28,243 research outputs found
Prima Facie Questions in Quantum Gravity
The long history of the study of quantum gravity has thrown up a complex web
of ideas and approaches. The aim of this article is to unravel this web a
little by analysing some of the {\em prima facie\/} questions that can be asked
of almost any approach to quantum gravity and whose answers assist in
classifying the different schemes. Particular emphasis is placed on (i) the
role of background conceptual and technical structure; (ii) the role of
spacetime diffeomorphisms; and (iii) the problem of time.Comment: 20,IC/TP/0
Testing Unimodular Gravity
We consider models of gravitation that are based on unimodular general
coordinate transformations (GCT). These transformations include only those
which do not change the determinant of the metric. We treat the determinant as
a separate field which transforms as a scalar under unimodular GCT. We consider
a class of such theories. In general, these theories do not transform
covariantly under the full GCT. We characterize the violation of general
coordinate invariance by introducing a new parameter. We show that the theory
is consistent with observations for a wide range of this parameter. This
parameter may serve as a test for possible violations of general coordinate
invariance. We also consider the cosmic evolution within the framework of these
models. We show that in general we do not obtain consistent cosmological
solutions if we assume the standard cosmological constant or the standard form
of non-relativistic matter. We propose a suitable generalization which is
consistent with cosmology. We fit the resulting model to the high redshift
supernova data. We find that we can obtain a good fit to this data even if
include only a single component, either cosmological constant or
non-relativistic matter.Comment: 14 pages one figure, significantly revised with several new results
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Decoherence induced by Smith-Purcell radiation
The interaction between charged particles and the vacuum fluctuations of the
electromagnetic field induces decoherence, and therefore affects the contrast
of fringes in an interference experiment. In this article we show that if a
double slit experiment is performed near a conducting grating, the fringe
visibility is reduced. We find that the reduction of contrast is proportional
to the number of grooves in the conducting surface, and that for realistic
values of the parameters it could be large enough to be observed. The effect
can be understood in terms of the Smith-Purcell radiation produced by the
surface currents induced in the conductor.Comment: 10 pages, 3 figures. Improved discussion on experimental
perspectives. References added. Version to appear in Phys. Rev.
The Equivalence Principle in the Non-baryonic Regime
We consider the empirical validity of the equivalence principle for
non-baryonic matter. Working in the context of the TH\epsilon\mu formalism, we
evaluate the constraints experiments place on parameters associated with
violation of the equivalence principle (EVPs) over as wide a sector of the
standard model as possible. Specific examples include new parameter constraints
which arise from torsion balance experiments, gravitational red shift,
variation of the fine structure constant, time-dilation measurements, and
matter/antimatter experiments. We find several new bounds on EVPs in the
leptonic and kaon sectors.Comment: 22 pages, late
Duality in Non-Trivially Compactified Heterotic Strings
We study the implications of duality symmetry on the analyticity properties
of the partition function as it depends upon the compactification length. In
order to obtain non-trivial compactifications, we give a physical prescription
to get the Helmholtz free energy for any heterotic string supersymmetric or
not. After proving that the free energy is always invariant under the duality
transformation and getting the zero temperature
theory whose partition function corresponds to the Helmholtz potential, we show
that the self-dual point is a generic singularity
as the Hagedorn one. The main difference between these two critical
compactification radii is that the term producing the singularity at the
self-dual point is finite for any . We see that this behavior at
actually implies a loss of degrees of freedom below that point.Comment: (Preprint No. FTUAM-92/12) 17 page
Determining fundamental parameters from the chargino sector in Left-Right Supersymmetric models
Analytical expressions relating the fundamental parameters describing the
chargino sector in the context of the Left-Right Supersymmetric model are
constructed. A general complex extension of the real non-symmetric chargino
mass matrix including all possible CP-violating phases is considered. The
method used for such a effects is the projector formalism based on the explicit
knowledge of two unitary matrices diagonalizing the chargino mass matrix. Some
possible scenarios allowing us to extract analytical and numerical values for
the unknown parameters are considered. Moreover, an algorithm allowing us to
disentangle the fundamental parameters of the chargino sector, based on
possible measurements of some class of cross-section observables related to the
chargino pair production in annihilation processes, is described.
Some comparisons with the corresponding results in the context of the Minimal
Supersymmetric Standard Model are given.Comment: 48 pages, 13 figure
Electric charge quantization without anomalies?
In gauge theories like the standard model, the electric charges of the
fermions can be heavily constrained from the classical structure of the theory
and from the cancellation of anomalies. We argue that the anomaly conditions
are not quite as well motivated as the classical constraints, since it is
possible that new fermions could exist which cancel potential anomalies. For
this reason we examine the classically allowed electric charges of the known
fermions and we point out that the electric charge of the tau neutrino is
classically allowed to be non-zero. The experimental bound on the electric
charge of the tau neutrino is many orders of magnitude weaker than for any
other known neutrino. We discuss possible modifications of the minimal standard
model such that electric charge is quantized classically.Comment: 10 McGill/93-3
Pokrovsky-Talapov commensurate-incommensurate transition in the CO/Pd(100) system
X-ray-diffraction measurements have been carried out for a monolayer of CO on the Pd(100) surface. Because of equilibrium with the gas phase, at constant pressure a series of structures is formed with varying temperature (varying coverage). One structure at 350 K selected for crystallographic analysis, is found to be a commensurate lattice with p2gg symmetry of CO molecules bound in substrate-bridge sites. The vibration amplitudes of the molecules are substantially larger in-plane than out-of-plane. Upon cooling, a phase transition is crossed beyond which the structure becomes incommensurate. Symmetry considerations and the measured exponent of 0.5±0.05 establish this transition to be in the Pokrovsky-Talapov universality class
Prospects for observations of high-energy cosmic tau neutrinos
We study prospects for the observations of high-energy cosmic tau neutrinos
(E \geq 10^6 GeV) originating from proton acceleration in the cores of active
galactic nuclei. We consider the possibility that vacuum flavor neutrino
oscillations induce a tau to muon neutrino flux ratio greatly exceeding the
rather small value expected from intrinsic production. The criterias and event
rates for under water/ice light Cerenkov neutrino telescopes are given by
considering the possible detection of downgoing high-energy cosmic tau
neutrinos through characteristic double shower events.Comment: 10 pages, Revtex, 3 figures included with eps
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