1,476 research outputs found
Behaviour of Charged Spinning Massless Particles
We revisit the classical theory of a relativistic massless charged point
particle with spin and interacting with an external electromagnetic field. In
particular, we give a proper definition of its kinetic energy and its total
energy, the latter being conserved when the external field is stationary. We
also write the conservation laws for the linear and angular momenta. Finally,
we find that the particle's velocity may differ from as a result of the
spin---electromagnetic field interaction, without jeopardizing Lorentz
invariance.Comment: PDFtex file, 20 pages, 3 figures Revised version published in
Symmetry (Basel
Scattering Amplitudes For All Masses and Spins
We introduce a formalism for describing four-dimensional scattering
amplitudes for particles of any mass and spin. This naturally extends the
familiar spinor-helicity formalism for massless particles to one where these
variables carry an extra SU(2) little group index for massive particles, with
the amplitudes for spin S particles transforming as symmetric rank 2S tensors.
We systematically characterise all possible three particle amplitudes
compatible with Poincare symmetry. Unitarity, in the form of consistent
factorization, imposes algebraic conditions that can be used to construct all
possible four-particle tree amplitudes. This also gives us a convenient basis
in which to expand all possible four-particle amplitudes in terms of what can
be called "spinning polynomials". Many general results of quantum field theory
follow the analysis of four-particle scattering, ranging from the set of all
possible consistent theories for massless particles, to spin-statistics, and
the Weinberg-Witten theorem. We also find a transparent understanding for why
massive particles of sufficiently high spin can not be "elementary". The Higgs
and Super-Higgs mechanisms are naturally discovered as an infrared unification
of many disparate helicity amplitudes into a smaller number of massive
amplitudes, with a simple understanding for why this can't be extended to
Higgsing for gravitons. We illustrate a number of applications of the formalism
at one-loop, giving few-line computations of the electron (g-2) as well as the
beta function and rational terms in QCD. "Off-shell" observables like
correlation functions and form-factors can be thought of as scattering
amplitudes with external "probe" particles of general mass and spin, so all
these objects--amplitudes, form factors and correlators, can be studied from a
common on-shell perspective.Comment: 79 page
Schwarzschild-like black holes: Light-like trajectories and massless scalar absorption
Black holes are among the most intriguing objects in nature. They are
believed to be fully described by General Relativity (GR), and the
astrophysical black holes are expected to belong to the Kerr family, obeying
the no-hair theorems. Alternative theories of gravity or parameterized
deviations of GR allow black hole solutions, which have additional parameters
other than mass and angular momentum. We analyze a Schwarzschild-like metric,
proposed by Johannsen and Psaltis, characterized by its mass and a deformation
parameter. We compute the absorption cross section of massless scalar waves for
different values of this deformation parameter and compare it with the
corresponding scalar absorption cross section of the Schwarzschild black hole.
We also present analytical approximations for the absorption cross section in
the high-frequency regime. We check the consistence of our results comparing
the numerical and analytical approaches, finding excellent agreement.Comment: 8 pages, 14 figure
Global Strings in High Density QCD
We show that several types of global strings occur in colour superconducting
quark matter due to the spontaneous violation of relevant U(1) symmetries.
These include the baryon U(1)_B, and approximate axial U(1)_A symmetries as
well as an approximate U(1)_S arising from kaon condensation. We discuss some
general properties of these strings and their interactions. In particular, we
demonstrate that the U(1)_A strings behave as superconducting strings. We draw
some parallels between these strings and global cosmological strings and
discuss some possible implications of these strings to the physics in neutron
star cores.Comment: LaTeX JHEP-format (26 pages) Option in source for REVTeX4 forma
Hawking radiation: a particle physics perspective
It has recently become fashionable to regard black holes as elementary
particles. By taking this suggestion seriously it is possible to cobble
together an elementary particle physics based estimate for the decay rate
.
This estimate of the spontaneous emission rate contains two free parameters
which may be fixed by demanding that the high energy end of the spectrum of
emitted quanta match a blackbody spectrum at the Hawking temperature. The
calculation, though technically trivial, has important conceptual implications:
(1) The existence of Hawking radiation from black holes is ultimately dependent
only on the fact that massless quanta (and all other forms of matter) couple to
gravity. (2) The thermal nature of the Hawking spectrum depends only on the
fact that the number of internal states of a large mass black hole is enormous.
(3) Remarkably, the resulting formula for the decay rate gives meaningful
answers even when extrapolated to low mass black holes. The analysis strongly
supports the scenario of complete evaporation as the endpoint of the Hawking
radiation process (no naked singularity, no stable massive remnant).Comment: (15 pages) RevTe
Dynamical Boson Stars
The idea of stable, localized bundles of energy has strong appeal as a model
for particles. In the 1950s John Wheeler envisioned such bundles as smooth
configurations of electromagnetic energy that he called {\em geons}, but none
were found. Instead, particle-like solutions were found in the late 1960s with
the addition of a scalar field, and these were given the name {\em boson
stars}. Since then, boson stars find use in a wide variety of models as sources
of dark matter, as black hole mimickers, in simple models of binary systems,
and as a tool in finding black holes in higher dimensions with only a single
killing vector. We discuss important varieties of boson stars, their dynamic
properties, and some of their uses, concentrating on recent efforts.Comment: 79 pages, 25 figures, invited review for Living Reviews in
Relativity; major revision in 201
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