23,152 research outputs found
Inhomogeneous Reionization Regulated by Radiative and Stellar Feedbacks
We study the inhomogeneous reionization in a critical density CDM universe
due to stellar sources, including Population III objects. The spatial
distribution of the sources is obtained from high resolution numerical N-body
simulations. We calculate the source properties taking into account a
self-consistent treatment of both radiative (ie ionizing and H2
-photodissociating photons) and stellar (ie SN explosions) feedbacks regulated
by massive stars. This allows us to describe the topology of the ionized and
dissociated regions at various cosmic epochs and derive the evolution of H, He,
and H2 filling factors, soft UV background, cosmic star formation rate and the
final fate of ionizing objects. The main results are: (i) galaxies reionize the
IGM by z~10 (with some uncertainty related to the gas clumping factor), whereas
H2 is completely dissociated already by z~25; (ii) reionization is mostly due
to the relatively massive objects which collapse via H line cooling, while
objects whose formation relies on H2 cooling alone are insufficient to this
aim; (iii) the diffuse soft UV background is the major source of radiative
feedback effects for z<15; at higher z direct flux from neighboring objects
dominates; (iv) the match of the calculated cosmic star formation history with
the one observed at lower redshifts suggests that the conversion efficiency of
baryons into stars is ~1%; (v) we find that a very large population of dark
objects which failed to form stars is present by z~8. We discuss and compare
our results with similar previous studies.Comment: 34 pages, emulateapj.sty, LaTeX, 13 figures. MNRAS, submitte
Multiplexed Memory-Insensitive Quantum Repeaters
Long-distance quantum communication via distant pairs of entangled quantum
bits (qubits) is the first step towards more secure message transmission and
distributed quantum computing. To date, the most promising proposals require
quantum repeaters to mitigate the exponential decrease in communication rate
due to optical fiber losses. However, these are exquisitely sensitive to the
lifetimes of their memory elements. We propose a multiplexing of quantum nodes
that should enable the construction of quantum networks that are largely
insensitive to the coherence times of the quantum memory elements.Comment: 5 pages, 4 figures. Accepted for publication in PR
Hilbert Series for Flavor Invariants of the Standard Model
The Hilbert series is computed for the lepton flavor invariants of the
Standard Model with three generations including the right-handed neutrino
sector needed to generate light neutrino masses via the see-saw mechanism. We
also compute the Hilbert series of the quark flavor invariants for the case of
four generations.Comment: 6 page
Analysis of General Power Counting Rules in Effective Field Theory
We derive the general counting rules for a quantum effective field theory
(EFT) in dimensions. The rules are valid for strongly and weakly
coupled theories, and predict that all kinetic energy terms are canonically
normalized. They determine the energy dependence of scattering cross sections
in the range of validity of the EFT expansion. We show that the size of cross
sections is controlled by the power counting of EFT, not by chiral
counting, even for chiral perturbation theory (PT). The relation between
and is generalized to dimensions. We show that the
naive dimensional analysis counting is related to counting. The
EFT counting rules are applied to PT, low-energy weak interactions,
Standard Model EFT and the non-trivial case of Higgs EFT.Comment: V2: more details and examples added; version published in journal. 17
pages, 4 figures, 2 table
Hyperon Nonleptonic Decays in Chiral Perturbation Theory Reexamined
We recalculate the leading nonanalytic contributions to the amplitudes for
hyperon nonleptonic decays in chiral perturbation theory. Our results partially
disagree with those calculated before, and include new terms previously omitted
in the P-wave amplitudes. Although these modifications are numerically
significant, they do not change the well-known fact that good agreement with
experiment cannot be simultaneously achieved using one-loop S- and P-wave
amplitudes.Comment: 14 pages, latex, 3 figures, uses axodraw.sty, minor additions, to
appear in Phys. Rev.
Supporting Pluralism by Artificial Intelligence: Conceptualizing Epistemic Disagreements as Digital Artifacts
A crucial concept in philosophy and social sciences, epistemic disagreement, has not yet been adequately reflected in the Web. In this paper, we call for development of intelligent tools dealing with epistemic disagreements on the Web to support pluralism. As a first step, we present Polyphony, an ontology for representing and annotating epistemic disagreements
Electromagnetic Moments of the Baryon Decuplet
We compute the leading contributions to the magnetic dipole and electric
quadrupole moments of the baryon decuplet in chiral perturbation theory. The
measured value for the magnetic moment of the is used to determine
the local counterterm for the magnetic moments. We compare the chiral
perturbation theory predictions for the magnetic moments of the decuplet with
those of the baryon octet and find reasonable agreement with the predictions of
the large-- limit of QCD. The leading contribution to the quadrupole
moment of the and other members of the decuplet comes from one--loop
graphs. The pionic contribution is shown to be proportional to (and so
will not contribute to the quadrupole moment of nuclei), while the
contribution from kaons has both isovector and isoscalar components. The chiral
logarithmic enhancement of both pion and kaon loops has a coefficient that
vanishes in the limit. The third allowed moment, the magnetic octupole,
is shown to be dominated by a local counterterm with corrections arising at two
loops. We briefly mention the strange counterparts of these moments.Comment: Uses harvmac.tex, 15 pages with 3 PostScript figures packed using
uufiles. UCSD/PTH 93-22, QUSTH-93-05, Duke-TH-93-5
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