1,145 research outputs found
Non-linear terms in 2D cosmology
In this work we investigate the behavior of two-dimensional (2D) cosmological
models, starting with the Jackiw-Teitelboim (JT) theory of gravitation. A
geometrical term, non-linear in the scalar curvature , is added to the JT
dynamics to test if it could play the role of dark energy in a 2D expanding
universe. This formulation makes possible, first, the description of an early
(inflationary) 2D universe, when the van der Waals (vdW) equation of state is
used to construct the energy-momentum tensor of the gravitational sources.
Second, it is found that for later times the non-linear term in can
generate an old 2D universe in accelerated expansion, where an ordinary matter
dominated era evolves into a decelerated/accelerated transition, giving to the
dark energy effects a geometrical origin. The results emerge through numerical
analysis, following the evolution in time of the scale factor, its
acceleration, and the energy densities of constituents.Comment: tex file plus figures in two zipped files. To appear in Europhys.
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Roots of the derivative of the Riemann zeta function and of characteristic polynomials
We investigate the horizontal distribution of zeros of the derivative of the
Riemann zeta function and compare this to the radial distribution of zeros of
the derivative of the characteristic polynomial of a random unitary matrix.
Both cases show a surprising bimodal distribution which has yet to be
explained. We show by example that the bimodality is a general phenomenon. For
the unitary matrix case we prove a conjecture of Mezzadri concerning the
leading order behavior, and we show that the same follows from the random
matrix conjectures for the zeros of the zeta function.Comment: 24 pages, 6 figure
Risk Assessment to the Health of Amazonian Indigenous For the Consumption of Fish, Meat of Hunts and Vegetables Containing Metylmercury
This study was to evaluate the exposure to methylmercury (HgMe) and the potential health risk of Tupari Indians through the consumption of their main foods. Were collection of samples of plant foods and muscle tissue from different species of fish and wild animals consumed in three villages of the Rio Branco Indigenous Land in Rondônia, in the Brazilian Amazon. The HgMe was measured in an atomic fluorescence spectrophotometer with gas chromatography. The statistical treatment of the data was performed by software R. Of the six different plant species, only sweet potato (Ipomoea batatas) had mean concentrations of HgMe above the limit of detection of the analytical technique for the three villages. There was a significant difference in the levels of HgMe between the species of wild animals and fish belonging to the same alimentary habit. Carnivores presented higher levels of HgMe than those obtained for non-predators, both for fish and for wild animals. The results of the assessment of the potential risk to indigenous health indicated a total HgMe of the weekly ingestion rate (WIR) of between 8.4 and 15.0 μg / kg of body weight for the villages evaluated, extrapolating all reference doses ( RfD) regarding for the Provisional Tolerable Weekly Intake (PTWI). The risk quotients (RQ) varied from 5.3 to 21.4, considerably exceeding the limit (RQ ≤ 1), which allows to consider the impossibility of toxic effects of HgMe. The fishes accounted for the highest percentage of WIR of HgMe for all villages, with an emphasis on predatory species. Considering the nutritional value of fish meat, it is suggested the continuity of the consumption of this meat in the villages with preference for non-carnivores
Dynamical Properties of small Polarons
On the basis of the two-site polaron problem, which we solve by exact
diagonalization, we analyse the spectral properties of polaronic systems in
view of discerning localized from itinerant polarons and bound polaron pairs
from an ensemble of single polarons. The corresponding experimental techniques
for that concern photoemission and inverse photoemission spectroscopy. The
evolution of the density of states as a function of concentration of charge
carriers and strength of the electron-phonon interaction clearly shows the
opening up of a gap between single polaronic and bi-polaronic states, in
analogy to the Hubbard problem for strongly correlated electron systems. The
crossover regime between adiabatic and anti-adiabatic small polarons is
triggered by two characteristic time scales: the renormalized electron hopping
rate and the renormalized vibrational frequency becoming equal. This crossover
regime is then characterized by temporarily alternating self- localization and
delocalization of the charge carriers which is accompanied by phase slips in
the charge and molecular deformation oscillations and ultimately leads to a
dephasing between these two dynamical components of the polaron problem. We
visualize these features by a study of the temporal evolution of the charge
redistribution and the change in molecular deformations. The spectral and
dynamical properties of polarons discussed here are beyond the applicability of
the standard Lang Firsov approach to the polaron problem.Comment: 31 pages and 23 figs.(eps), accepted in the Phys. Rev.
Composition and diversity of Ephemeroptera (Insecta) nymph communities in the middle section of the Jacuí River and some tributaries, southern Brazil
Indication of a deeply bound compact K-pp state formed in the pp -> p Lambda K+ reaction at 2.85 GeV
We have analyzed data of the DISTO experiment on the exclusive pp -> p Lambda
K+ reaction at 2.85 GeV to search for a strongly bound compact K-pp (= X) state
to be formed in the pp -> K+ + X reaction. The observed spectra of the K+
missing-mass and the p Lambda invariant-mass with high transverse momenta of p
and K+ revealed a broad distinct peak with a mass M_X = 2265 +- 2 (stat) +- 5
(syst) MeV/c2 and a width Gamma_X = 118 +- 8 (stat) +- 10 (syst) MeV.Comment: 4 pages, 4 figure
Quantum Monte Carlo and variational approaches to the Holstein model
Based on the canonical Lang-Firsov transformation of the Hamiltonian we
develop a very efficient quantum Monte Carlo algorithm for the Holstein model
with one electron. Separation of the fermionic degrees of freedom by a
reweighting of the probability distribution leads to a dramatic reduction in
computational effort. A principal component representation of the phonon
degrees of freedom allows to sample completely uncorrelated phonon
configurations. The combination of these elements enables us to perform
efficient simulations for a wide range of temperature, phonon frequency and
electron-phonon coupling on clusters large enough to avoid finite-size effects.
The algorithm is tested in one dimension and the data are compared with
exact-diagonalization results and with existing work. Moreover, the ideas
presented here can also be applied to the many-electron case. In the
one-electron case considered here, the physics of the Holstein model can be
described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte
Inclusive pion and eta production in p+Nb collisions at 3.5 GeV beam energy
Data on inclusive pion and eta production measured with the dielectron
spectrometer HADES in the reaction p+93Nb at a kinetic beam energy of 3.5 GeV
are presented. Our results, obtained with the photon conversion method,
supplement the rather sparse information on neutral meson production in
proton-nucleus reactions existing for this bombarding energy regime. The
reconstructed e+e-e+e- transverse-momentum and rapidity distributions are
confronted with transport model calculations, which account fairly well for
both pi0 and eta production.Comment: 12 pages, 9 figures, submitted to Physical Review
Searching a Dark Photon with HADES
We present a search for the e+e- decay of a hypothetical dark photon, also
names U vector boson, in inclusive dielectron spectra measured by HADES in the
p (3.5 GeV) + p, Nb reactions, as well as the Ar (1.756 GeV/u) + KCl reaction.
An upper limit on the kinetic mixing parameter squared epsilon^{2} at 90% CL
has been obtained for the mass range M(U) = 0.02 - 0.55 GeV/c2 and is compared
with the present world data set. For masses 0.03 - 0.1 GeV/c^2, the limit has
been lowered with respect to previous results, allowing now to exclude a large
part of the parameter region favoured by the muon g-2 anomaly. Furthermore, an
improved upper limit on the branching ratio of 2.3 * 10^{-6} has been set on
the helicity-suppressed direct decay of the eta meson, eta-> e+e-, at 90% CL
Simulation and background characterisation of the SABRE South experiment
SABRE (Sodium iodide with Active Background REjection) is a direct detection
dark matter experiment based on arrays of radio-pure NaI(Tl) crystals. The
experiment aims at achieving an ultra-low background rate and its primary goal
is to confirm or refute the results from the DAMA/LIBRA experiment. The SABRE
Proof-of-Principle phase was carried out in 2020-2021 at the Gran Sasso
National Laboratory (LNGS), in Italy. The next phase consists of two full-scale
experiments: SABRE South at the Stawell Underground Physics Laboratory, in
Australia, and SABRE North at LNGS. This paper focuses on SABRE South and
presents a detailed simulation of the detector, which is used to characterise
the background for dark matter searches including DAMA/LIBRA-like modulation.
We estimate an overall background of 0.72 cpd/kg/keV in the energy range
16 keV primarily due to radioactive contamination in the crystals.
Given this level of background and considering that the SABRE South has a
target mass of 50 kg, we expect to exclude (confirm) DAMA/LIBRA modulation at
within 2.5 years of data taking
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