2,592 research outputs found
Photon Emission from Ultrarelativistic Plasmas
The emission rate of photons from a hot, weakly coupled ultrarelativistic
plasma is analyzed. Leading-log results, reflecting the sensitivity of the
emission rate to scattering events with momentum transfers from to ,
have previously been obtained. But a complete leading-order treatment requires
including collinearly enhanced, inelastic processes such as bremsstrahlung.
These inelastic processes receive O(1) modifications from multiple scattering
during the photon emission process, which limits the coherence length of the
emitted radiation (the Landau-Pomeranchuk-Migdal effect). We perform a
diagrammatic analysis to identify, and sum, all leading-order contributions. We
find that the leading-order photon emission rate is not sensitive to
non-perturbative scale dynamics. We derive an integral equation for the
photon emission rate which is very similar to the result of Migdal in his
original discussion of the LPM effect. The accurate solution of this integral
equation for specific theories of interest will be reported in a companion
paper.Comment: 50 pages, 20 figures. Added references and minor rewordings:
published versio
Large N Quantum Time Evolution Beyond Leading Order
For quantum theories with a classical limit (which includes the large N
limits of typical field theories), we derive a hierarchy of evolution equations
for equal time correlators which systematically incorporate corrections to the
limiting classical evolution. Explicit expressions are given for
next-to-leading order, and next-to-next-to-leading order time evolution. The
large N limit of N-component vector models, and the usual semiclassical limit
of point particle quantum mechanics are used as concrete examples. Our
formulation directly exploits the appropriate group structure which underlies
the construction of suitable coherent states and generates the classical phase
space. We discuss the growth of truncation error with time, and argue that
truncations of the large-N evolution equations are generically expected to be
useful only for times short compared to a ``decoherence'' time which scales
like N^{1/2}.Comment: 36 pages, 2 eps figures, latex, uses revtex, epsfig, float
Non-perturbative equivalences among large N gauge theories with adjoint and bifundamental matter fields
We prove an equivalence, in the large N limit, between certain U(N) gauge
theories containing adjoint representation matter fields and their orbifold
projections. Lattice regularization is used to provide a non-perturbative
definition of these theories; our proof applies in the strong coupling, large
mass phase of the theories. Equivalence is demonstrated by constructing and
comparing the loop equations for a parent theory and its orbifold projections.
Loop equations for both expectation values of single-trace observables, and for
connected correlators of such observables, are considered; hence the
demonstrated non-perturbative equivalence applies to the large N limits of both
string tensions and particle spectra.Comment: 40 pages, JHEP styl
Detecting Mens Rea in the Brain
What if the widely used Model Penal Code (MPC) assumes a distinction between mental states that doesn’t actually exist? The MPC assumes, for instance, that there is a real distinction in real people between the mental states it defines as “knowing” and “reckless.” But is there?
If there are such psychological differences, there must also be brain differences. Consequently, the moral legitimacy of the Model Penal Code’s taxonomy of culpable mental states – which punishes those in defined mental states differently – depends on whether those mental states actually correspond to different brain states in the way the MPC categorization assumes
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