576 research outputs found
Analytic approximation and an improved method for computing the stress-energy of quantized scalar fields in Robertson-Walker spacetimes
An improved method is given for the computation of the stress-energy tensor
of a quantized scalar field using adiabatic regularization. The method works
for fields with arbitrary mass and curvature coupling in Robertson-Walker
spacetimes and is particularly useful for spacetimes with compact spatial
sections. For massless fields it yields an analytic approximation for the
stress-energy tensor that is similar in nature to those obtained previously for
massless fields in static spacetimes.Comment: RevTeX, 8 pages, no figure
Method to compute the stress-energy tensor for the massless spin 1/2 field in a general static spherically symmetric spacetime
A method for computing the stress-energy tensor for the quantized, massless,
spin 1/2 field in a general static spherically symmetric spacetime is
presented. The field can be in a zero temperature state or a non-zero
temperature thermal state. An expression for the full renormalized
stress-energy tensor is derived. It consists of a sum of two tensors both of
which are conserved. One tensor is written in terms of the modes of the
quantized field and has zero trace. In most cases it must be computed
numerically. The other tensor does not explicitly depend on the modes and has a
trace equal to the trace anomaly. It can be used as an analytic approximation
for the stress-energy tensor and is equivalent to other approximations that
have been made for the stress-energy tensor of the massless spin 1/2 field in
static spherically symmetric spacetimes.Comment: 34 pages, no figure
Charged black holes in quadratic gravity
Iterative solutions to fourth-order gravity describing static and
electrically charged black holes are constructed. Obtained solutions are
parametrized by two integration constants which are related to the electric
charge and the exact location of the event horizon. Special emphasis is put on
the extremal black holes. It is explicitly demonstrated that in the extremal
limit, the exact location of the (degenerate) event horizon is given by \rp =
|e|. Similarly to the classical Reissner-Nordstr\"om solution, the
near-horizon geometry of the charged black holes in quadratic gravity, when
expanded into the whole manifold, is simply that of Bertotti and Robinson.
Similar considerations have been carried out for the boundary conditions of
second type which employ the electric charge and the mass of the system as seen
by a distant observer. The relations between results obtained within the
framework of each method are briefly discussed
Regular black holes in quadratic gravity
The first-order correction of the perturbative solution of the coupled
equations of the quadratic gravity and nonlinear electrodynamics is
constructed, with the zeroth-order solution coinciding with the ones given by
Ay\'on-Beato and Garc{\'\i}a and by Bronnikov. It is shown that a simple
generalization of the Bronnikov's electromagnetic Lagrangian leads to the
solution expressible in terms of the polylogarithm functions. The solution is
parametrized by two integration constants and depends on two free parameters.
By the boundary conditions the integration constants are related to the charge
and total mass of the system as seen by a distant observer, whereas the free
parameters are adjusted to make the resultant line element regular at the
center. It is argued that various curvature invariants are also regular there
that strongly suggests the regularity of the spacetime. Despite the complexity
of the problem the obtained solution can be studied analytically. The location
of the event horizon of the black hole, its asymptotics and temperature are
calculated. Special emphasis is put on the extremal configuration
Energy-Momentum Tensor of Particles Created in an Expanding Universe
We present a general formulation of the time-dependent initial value problem
for a quantum scalar field of arbitrary mass and curvature coupling in a FRW
cosmological model. We introduce an adiabatic number basis which has the virtue
that the divergent parts of the quantum expectation value of the
energy-momentum tensor are isolated in the vacuum piece of , and
may be removed using adiabatic subtraction. The resulting renormalized
is conserved, independent of the cutoff, and has a physically transparent,
quasiclassical form in terms of the average number of created adiabatic
`particles'. By analyzing the evolution of the adiabatic particle number in de
Sitter spacetime we exhibit the time structure of the particle creation
process, which can be understood in terms of the time at which different
momentum scales enter the horizon. A numerical scheme to compute as a
function of time with arbitrary adiabatic initial states (not necessarily de
Sitter invariant) is described. For minimally coupled, massless fields, at late
times the renormalized goes asymptotically to the de Sitter invariant
state previously found by Allen and Folacci, and not to the zero mass limit of
the Bunch-Davies vacuum. If the mass m and the curvature coupling xi differ
from zero, but satisfy m^2+xi R=0, the energy density and pressure of the
scalar field grow linearly in cosmic time demonstrating that, at least in this
case, backreaction effects become significant and cannot be neglected in de
Sitter spacetime.Comment: 28 pages, Revtex, 11 embedded .ps figure
Big bounce from spin and torsion
The Einstein-Cartan-Sciama-Kibble theory of gravity naturally extends general
relativity to account for the intrinsic spin of matter. Spacetime torsion,
generated by spin of Dirac fields, induces gravitational repulsion in fermionic
matter at extremely high densities and prevents the formation of singularities.
Accordingly, the big bang is replaced by a bounce that occurred when the energy
density was on the order of (in
natural units), where is the fermion number density and is
the number of thermal degrees of freedom. If the early Universe contained only
the known standard-model particles (), then the energy density at
the big bounce was about 15 times larger than the Planck energy. The minimum
scale factor of the Universe (at the bounce) was about times smaller
than its present value, giving \approx 50 \mum. If more fermions existed in
the early Universe, then the spin-torsion coupling causes a bounce at a lower
energy and larger scale factor. Recent observations of high-energy photons from
gamma-ray bursts indicate that spacetime may behave classically even at scales
below the Planck length, supporting the classical spin-torsion mechanism of the
big bounce. Such a classical bounce prevents the matter in the contracting
Universe from reaching the conditions at which a quantum bounce could possibly
occur.Comment: 6 pages; published versio
Off-Diagonal Elements of the DeWitt Expansion from the Quantum Mechanical Path Integral
The DeWitt expansion of the matrix element M_{xy} = \left\langle x \right|
\exp -[\case{1}{2} (p-A)^2 + V]t \left| y \right\rangle, in
powers of can be made in a number of ways. For (the case of interest
when doing one-loop calculations) numerous approaches have been employed to
determine this expansion to very high order; when (relevant for
doing calculations beyond one-loop) there appear to be but two examples of
performing the DeWitt expansion. In this paper we compute the off-diagonal
elements of the DeWitt expansion coefficients using the Fock-Schwinger gauge.
Our technique is based on representing by a quantum mechanical path
integral. We also generalize our method to the case of curved space, allowing
us to determine the DeWitt expansion of \tilde M_{xy} = \langle x| \exp
\case{1}{2} [\case{1}{\sqrt {g}} (\partial_\mu - i
A_\mu)g^{\mu\nu}{\sqrt{g}}(\partial_\nu - i A_\nu) ] t| y \rangle by use of
normal coordinates. By comparison with results for the DeWitt expansion of this
matrix element obtained by the iterative solution of the diffusion equation,
the relative merit of different approaches to the representation of as a quantum mechanical path integral can be assessed. Furthermore, the
exact dependence of on some geometric scalars can be
determined. In two appendices, we discuss boundary effects in the
one-dimensional quantum mechanical path integral, and the curved space
generalization of the Fock-Schwinger gauge.Comment: 16pp, REVTeX. One additional appendix concerning end-point effects
for finite proper-time intervals; inclusion of these effects seem to make our
results consistent with those from explicit heat-kernel method
Schwinger Pair Production via Instantons in Strong Electric Fields
In the space-dependent gauge, each mode of the Klein-Gordon equation in a
strong electric field takes the form of a time-independent Schr\"{o}dinger
equation with a potential barrier. We propose that the single- and
multi-instantons of quantum tunneling may be related with the single- and
multi-pair production of bosons and the relative probability for the no-pair
production is determined by the total tunneling probability via instantons. In
the case of a uniform electric field, the instanton interpretation recovers
exactly the well-known pair production rate for bosons and when the Pauli
blocking is taken into account, it gives the correct fermion production rate.
The instanton is used to calculate the pair production rate even in an
inhomogeneous electric field. Furthermore, the instanton interpretation
confirms the fact that bosons and fermions can not be produced by a static
magnetic field only.Comment: RevTex 7 Pages, No figure; Formulae for the production rate in very
strong fields and references added; the final version accepted in Phys. Rev.
Probing exotic phenomena at the interface of nuclear and particle physics with the electric dipole moments of diamagnetic atoms: A unique window to hadronic and semi-leptonic CP violation
The current status of electric dipole moments of diamagnetic atoms which
involves the synergy between atomic experiments and three different theoretical
areas -- particle, nuclear and atomic is reviewed. Various models of particle
physics that predict CP violation, which is necessary for the existence of such
electric dipole moments, are presented. These include the standard model of
particle physics and various extensions of it. Effective hadron level combined
charge conjugation (C) and parity (P) symmetry violating interactions are
derived taking into consideration different ways in which a nucleon interacts
with other nucleons as well as with electrons. Nuclear structure calculations
of the CP-odd nuclear Schiff moment are discussed using the shell model and
other theoretical approaches. Results of the calculations of atomic electric
dipole moments due to the interaction of the nuclear Schiff moment with the
electrons and the P and time-reversal (T) symmetry violating
tensor-pseudotensor electron-nucleus are elucidated using different
relativistic many-body theories. The principles of the measurement of the
electric dipole moments of diamagnetic atoms are outlined. Upper limits for the
nuclear Schiff moment and tensor-pseudotensor coupling constant are obtained
combining the results of atomic experiments and relativistic many-body
theories. The coefficients for the different sources of CP violation have been
estimated at the elementary particle level for all the diamagnetic atoms of
current experimental interest and their implications for physics beyond the
standard model is discussed. Possible improvements of the current results of
the measurements as well as quantum chromodynamics, nuclear and atomic
calculations are suggested.Comment: 46 pages, 19 tables and 16 figures. A review article accepted for
EPJ
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
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