11,721 research outputs found
On the reduction of hypercubic lattice artifacts
This note presents a comparative study of various options to reduce the
errors coming from the discretization of a Quantum Field Theory in a lattice
with hypercubic symmetry. We show that it is possible to perform an
extrapolation towards the continuum which is able to eliminate systematically
the artifacts which break the O(4) symmetry.Comment: 15 pages. 4 figures. Minor changes (Appendix and refs added
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Effect of the nearby levels on the resonance fluorescence spectrum of the atom-field interaction
We study the resonance fluorescence in the Jaynes-Cummings model when nearby
levels are taking into account. We show that the Stark shift produced by such
levels generates a displacement of the peaks of the resonance fluorescence due
to an induced effective detuning and also induces an asymmetry. Specific
results are presented assuming a coherent and a thermal fields
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