43 research outputs found
Hard thermal loops for soft or collinear external momenta
We consider finite temperature 1-loop diagrams with hard loop momenta and an
arbitrary number of external gauge fields when the external momenta are either
soft, or near the light cone and nearly collinear with the loop momentum. We
obtain a recursion relation for these diagrams which we translate into an
equation for their generating functional. By integrating out the soft fields
while keeping two collinear ones we find an integral equation, originally due
to Arnold, Moore, and Yaffe, which sums the bremsstrahlung and pair
annihilation contribution to the thermal photon production rate.Comment: 17 pages, title corrected, clarifying paragraph added to the
appendix, version to appear in JHE
Thermal production of ultrarelativistic right-handed neutrinos: Complete leading-order results
The thermal production of relativistic right-handed Majorana neutrinos is of
importance for models of thermal leptogenesis in the early Universe.
Right-handed neutrinos can be produced both by 1 2 decay or inverse decay
and by 2 -> 2 scattering processes. In a previous publication, we have studied
the production via 1 2 (inverse) decay processes. There we have shown that
multiple scattering mediated by soft gauge boson exchange also contributes to
the production rate at leading order, and gives a strong enhancement. Here we
complete the leading order calculation by adding 2 -> 2 scattering processes
involving either electroweak gauge bosons or third-generation quarks. We find
that processes with gauge interactions give the most important contributions.
We also obtain a new sum rule for the Hard Thermal Loop resummed fermion
propagator.Comment: 27 pages, 7 figures. Error in the matrix element for the
(subdominant) subprocess with s-channel fermion exchange corrected. This
changes the corresponding phase space integral and the constant c_V.
Numerically it increases the total 2 -> 2 rate by about 2 percent and the
complete rate by about 1 percent. The main results and conclusions are
unaffecte
The Boltzmann Equation from Quantum Field Theory
We show from first principles the emergence of classical Boltzmann equations
from relativistic nonequilibrium quantum field theory as described by the
Kadanoff-Baym equations. Our method applies to a generic quantum field, coupled
to a collection of background fields and sources, in a homogeneous and
isotropic spacetime. The analysis is based on analytical solutions to the full
Kadanoff-Baym equations, using the WKB approximation. This is in contrast to
previous derivations of kinetic equations that rely on similar physical
assumptions, but obtain approximate equations of motion from a gradient
expansion in momentum space. We show that the system follows a generalized
Boltzmann equation whenever the WKB approximation holds. The generalized
Boltzmann equation, which includes off-shell transport, is valid far from
equilibrium and in a time dependent background, such as the expanding universe.Comment: title changed, discussion extended and example added, references
adde
Thermal production of relativistic Majorana neutrinos: Strong enhancement by multiple soft scattering
The production rate of heavy Majorana neutrinos is relevant for models of
thermal leptogenesis in the early Universe. In the high temperature limit the
production can proceed via the 1 2 (inverse) decays which are allowed by
the thermal masses. We consider new production mechanisms which are obtained by
including additional soft gauge interactions with the plasma. We show that an
arbitrary number of such interactions gives leading order contributions, and we
sum all of them. The rate turns out to be smooth in the region where the 1
2 processes are kinematically forbidden. At higher temperature it is enhanced
by a factor 3 compared to the 1 2 rate.Comment: 26 pages, 8 figures; added references, added comments on 2 to 2
scattering processes, improved appearance of fig. 8, corrected typos; matches
published versio
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe