3,766 research outputs found
Extraction of the atmospheric neutrino fluxes from experimental event rate data
The precise knowledge of the atmospheric neutrino fluxes is a key ingredient
in the interpretation of the results from any atmospheric neutrino experiment.
In the standard atmospheric neutrino data analysis, these fluxes are
theoretical inputs obtained from sophisticated numerical calculations. In this
contribution we present an alternative approach to the determination of the
atmospheric neutrino fluxes based on the direct extraction from the
experimental data on neutrino event rates. The extraction is achieved by means
of a combination of artificial neural networks as interpolants and Monte Carlo
methods.Comment: 6 pages, 2 figs, to appear in the proceedings of the 2nd
International Conference on Quantum Theories and Renormalization Group in
Gravity and Cosmology, Barcelona, July 200
Microscopic origin of granular ratcheting
Numerical simulations of assemblies of grains under cyclic loading exhibit
``granular ratcheting'': a small net deformation occurs with each cycle,
leading to a linear accumulation of deformation with cycle number. We show that
this is due to a curious property of the most frequently used models of the
particle-particle interaction: namely, that the potential energy stored in
contacts is path-dependent. There exist closed paths that change the stored
energy, even if the particles remain in contact and do not slide. An
alternative method for calculating the tangential force removes granular
ratcheting.Comment: 13 pages, 18 figure
Mechanical Unfolding of a Simple Model Protein Goes Beyond the Reach of One-Dimensional Descriptions
We study the mechanical unfolding of a simple model protein. The Langevin
dynamics results are analyzed using Markov-model methods which allow to
describe completely the configurational space of the system. Using transition
path theory we also provide a quantitative description of the unfolding
pathways followed by the system. Our study shows a complex dynamical scenario.
In particular, we see that the usual one-dimensional picture: free-energy vs
end-to-end distance representation, gives a misleading description of the
process. Unfolding can occur following different pathways and configurations
which seem to play a central role in one-dimensional pictures are not the
intermediate states of the unfolding dynamics.Comment: 10 pages, 6 figure
Neural network determination of the non-singlet quark distribution
We summarize the main features of our approach to parton fitting, and we show
a preliminary result for the non-singlet structure function. When comparing our
result to other PDF sets, we find a better description of large x data and
larger error bands in the extrapolation regions.Comment: 4 pages, 1 eps figure. Presented at the XIV International Workshop on
Deep Inelastic Scattering (DIS2006), Tsukuba, Japan, 20-24 April 200
Narrow-escape-time problem: the imperfect trapping case
We present a master equation approach to the \emph{narrow escape time} (NET)
problem, i.e. the time needed for a particle contained in a confining domain
with a single narrow opening, to exit the domain for the first time. We
introduce a finite transition probability, , at the narrow escape window
allowing the study of the imperfect trapping case. Ranging from 0 to ,
allowed the study of both extremes of the trapping process: that of a
highly deficient capture, and situations where escape is certain ("perfect
trapping" case). We have obtained analytic results for the basic quantity
studied in the NET problem, the \emph{mean escape time} (MET), and we have
studied its dependence in terms of the transition (desorption) probability over
(from) the surface boundary, the confining domain dimensions, and the finite
transition probability at the escape window. Particularly we show that the
existence of a global minimum in the NET depends on the `imperfection' of the
trapping process. In addition to our analytical approach, we have implemented
Monte Carlo simulations, finding excellent agreement between the theoretical
results and simulations.Comment: 9 page
Giant planets around two intermediate-mass evolved stars and confirmation of the planetary nature of HIP67851 c
Precision radial velocities are required to discover and characterize planets
orbiting nearby stars. Optical and near infrared spectra that exhibit many
hundreds of absorption lines can allow the m/s precision levels required for
such work. However, this means that studies have generally focused on
solar-type dwarf stars. After the main-sequence, intermediate-mass stars
(former A-F stars) expand and rotate slower than their progenitors, thus
thousands of narrow absorption lines appear in the optical region, permitting
the search for planetary Doppler signals in the data for these types of stars.
We present the discovery of two giant planets around the intermediate-mass
evolved star HIP65891 and HIP107773. The best Keplerian fit to the HIP65891 and
HIP107773 radial velocities leads to the following orbital parameters: P=1084.5
d; msin = 6.0 M; =0.13 and P=144.3 d; msin = 2.0
M; =0.09, respectively. In addition, we confirm the planetary nature
of the outer object orbiting the giant star HIP67851. The orbital parameters of
HIP67851c are: P=2131.8 d, msin = 6.0 M and =0.17. With
masses of 2.5 M and 2.4 M HIP65891 and HIP107773 are two of the
most massive stars known to host planets. Additionally, HIP67851 is one of five
giant stars that are known to host a planetary system having a close-in planet
( 0.7 AU). Based on the evolutionary states of those five stars, we
conclude that close-in planets do exist in multiple systems around subgiants
and slightly evolved giants stars, but probably they are subsequently destroyed
by the stellar envelope during the ascent of the red giant branch phase. As a
consequence, planetary systems with close-in objects are not found around
horizontal branch stars.Comment: Accepted for publication in A&
An eccentric companion at the edge of the brown dwarf desert orbiting the 2.4 Msun giant star HIP67537
We report the discovery of a substellar companion around the giant star
HIP67537. Based on precision radial velocity measurements from CHIRON and FEROS
high-resolution spectroscopic data, we derived the following orbital elements
for HIP67537: msin = 11.1 M,
= 4.9 AU and = 0.59. Considering
random inclination angles, this object has 65% probability to be
above the theoretical deuterium-burning limit, thus it is one of the few known
objects in the planet to brown-dwarf transition region. In addition, we
analyzed the Hipparcos astrometric data of this star, from which we derived a
minimum inclination angle for the companion of 2 deg. This value
corresponds to an upper mass limit of 0.3 M, therefore the
probability that HIP67537 is stellar in nature is 7%. The large
mass of the host star and the high orbital eccentricity makes HIP67537 a
very interesting and rare substellar object. This is the second candidate
companion in the brown dwarf desert detected in the sample of intermediate-mass
stars targeted by the EXPRESS radial velocity program, which corresponds to a
detection fraction of = 1.6%. This value is larger than the
fraction observed in solar-type stars, providing new observational evidence of
an enhanced formation efficiency of massive substellar companions in massive
disks. Finally, we speculate about different formation channels for this
object.Comment: Accepted for publication to A&
Recent progress on NNPDF for LHC
We present recent results of the NNPDF collaboration on a full DIS analysis
of Parton Distribution Functions (PDFs). Our method is based on the idea of
combining a Monte Carlo sampling of the probability measure in the space of
PDFs with the use of neural networks as unbiased universal interpolating
functions. The general structure of the project and the features of the fit are
described and compared to those of the traditional approaches.Comment: 4 pages, 6 figures, contribution for the proceedings of the
conference "Rencontres de Moriond, QCD and High Energy Interactions
Progress on neural parton distributions
We give a status report on the determination of a set of parton distributions
based on neural networks. In particular, we summarize the determination of the
nonsinglet quark distribution up to NNLO, we compare it with results obtained
using other approaches, and we discuss its use for a determination of
.Comment: 4 pages, 2 figs, uses dis2007.cls, to appear in the DIS 2007 workshop
proceeding
The impact of heavy quark mass effects in the NNPDF global analysis
We discuss the implementation of the FONLL general-mass scheme for heavy
quarks in deep-inelastic scattering in the FastKernel framework, used in the
NNPDF series of global PDF analysis. We present the general features of FONLL
and benchmark the accuracy of its implementation in FastKernel comparing with
the Les Houches heavy quark benchmark tables. We then show preliminary results
of the NNPDF2.1 analysis, in which heavy quark mass effects are included
following the FONLL-A GM scheme.Comment: 5 pages, 3 figures; to appear in the proceedings of DIS 2010, Firenz
- …