3,392 research outputs found
Spectral geometry as a probe of quantum spacetime
Employing standard results from spectral geometry, we provide strong evidence
that in the classical limit the ground state of three-dimensional causal
dynamical triangulations is de Sitter spacetime. This result is obtained by
measuring the expectation value of the spectral dimension on the ensemble of
geometries defined by these models, and comparing its large scale behaviour to
that of a sphere (Euclidean de Sitter). From the same measurement we are also
able to confirm the phenomenon of dynamical dimensional reduction observed in
this and other approaches to quantum gravity -- the first time this has been
done for three-dimensional causal dynamical triangulations. In this case, the
value for the short-scale limit of the spectral dimension that we find is
approximately 2. We comment on the relevance of these results for the
comparison to asymptotic safety and Horava-Lifshitz gravity, among other
approaches to quantum gravity.Comment: 25 pages, 6 figures. Version 2: references to figures added,
acknowledgment added
Tuning the shape of the condensate in spontaneous symmetry breaking
We investigate what determines the shape of a particle condensate in
situations when it emerges as a result of spontaneous breaking of translational
symmetry. We consider a model with particles hopping between sites of a
one-dimensional grid and interacting if they are at the same or at neighboring
nodes. We predict the envelope of the condensate and the scaling of its width
with the system size for various interaction potentials and show how to tune
the shape from a delta-peak to a rectangular or a parabolic-like form.Comment: 4 pages, 2 figures, major revision, the title has been change
Spectroscopic Study of IRAS 19285+0517(PDS 100): A Rapidly Rotating Li-Rich K Giant
We report on photometry and high-resolution spectroscopy for IRAS 19285+0517.
The spectral energy distribution based on visible and near-IR photometry and
far-IR fluxes shows that the star is surrounded by dust at a temperature of
250 K. Spectral line analysis shows that the star is a K
giant with a projected rotational velocity = 9 2 km s.
We determined the atmospheric parameters: = 4500 K, log =
2.5, = 1.5 km s, and [Fe/H] = 0.14 dex. The LTE abundance
analysis shows that the star is Li-rich (log (Li) = 2.50.15),
but with essentially normal C, N, and O, and metal abundances. Spectral
synthesis of molecular CN lines yields the carbon isotopic ratio
C/C = 9 3, a signature of post-main sequence evolution and
dredge-up on the RGB. Analysis of the Li resonance line at 6707 \AA for
different ratios Li/Li shows that the Li profile can be fitted best
with a predicted profile for pure Li. Far-IR excess, large Li abundance,
and rapid rotation suggest that a planet has been swallowed or, perhaps, that
an instability in the RGB outer layers triggered a sudden enrichment of Li and
caused mass-loss.Comment: To appear in AJ; 40 pages, 9 figure
RQM description of the charge form factor of the pion and its asymptotic behavior
The pion charge and scalar form factors, and , are first
calculated in different forms of relativistic quantum mechanics. This is done
using the solution of a mass operator that contains both confinement and
one-gluon-exchange interactions. Results of calculations, based on a one-body
current, are compared to experiment for the first one. As it could be expected,
those point-form, and instant and front-form ones in a parallel momentum
configuration fail to reproduce experiment. The other results corresponding to
a perpendicular momentum configuration (instant form in the Breit frame and
front form with ) do much better. The comparison of charge and scalar
form factors shows that the spin-1/2 nature of the constituents plays an
important role. Taking into account that only the last set of results
represents a reasonable basis for improving the description of the charge form
factor, this one is then discussed with regard to the asymptotic QCD-power-law
behavior . The contribution of two-body currents in achieving the right
power law is considered while the scalar form factor, , is shown to
have the right power-law behavior in any case. The low- behavior of the
charge form factor and the pion-decay constant are also discussed.}Comment: 30 pages, 10 figure
On partial order semantics for SAT/SMT-based symbolic encodings of weak memory concurrency
Concurrent systems are notoriously difficult to analyze, and technological
advances such as weak memory architectures greatly compound this problem. This
has renewed interest in partial order semantics as a theoretical foundation for
formal verification techniques. Among these, symbolic techniques have been
shown to be particularly effective at finding concurrency-related bugs because
they can leverage highly optimized decision procedures such as SAT/SMT solvers.
This paper gives new fundamental results on partial order semantics for
SAT/SMT-based symbolic encodings of weak memory concurrency. In particular, we
give the theoretical basis for a decision procedure that can handle a fragment
of concurrent programs endowed with least fixed point operators. In addition,
we show that a certain partial order semantics of relaxed sequential
consistency is equivalent to the conjunction of three extensively studied weak
memory axioms by Alglave et al. An important consequence of this equivalence is
an asymptotically smaller symbolic encoding for bounded model checking which
has only a quadratic number of partial order constraints compared to the
state-of-the-art cubic-size encoding.Comment: 15 pages, 3 figure
Emergence of a 4D World from Causal Quantum Gravity
Causal Dynamical Triangulations in four dimensions provide a
background-independent definition of the sum over geometries in nonperturbative
quantum gravity, with a positive cosmological constant. We present evidence
that a macroscopic four-dimensional world emerges from this theory dynamically.Comment: 11 pages, 3 figures; some short clarifying comments added; final
version to appear in Phys. Rev. Let
Covariant Light-Front Approach for s-wave and p-wave Mesons: Its Application to Decay Constants and Form Factors
We study the decay constants and form factors of the ground-state s-wave and
low-lying p-wave mesons within a covariant light-front approach. Numerical
results of the form factors for transitions between a heavy pseudoscalar meson
and an s-wave or p-wave meson and their momentum dependence are presented in
detail. In particular, form factors for heavy-to-light and B to D**
transitions, where D** denotes generically a p-wave charmed meson, are compared
with other model calculations. The experimental measurements of the decays B^-
to D** pi^- and B to D D**_s are employed to test the decay constants of D**_s
and the B to D** transition form factors. The heavy quark limit behavior of the
decay constants and form factors is examined and it is found that the
requirement of heavy quark symmetry is satisfied. The universal Isgur-Wise (IW)
functions, one for s-wave to s-wave and two for s-wave to p-wave transitions,
are obtained. The values of IW functions at zero recoil and their slope
parameters can be used to test the Bjorken and Uraltsev sum rules.Comment: 59 pages, 6 figures. Version to appear in Phys. Rev. D. Changes are:
(i) D_s to phi transition form factors are discussed and compared with the
recent FOCUS measurements and (ii) zero mode effects are clarifie
Generalised quantum weakest preconditions
Generalisation of the quantum weakest precondition result of D'Hondt and
Panangaden is presented. In particular the most general notion of quantum
predicate as positive operator valued measure (POVM) is introduced. The
previously known quantum weakest precondition result has been extended to cover
the case of POVM playing the role of a quantum predicate. Additionally, our
result is valid in infinite dimension case and also holds for a quantum
programs defined as a positive but not necessary completely positive
transformations of a quantum states.Comment: 7 pages, no figures, added references, changed conten
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