13,089 research outputs found
Thermal field theories and shifted boundary conditions
The analytic continuation to an imaginary velocity of the canonical partition
function of a thermal system expressed in a moving frame has a natural
implementation in the Euclidean path-integral formulation in terms of shifted
boundary conditions. The Poincare' invariance underlying a relativistic theory
implies a dependence of the free-energy on the compact length L_0 and the shift
xi only through the combination beta=L_0(1+xi^2)^(1/2). This in turn implies
that the energy and the momentum distributions of the thermal theory are
related, a fact which is encoded in a set of Ward identities among the
correlators of the energy-momentum tensor. The latter have interesting
applications in lattice field theory: they offer novel ways to compute
thermodynamic potentials, and a set of identities to renormalize
non-perturbatively the energy-momentum tensor. At fixed bare parameters the
shifted boundary conditions also provide a simple method to vary the
temperature in much smaller steps than with the standard procedure.Comment: 7 pages, 1 figure, talk presented at the 31st International Symposium
on Lattice Field Theory - Lattice 2013, Mainz, German
Young massive stars in the ISOGAL survey I. VLA observations of the ISOGAL l=+45 field
We present VLA radio continuum observations at 3.6 and 6 cm of a ~0.65
sq.deg. field in the galactic plane at l=+45deg . These observations are meant
to be used in a comparison with ISO observations at 7 and 15 um of the same
region. In this paper we compare the radio results with other radio surveys and
with the IRAS-PSC.
At 3.6 and/or 6 cm we detect a total of 34 discrete sources, 13 of which are
found in five separate extended complexes. These are all multiple or single
extended thermal ultra-compact HII (UCHII) regions. While for each of these
complexes an IRAS counterpart could be reliably found, no IRAS counterpart
could be reliably identified for any of the remaining 21 sources. Of these 21
compact sources, six are candidate UCHII regions, and the other 15 are most
probably background extragalactic non-thermal sources.
The five IRAS sources associated with the radio continuum complexes all
satisfy the Wood & Churchwell (1989; WC89) color criteria for UCHII. None of
the other 38 IRAS point sources present in our surveyed field show the same
colors. This fraction of WC89 type to total IRAS sources is consistent with
what is found over the entire galactic plane. The fact that, when observed with
a compact VLA configuration, the IRAS sources with "UCHII colors" are found to
be associated with arcminute-scale extended sources, rather than with compact
or unresolved radio sources, may have important implications on the estimated
lifetime of UCHII regions.Comment: 15 pages, 22 eps figures, A&A Supp. in press, higher resolution
figures available at http://www.arcetri.astro.it/~lt/preprints/preprints.htm
Comparative Enumeration Gene Expression
This paper is about differential gene expression measured by transcript counting methods such as SAGE or MPSS. It introduces two significance tests for detection of differential expressed tags: frequentist and Bayesian. Under the frequentist view, it is proposed a test that computes the critical level as a function of each tag total frequency. Under the Bayesian view the Full Bayesian Significance Test is used considering the logistic normal distribution. The two proposed significance levels, the frequentist and the Bayesian, are compared for a data set with four libraries. The linking function between them is a Beta distribution function with mean 0.39 and standard deviation 0.30
Properties of Reactive Oxygen Species by Quantum Monte Carlo
The electronic properties of the oxygen molecule, in its singlet and triplet
states, and of many small oxygen-containing radicals and anions have important
roles in different fields of Chemistry, Biology and Atmospheric Science.
Nevertheless, the electronic structure of such species is a challenge for
ab-initio computational approaches because of the difficulties to correctly
describe the statical and dynamical correlation effects in presence of one or
more unpaired electrons. Only the highest-level quantum chemical approaches can
yield reliable characterizations of their molecular properties, such as binding
energies, equilibrium structures, molecular vibrations, charge distribution and
polarizabilities. In this work we use the variational Monte Carlo (VMC) and the
lattice regularized Monte Carlo (LRDMC) methods to investigate the equilibrium
geometries and molecular properties of oxygen and oxygen reactive species.
Quantum Monte Carlo methods are used in combination with the Jastrow
Antisymmetrized Geminal Power (JAGP) wave function ansatz, which has been
recently shown to effectively describe the statical and dynamical correlation
of different molecular systems. In particular we have studied the oxygen
molecule, the superoxide anion, the nitric oxide radical and anion, the
hydroxyl and hydroperoxyl radicals and their corresponding anions, and the
hydrotrioxyl radical. Overall, the methodology was able to correctly describe
the geometrical and electronic properties of these systems, through compact but
fully-optimised basis sets and with a computational cost which scales as
, where is the number of electrons. This work is therefore opening
the way to the accurate study of the energetics and of the reactivity of large
and complex oxygen species by first principles
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