289 research outputs found
Shining a Gluon Beam Through Quark-Gluon Plasma
We compute the energy density radiated by a quark undergoing circular motion
in strongly coupled supersymmetric Yang-Mills plasma. If it
were in vacuum, this quark would radiate a beam of strongly coupled radiation
whose angular distribution has been characterized and is very similar to that
of synchrotron radiation produced by an electron in circular motion in
electrodynamics. Here, we watch this beam of gluons getting quenched by the
strongly coupled plasma. We find that a beam of gluons of momenta is attenuated rapidly, over a distance in
a plasma with temperature . As the beam propagates through the plasma at the
speed of light, it sheds trailing sound waves with momenta .
Presumably these sound waves would thermalize in the plasma if they were not
hit soon after their production by the next pulse of gluons from the
lighthouse-like rotating quark. At larger and larger , the trailing sound
wave becomes less and less prominent. The outward going beam of gluon radiation
itself shows no tendency to spread in angle or to shift toward larger
wavelengths, even as it is completely attenuated. In this regard, the behavior
of the beam of gluons that we analyze is reminiscent of the behavior of jets
produced in heavy ion collisions at the LHC that lose a significant fraction of
their energy without appreciable change in their angular distribution or their
momentum distribution as they plow through the strongly coupled quark-gluon
plasma produced in these collisions.Comment: 16 pages, 4 figure
Instability of the massive Klein-Gordon field on the Kerr spacetime
We investigate the instability of the massive scalar field in the vicinity of
a rotating black hole. The instability arises from amplification caused by the
classical superradiance effect. The instability affects bound states: solutions
to the massive Klein-Gordon equation which tend to zero at infinity. We
calculate the spectrum of bound state frequencies on the Kerr background using
a continued fraction method, adapted from studies of quasinormal modes. We
demonstrate that the instability is most significant for the ,
state, for . For a fast rotating hole () we find
a maximum growth rate of ,
at . The physical implications are discussed.Comment: Added references. 27 pages, 7 figure
Coarsening dynamics at unstable crystal surfaces
In this paper we focus on crystal surfaces led out of equilibrium by a growth
or erosion process. As a consequence of that the surface may undergo
morphological instabilities and develop a distinct structure: ondulations,
mounds or pyramids, bunches of steps, ripples. The typical size of the emergent
pattern may be fixed or it may increase in time through a coarsening process
which in turn may last forever or it may be interrupted at some relevant length
scale. We study dynamics in three different cases, stressing the main physical
ingredients and the main features of coarsening: a kinetic instability, an
energetic instability, and an athermal instability.Comment: 12 pages. Several minor changes. To appear in a Comptes Rendus
Physique special issue on "Coarsening Dynamics", see
https://sites.google.com/site/ppoliti/crp-special-issu
Fast Magnetic Reconnection and Spontaneous Stochasticity
Magnetic field-lines in astrophysical plasmas are expected to be frozen-in at
scales larger than the ion gyroradius. The rapid reconnection of magnetic flux
structures with dimensions vastly larger than the gyroradius requires a
breakdown in the standard Alfv\'en flux-freezing law. We attribute this
breakdown to ubiquitous MHD plasma turbulence with power-law scaling ranges of
velocity and magnetic energy spectra. Lagrangian particle trajectories in such
environments become "spontaneously stochastic", so that infinitely-many
magnetic field-lines are advected to each point and must be averaged to obtain
the resultant magnetic field. The relative distance between initial magnetic
field lines which arrive to the same final point depends upon the properties of
two-particle turbulent dispersion. We develop predictions based on the
phenomenological Goldreich & Sridhar theory of strong MHD turbulence and on
weak MHD turbulence theory. We recover the predictions of the Lazarian &
Vishniac theory for the reconnection rate of large-scale magnetic structures.
Lazarian & Vishniac also invoked "spontaneous stochasticity", but of the
field-lines rather than of the Lagrangian trajectories. More recent theories of
fast magnetic reconnection appeal to microscopic plasma processes that lead to
additional terms in the generalized Ohm's law, such as the collisionless Hall
term. We estimate quantitatively the effect of such processes on the
inertial-range turbulence dynamics and find them to be negligible in most
astrophysical environments. For example, the predictions of the
Lazarian-Vishniac theory are unchanged in Hall MHD turbulence with an extended
inertial range, whenever the ion skin depth is much smaller than the
turbulent integral length or injection-scale Comment: 31 pages, 5 figure
Dynamics of Waves and Patterns (hybrid meeting)
The dynamics of waves and patterns play a significant role in the sciences, especially in fluid mechanics, material science, neuroscience and ecology. The mathematical treatment interconnects several areas, ranging from evolution equations and functional analysis to dynamical systems, geometry, topology, and stochastic as well as numerical analysis. This workshop has specifically focussed on dynamic stability on extended domains, bifurcations of waves and patterns, effects of stochastic driving, and spatio-temporal inhomogenities. During the workshop, multiple new directions, collaborations, and very interesting scientific conversations arose across the entire field
Gauge Fields and Strings
Based on his own work, the author synthesizes the most promising approaches and ideals in field theory today. He presents such subjects as statistical mechanics, quantum field theory and their interrelation, continuous global symmetry, non-Abelian gauge fields, instantons and the quantam theory of loops, and quantum strings and random surfaces. This book is aimed at postgraduate students studying field theory and statistical mechanics, and for research workers in continuous global theory
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