314 research outputs found
The Relativistic Particle: Dirac observables and Feynman propagator
We analyze the algebra of Dirac observables of the relativistic particle in
four space-time dimensions. We show that the position observables become
non-commutative and the commutation relations lead to a structure very similar
to the non-commutative geometry of Deformed Special Relativity (DSR). In this
framework, it appears natural to consider the 4d relativistic particle as a
five dimensional massless particle. We study its quantization in terms of wave
functions on the 5d light cone. We introduce the corresponding five-dimensional
action principle and analyze how it reproduces the physics of the 4d
relativistic particle. The formalism is naturally subject to divergences and we
show that DSR arises as a natural regularization: the 5d light cone is
regularized as the de Sitter space. We interpret the fifth coordinate as the
particle's proper time while the fifth moment can be understood as the mass.
Finally, we show how to formulate the Feynman propagator and the Feynman
amplitudes of quantum field theory in this context in terms of Dirac
observables. This provides new insights for the construction of observables and
scattering amplitudes in DSR.Comment: 14 pages, Revtex
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
Doubly Special Relativity and de Sitter space
In this paper we recall the construction of Doubly Special Relativity (DSR)
as a theory with energy-momentum space being the four dimensional de Sitter
space. Then the bases of the DSR theory can be understood as different
coordinate systems on this space. We investigate the emerging geometrical
picture of Doubly Special Relativity by presenting the basis independent
features of DSR that include the non-commutative structure of space-time and
the phase space algebra. Next we investigate the relation between our geometric
formulation and the one based on quantum -deformations of the
Poincar\'e algebra. Finally we re-derive the five-dimensional differential
calculus using the geometric method, and use it to write down the deformed
Klein-Gordon equation and to analyze its plane wave solutions.Comment: 26 pages, one formula (67) corrected; some remarks adde
Snyder's Quantized Space-time and De Sitter Special Relativity
There is a one-to-one correspondence between Snyder's model in de Sitter
space of momenta and the \dS-invariant special relativity. This indicates that
physics at the Planck length and the scale should be
dual to each other and there is in-between gravity of local \dS-invariance
characterized by a dimensionless coupling constant .Comment: 8 page
Signals from the Noise: Image Stacking for Quasars in the FIRST Survey
We present a technique to explore the radio sky into the nanoJansky regime by
employing image stacking using the FIRST survey. We first discuss the
non-intuitive relationship between the mean and median values of a distribution
that is dominated by noise, followed by an analysis of the systematic effects
present in FIRST's 20cm VLA snapshot images. Image stacking allows us to
recover the properties of source populations with fluxes a factor of 30 or more
below the rms noise level. Mean estimates of radio flux density, luminosity,
etc., are derivable for any source class having arcsecond positional accuracy.
We use this technique to compute the mean radio properties for 41,295 quasars
from the SDSS DR3 catalog. There is a tight correlation between optical and
radio luminosity, with the radio luminosity increasing as the 0.85 power of
optical luminosity. This implies declining radio-loudness with optical
luminosity: the most luminous objects (M=-28.5) have average radio-to-optical
ratios 3 times lower than the least luminous objects (M=-20). There is also a
striking correlation between optical color and radio loudness: quasars that are
either redder or bluer than the norm are brighter radio sources, with objects
0.8 magnitudes redder than the SDSS composite spectrum having radio-loudness
ratios that are higher by a factor of 10. We explore the longstanding question
of whether a radio-loud/radio-quiet dichotomy exists in quasars, finding that
optical selection effects probably dominate the distribution function of radio
loudness, which has at most a modest (~20%) inflection between the radio-loud
and radio-quiet ends of the distribution. We also find, surprisingly, that
broad absorption line quasars have higher mean radio flux densities, with the
greatest disparity arising in the rare low-ionization BAL subclass.Comment: 17 pages, 19 figures, accepted by ApJ; corrected error in absolute
magnitude
Scalar field propagation in the phi^4 kappa-Minkowski model
In this article we use the noncommutative (NC) kappa-Minkowski phi^4 model
based on the kappa-deformed star product, ({*}_h). The action is modified by
expanding up to linear order in the kappa-deformation parameter a, producing an
effective model on commutative spacetime. For the computation of the tadpole
diagram contributions to the scalar field propagation/self-energy, we
anticipate that statistics on the kappa-Minkowski is specifically
kappa-deformed. Thus our prescription in fact represents hybrid approach
between standard quantum field theory (QFT) and NCQFT on the kappa-deformed
Minkowski spacetime, resulting in a kappa-effective model. The propagation is
analyzed in the framework of the two-point Green's function for low,
intermediate, and for the Planckian propagation energies, respectively.
Semiclassical/hybrid behavior of the first order quantum correction do show up
due to the kappa-deformed momentum conservation law. For low energies, the
dependence of the tadpole contribution on the deformation parameter a drops out
completely, while for Planckian energies, it tends to a fixed finite value. The
mass term of the scalar field is shifted and these shifts are very different at
different propagation energies. At the Planckian energies we obtain the
direction dependent kappa-modified dispersion relations. Thus our
kappa-effective model for the massive scalar field shows a birefringence
effect.Comment: 23 pages, 2 figures; To be published in JHEP. Minor typos corrected.
Shorter version of the paper arXiv:1107.236
Kinematics of a relativistic particle with de Sitter momentum space
We discuss kinematical properties of a free relativistic particle with
deformed phase space in which momentum space is given by (a submanifold of) de
Sitter space. We provide a detailed derivation of the action, Hamiltonian
structure and equations of motion for such free particle. We study the action
of deformed relativistic symmetries on the phase space and derive explicit
formulas for the action of the deformed Poincare' group. Finally we provide a
discussion on parametrization of the particle worldlines stressing analogies
and differences with ordinary relativistic kinematics.Comment: RevTeX, 12 pages, no figure
1-D Harmonic Oscillator in Snyder Space, the Classic and the Quantum
The 1-D dimension harmonic oscillator in Snyder space is investigated in its
classical and quantum versions. The classical trajectory is obtained and the
semiclassical quantization from the phase space trajectories is discussed. In
the meanwhile, an effective cutoff to high frequencies is found. The quantum
version is developed and an equivalent usual harmonic oscillator is obtained
through an effective mass and an effective frequency introduced in the model.
This modified parameters give us an also modified energy spectra.Comment: 8 pages, 2 figure
Linearly-realised Worldsheet Supersymmetry in pp-wave Background
We study the linearly-realised worldsheet supersymmetries in the ``massive''
type II light-cone actions for pp-wave backgrounds. The pp-waves have have
16+N_sup Killing spinors, comprising 16 ``standard'' Killing spinors that occur
in any wave background, plus N_sup ``supernumerary'' Killing spinors (0\le
N_sup \le 16) that occur only for special backgrounds. We show that only the
supernumerary Killing spinors give rise to linearly-realised worldsheet
supersymmetries after light-cone gauge fixing, while the 16 standard Killing
spinors describe only non-linearly realised inhomogeneous symmetries. We also
study the type II actions in the physical gauge, and we show that although in
this case the actions are not free, there are now linearly-realised
supersymmetries coming both from the standard and the supernumerary Killing
spinors. In the physical gauge, there are no mass terms for any worldsheet
degrees of freedom, so the masses appearing in the light-cone gauge may be
viewed as gauge artefacts. We obtain type IIA and IIB supergravity solutions
describing solitonic strings in pp-wave backgrounds, and show how these are
related to the physical-gauge fundamental string actions. We study the
supersymmetries of these solutions, and find examples with various numbers of
Killing spinors, including total numbers that are odd.Comment: Latex, 35 page
Scalar field in the Bianchi I: Non commutative classical and Quantum Cosmology
Using the ADM formalism in the minisuperspace, we obtain the commutative and
noncommutative exact classical solutions and exact wave function to the
Wheeler-DeWitt equation with an arbitrary factor ordering, for the anisotropic
Bianchi type I cosmological model, coupled to a scalar field, cosmological term
and barotropic perfect fluid. We introduce noncommutative scale factors,
considering that all minisuperspace variables do not commute, so the
symplectic structure was modified. In the classical regime, it is shown that
the anisotropic parameter and the field , for some
value in the cosmological term and noncommutative
parameter, present a dynamical isotropization up to a critical cosmic time
; after this time, the effects of isotropization in the noncommutative
minisuperspace seems to disappear. In the quantum regimen, the probability
density presents a new structure that corresponds to the value of the
noncommutativity parameter.Comment: 17 pages, 6 figures, Acepted in IJT
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