5,365 research outputs found
InclusĂŁo do esterco bovino ao conceito de biorrefinarias: avaliação no uso como substrato para produção de enzimas hemi(celulolĂticas)
Pair distribution function in a two-dimensional electron gas
We calculate the pair distribution function, , in a two-dimensional
electron gas and derive a simple analytical expression for its value at the
origin as a function of . Our approach is based on solving the
Schr\"{o}dinger equation for the two-electron wave function in an appropriate
effective potential, leading to results that are in good agreement with Quantum
Monte Carlo data and with the most recent numerical calculations of . [C.
Bulutay and B. Tanatar, Phys. Rev. B {\bf 65}, 195116 (2002)] We also show that
the spin-up spin-down correlation function at the origin, , is mainly independent of the degree of spin polarization of
the electronic system.Comment: 5 figures, pair distribution dependence with distance is calculate
Dijet production as a centrality trigger for p-p collisions at CERN LHC
We demonstrate that a trigger on hard dijet production at small rapidities
allows to establish a quantitative distinction between central and peripheral
collisions in pbar-p and p-p collisions at Tevatron and LHC energies. Such a
trigger strongly reduces the effective impact parameters as compared to minimum
bias events. This happens because the transverse spatial distribution of hard
partons (x >~ 10^{-2}) in the proton is considerably narrower than that of soft
partons, whose collisions dominate the total cross section. In the central
collisions selected by the trigger, most of the partons with x >~ 10^{-2}
interact with a gluon field whose strength rapidly increases with energy. At
LHC (and to some extent already at Tevatron) energies the strength of this
interaction approaches the unitarity ('black-body') limit. This leads to
specific modifications of the final state, such as a higher probability of
multijet events at small rapidities, a strong increase of the transverse
momenta and depletion of the longitudinal momenta at large rapidities, and the
appearance of long-range correlations in rapidity between the forward/backward
fragmentation regions. The same pattern is expected for events with production
of new heavy particles (Higgs, SUSY). Studies of these phenomena would be
feasible with the CMS-TOTEM detector setup, and would have considerable impact
on the exploration of the physics of strong gluon fields in QCD, as well as the
search for new particles at LHC.Comment: 17 pages, Revtex 4, 14 EPS figures. Expanded discussion of some
points, added 3 new figures and new references. Included comment on
connection with cosmic ray physics near the GZK cutoff. To appear in Phys Rev
Analytical expressions for the charge-charge local-field factor and the exchange-correlation kernel of a two-dimensional electron gas
We present an analytical expression for the static many-body local field
factor of a homogeneous two-dimensional electron gas, which
reproduces Diffusion Monte Carlo data and embodies the exact asymptotic
behaviors at both small and large wave number . This allows us to also
provide a closed-form expression for the exchange and correlation kernel
, which represents a key input for density functional studies of
inhomogeneous systems.Comment: 5 pages, 3 figure
Temperature and Humidity Dependence of Air Fluorescence Yield measured by AIRFLY
The fluorescence detection of ultra high energy cosmic rays requires a
detailed knowledge of the fluorescence light emission from nitrogen molecules
over a wide range of atmospheric parameters, corresponding to altitudes typical
of the cosmic ray shower development in the atmosphere. We have studied the
temperature and humidity dependence of the fluorescence light spectrum excited
by MeV electrons in air. Results for the 313.6 nm, 337.1 nm, 353.7 nm and 391.4
nm bands are reported in this paper. We found that the temperature and humidity
dependence of the quenching process changes the fluorescence yield by a
sizeable amount (up to 20%) and its effect must be included for a precise
estimation of the energy of ultra high energy cosmic rays.Comment: presented at the 5th Fluorescence Workshop, El Escorial - Madrid,
Spain, 16 - 20 September 2007, to appear in Nuclear Instruments and Methods
On local linearization of control systems
We consider the problem of topological linearization of smooth (C infinity or
real analytic) control systems, i.e. of their local equivalence to a linear
controllable system via point-wise transformations on the state and the control
(static feedback transformations) that are topological but not necessarily
differentiable. We prove that local topological linearization implies local
smooth linearization, at generic points. At arbitrary points, it implies local
conjugation to a linear system via a homeomorphism that induces a smooth
diffeomorphism on the state variables, and, except at "strongly" singular
points, this homeomorphism can be chosen to be a smooth mapping (the inverse
map needs not be smooth). Deciding whether the same is true at "strongly"
singular points is tantamount to solve an intriguing open question in
differential topology
Ultra-High Energy Cosmic Rays in a Structured and Magnetized Universe
We simulate propagation of cosmic ray nucleons above 10^{19} eV in scenarios
where both the source distribution and magnetic fields within about 50 Mpc from
us are obtained from an unconstrained large scale structure simulation. We find
that consistency of predicted sky distributions with current data above 4 x
10^{19} eV requires magnetic fields of ~0.1 microGauss in our immediate
environment, and a nearby source density of ~10^{-4}-10^{-3} Mpc^{-3}. Radio
galaxies could provide the required sources, but only if both high and
low-luminosity radio galaxies are very efficient cosmic ray accelerators.
Moreover, at ~10^{19} eV an additional isotropic flux component, presumably of
cosmological origin, should dominate over the local flux component by about a
factor three in order to explain the observed isotropy. This argues against the
scenario in which local astrophysical sources of cosmic rays above ~10^{19} eV
reside in strongly magnetized (B~0.1 microGauss) and structured intergalactic
medium. Finally we discuss how future large scale full-sky detectors such as
the Pierre Auger project will allow to put much more stringent constraints on
source and magnetic field distributions.Comment: 11 revtex pages, 10 postscript figures included, final version to
appear in PR
Super-GZK Photons from Photon-Axion Mixing
We show that photons with energies above the GZK cutoff can reach us from
very distant sources if they mix with light axions in extragalactic magnetic
fields. The effect which enables this is the conversion of photons into axions,
which are sufficiently weakly coupled to travel large distances unimpeded.
These axions then convert back into high energy photons close to the Earth. We
show that photon-axion mixing facilitates the survival of super-GZK photons
most efficiently with a photon-axion coupling scale of order 10^11 GeV, which
is in the same range as the scale for the photon-axion mixing explanation for
the dimming of supernovae without cosmic acceleration. We discuss possible
observational consequences of this effect.Comment: 17 pages, 5 figures. Published versio
Ultra-High Energy Cosmic Ray Nuclei from Individual Magnetized Sources
We investigate the dependence of composition, spectrum and angular
distributions of ultra-high energy cosmic rays above 10^19 eV from individual
sources on their magnetization. We find that, especially for sources within a
few megaparsecs from the observer, observable spectra and composition are
severely modified if the source is surrounded by fields of ~ 10^-7 Gauss on
scales of a few megaparsecs. Low energy particles diffuse over larger distances
during their energy loss time. This leads to considerable hardening of the
spectrum up to the energy where the loss distance becomes comparable to the
source distance. Magnetized sources thus have very important consequences for
observations, even if cosmic rays arrive within a few degrees from the source
direction. At the same time, details in spectra and chemical composition may be
intrinsically unpredictable because they depend on the unknown magnetic field
structure. If primaries are predominantly nuclei of atomic mass A accelerated
up to a maximum energy E_max with spectra not much softer than E^-2, secondary
protons from photo-disintegration can produce a conspicuous peak in the
spectrum at energy ~ E_max/A. A related feature appears in the average mass
dependence on energy.Comment: 15 pages, 16 ps figures, published version with minor changes, see
http://stacks.iop.org/1475-7516/2004/i=08/a=01
Maximum Likelihood Analysis of Clusters of Ultra-High Energy Cosmic Rays
We present a numerical code designed to conduct a likelihood analysis for
clusters of nucleons above 10**19 eV originating from discrete astrophysical
sources such as powerful radio galaxies, gamma-ray bursts or topological
defects. The code simulates the propagation of nucleons in a large-scale
magnetic field and constructs the likelihood of a given observed event cluster
as a function of the average time delay due to deflection in the magnetic
field, the source activity time scale, the total fluence of the source, and the
power law index of the particle injection spectrum. Other parameters such as
the coherence length and the power spectrum of the magnetic field are also
considered. We apply it to the three pairs of events above 4X10**19 eV recently
reported by the Akeno Giant Air Shower Array (AGASA) experiment, assuming that
these pairs were caused by nucleon primaries which originated from a common
source. Although current data are too sparse to fully constrain each of the
parameters considered, and/or to discriminate models of the origin of
ultra-high energy cosmic rays, several tendencies are indicated. If the
clustering suggested by AGASA is real, next generation experiments with their
increased exposure should detect more than 10 particles per source over a few
years and our method will put strong constraints on both the large-scale
magnetic field parameters and the nature of these sources.Comment: 11 latex pages, 8 postscript figures included, uses revtex.sty in
two-column format and epsf.sty. Submitted to Physical Review
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