and antiquark, $1/r$, and at short distances, we derive the potential between the two static sources, and calculate their energy and thermal decay width. Two mechanisms contribute to the thermal decay width: the imaginary part of the gluon self-energy induced by the Landau damping phenomenon, and the quark-antiquark color-singlet to color-octet thermal breakup. Parametrically, the first mechanism dominates for temperatures such that the Debye mass is larger than the binding energy, while the latter, which we quantify here for the first time, dominates for temperatures such that the Debye mass is smaller than the binding energy. If the Debye mass is of the same order as $1/r$, our results are in agreement with a recent calculation of the static Wilson loop at finite temperature. For temperatures smaller than $1/r$, we find new contributions to the potential, both real and imaginary, which may be relevant to understand the onset of heavy quarkonium dissociation in a thermal medium. DOI: http://dx.doi.org/10.1103/PhysRevD.78.014017 © 2008 The American Physical Society

We calculate the heavy quarkonium energy levels and decay widths in a quark-gluon plasma, whose temperature T and screening mass m _D satisfy the hierarchy mα _s ≫ T ≫ mα _s ² ≫m _D (m being the heavy-quark mass), at order mα _s ⁵ . We first sequentially integrate out the scales m, mα _s and T, and, next, we carry out the calculations in the resulting effective theory using techniques of integration by regions. A collinear region is identified, which contributes at this order. We also discuss the implications of our results concerning heavy quarkonium suppression in heavy ion collisions.

We compute the production rate of the energy density carried by gravitational waves emitted by a Standard Model plasma in thermal equilibrium, consistently to leading order in coupling constants for momenta k ∼ πT. Summing up the contributions from the full history of the universe, the highest temperature of the radiation epoch can be constrained by the so-called N_eff parameter. The current theoretical uncertainty ∆N_eff≤ 10⁻³ corresponds to T_max≤ 2 × 10¹⁷ GeV. In the course of the computation, we show how a subpart of the production rate can be determined with the help of standard packages, even if subsequently an IR subtraction and thermal resummation need to be implemented.