110 research outputs found
Angular momentum non-conserving decays in isotropic media
Various processes that are forbidden in the vacuum due to angular momentum
conservation can occur in a medium that is isotropic and does not carry any
angular momentum. We illustrate this by considering explicitly two examples.
The first one is the decay of a spin-0 particle into a photon and another
spin-0 particle, using a model involving the Yukawa interactions of the scalar
particles with a charged fermion field. The second one involves the decay of a
neutrino into another neutrino and a graviton, in the standard model of
particle interactions augmented with the linearized gravitational couplings.Comment: 22 pages, Latex, uses axodraw.sty. 3 figures embedded in the tex
file. This paper contains the same material as that presented in two earlier
papers, arXiv:0901.2981 and arXiv:0901.2982, written by us. This version
supersedes those two paper
Gravitational couplings of charged leptons in a medium
We calculate the leading order matter-induced corrections to the
gravitational interactions of charged leptons and their antiparticles in a
medium that contains electrons but not the other charged leptons, such as
normal matter. The gravitational coupling, which is universal at the tree
level, is found to be flavor-dependent, and also different for the
corresponding antiparticles, when the corrections of are taken into
account. General expressions are obtained for the matter-induced corrections to
the gravitational mass in a generic matter background, and explicit formulas
for those corrections are given in terms of the macroscopic parameters of the
medium for particular conditions of the background gases.Comment: Latex, axodraw, 39 pages. Added a few stylistic corrections and
clarifying statements in the treatment of the photon tadpole diagra
Soft Photon Production Rate in Resummed Perturbation Theory of High Temperature QCD
We calculate the production rate of soft real photons from a hot quark --
gluon plasma using Braaten -- Pisarski's perturbative resummation method. To
leading order in the QCD coupling constant we find a logarithmically
divergent result for photon energies of order , where is the plasma
temperature. This divergent behaviour is due to unscreened mass singularities
in the effective hard thermal loop vertices in the case of a massless external
photon.Comment: 13 pages (2 figures not included), PLAINTEX, LPTHE-Orsay 93/46, BI-TP
93/5
Thermal quark production in ultra-relativistic nuclear collisions
We calculate thermal production of u, d, s, c and b quarks in
ultra-relativistic heavy ion collisions. The following processes are taken into
account: thermal gluon decay (g to ibar i), gluon fusion (g g to ibar i), and
quark-antiquark annihilation (jbar j to ibar i), where i and j represent quark
species. We use the thermal quark masses, ,
in all the rates. At small mass (), the production is largely
dominated by the thermal gluon decay channel. We obtain numerical and analytic
solutions of one-dimensional hydrodynamic expansion of an initially pure glue
plasma. Our results show that even in a quite optimistic scenario, all quarks
are far from chemical equilibrium throughout the expansion. Thermal production
of light quarks (u, d and s) is nearly independent of species. Heavy quark (c
and b) production is quite independent of the transition temperature and could
serve as a very good probe of the initial temperature. Thermal quark production
measurements could also be used to determine the gluon damping rate, or
equivalently the magnetic mass.Comment: 14 pages (latex) plus 6 figures (uuencoded postscript files);
CERN-TH.7038/9
Two-Loop Self-Energy and Multiple Scattering at Finite Temperature
One and two loop self-energies are worked out explicitly for a heavy scalar
field interacting weakly with a light self-interacting scalar field at finite
temperature. The ring/daisy diagrams and a set of necklace diagrams can be
summed simultaneously. This simple model serves to illustrate the connection
between multi-loop self-energy diagrams and multiple scattering in a medium.Comment: 15 pages including 7 figures; v2. with appendix added, accepted by
PR
Structure of the Quark Propagator at High Temperature
In the high temperature, chirally invariant phase of QCD, the quark
propagator is shown to have two sets of poles with different dispersion
relations. A reflection property in momentum space relates all derivatives at
zero-momentum of the particle and hole energies, the particle and hole damping
rates, and the particle and hole residues. No use is made of perturbation
theory.Comment: 8 pages, Latex twocolum
Two-loop Compton and annihilation processes in thermal QCD
We calculate the Compton and annihilation production of a soft static lepton
pair in a quark-gluon plasma in the two-loop approximation. We work in the
context of the effective perturbative expansion based on the resummation of
hard thermal loops. Double counting is avoided by subtracting appropriate
counterterms. It is found that the two-loop diagrams give contributions of the
same order as the one-loop diagram. Furthermore, these contributions are
necessary to obtain agreement with the naive perturbative expansion in the
limit of vanishing thermal masses.Comment: Latex, 24 pages, postscript figures included with the package
graphic
Reply to "Comment on 'Light-Front Schwinger Model at Finite Temperature'"
In hep-th/0310278, Blankleider and Kvinikhidze propose an alternate thermal
propagator for the fermions in the light-front Schwinger model. We show that
such a propagator does not describe correctly the thermal behavior of fermions
in this theory and, as a consequence, the claims made in their paper are not
correct.Comment: 3pages, version to be published in Phys. Rev.
Two mechanisms for the elimination of pinch singularities in out of equilibrium thermal field theories
We analyze ill-defined pinch singularities characteristic of out of
equilibrium thermal field theories. We identify two mechanisms that eliminate
pinching even at the single self-energy insertion approximation to the
propagator: the first is based on the vanishing of phase space at the singular
point (threshold effect). It is effective in QED with a massive electron and a
massless photon. In massless QCD, this mechanism fails, but the pinches cancel
owing to the second mechanism, i.e., owing to the spinor/tensor structure of
the single self-energy insertion contribution to the propagator. The
constraints imposed on distribution functions are very reasonable.Comment: 24 pages, Latex, no figures, revised version, many minor changes and
correction
Thermal Field Theory and Generalized Light Front Coordinates
The dependence of thermal field theory on the surface of quantization and on
the velocity of the heat bath is investigated by working in general coordinates
that are arbitrary linear combinations of the Minkowski coordinates. In the
general coordinates the metric tensor is non-diagonal. The
Kubo, Martin, Schwinger condition requires periodicity in thermal correlation
functions when the temporal variable changes by an amount
. Light front quantization fails since
, however various related quantizations are possible.Comment: 10 page
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