1,836 research outputs found
APFELgrid: a high performance tool for parton density determinations
We present a new software package designed to reduce the computational burden
of hadron collider measurements in Parton Distribution Function (PDF) fits. The
APFELgrid package converts interpolated weight tables provided by APPLgrid
files into a more efficient format for PDF fitting by the combination with PDF
and evolution factors provided by APFEL. This combination
significantly reduces the number of operations required to perform the
calculation of hadronic observables in PDF fits and simplifies the structure of
the calculation into a readily optimised scalar product. We demonstrate that
our technique can lead to a substantial speed improvement when compared to
existing methods without any reduction in numerical accuracy.Comment: 13 pages, 2 figures. Submitted to CPC. Code available from
https://github.com/nhartland/APFELgri
Antiproton and Positron Signal Enhancement in Dark Matter Mini-Spikes Scenarios
The annihilation of dark matter (DM) in the Galaxy could produce specific
imprints on the spectra of antimatter species in Galactic cosmic rays, which
could be detected by upcoming experiments such as PAMELA and AMS02. Recent
studies show that the presence of substructures can enhance the annihilation
signal by a "boost factor" that not only depends on energy, but that is
intrinsically a statistical property of the distribution of DM substructures
inside the Milky Way. We investigate a scenario in which substructures consist
of "mini-spikes" around intermediate-mass black holes. Focusing on
primary positrons and antiprotons, we find large boost factors, up to a few
thousand, that exhibit a large variance at high energy in the case of positrons
and at low energy in the case of antiprotons. As a consequence, an estimate of
the DM particle mass based on the observed cut-off in the positron spectrum
could lead to a substantial underestimate of its actual value.Comment: 13 pages, 9 figures, minor changes, version accepted for publication
in PR
Test of the gravitational redshift with stable clocks in eccentric orbits: application to Galileo satellites 5 and 6
The Einstein Equivalence Principle (EEP) is one of the foundations of the
theory of General Relativity and several alternative theories of gravitation
predict violations of the EEP. Experimental constraints on this fundamental
principle of nature are therefore of paramount importance. The EEP can be split
in three sub-principles: the Universality of Free Fall (UFF), the Local Lorentz
Invariance (LLI) and the Local Position Invariance (LPI). In this paper we
propose to use stable clocks in eccentric orbits to perform a test of the
gravitational redshift, a consequence of the LPI. The best test to date was
performed with the Gravity Probe A (GP-A) experiment in 1976 with an
uncertainty of . Our proposal considers the opportunity of
using Galileo satellites 5 and 6 to improve on the GP-A test uncertainty. We
show that considering realistic noise and systematic effects, and thanks to a
highly eccentric orbit, it is possible to improve on the GP-A limit to an
uncertainty around after one year of integration of
Galileo 5 and 6 data.Comment: 13 pages, 5 figures, accepted in Classical and Quantum Gravity as a
Fast Track Communicatio
Neutron Measurements at the Lunar Surface (NMLS)
The Neutron Measurement System (NMS-Lunar) is an instrument payload manifested on Astrobotics Peregrine Mission One (M1). Astrobotic Mission One will land at Lacus Mortis (~44oN, 254oE). Astrobotic will fly up to fourteen NASA payloads to the lunar surface in addition to other payload customers on M1. NMS-Lunar is a re-design of the MSFC Fast Neutron Spectrometer (FNS) currently operating on the ISS. The design of NMS-Lunar enables operation on the lunar surface, integration onto the Peregrine lander, and measurement of thermal neutron count rates on the lunar surface. The primary science objectives for NMS-Lunar is to provide ground truth of mapped neutron data from the Lunar Reconnaissance Orbiter and Lunar Prospector missions. Neutrons are created when galactic cosmic rays interact with the lunar regolith, and can provide valuable elemental composition information
A determination of the fragmentation functions of pions, kaons, and protons with faithful uncertainties
We present NNFF1.0, a new determination of the fragmentation functions (FFs)
of charged pions, charged kaons, and protons/antiprotons from an analysis of
single-inclusive hadron production data in electron-positron annihilation. This
determination, performed at leading, next-to-leading, and
next-to-next-to-leading order in perturbative QCD, is based on the NNPDF
methodology, a fitting framework designed to provide a statistically sound
representation of FF uncertainties and to minimise any procedural bias. We
discuss novel aspects of the methodology used in this analysis, namely an
optimised parametrisation of FFs and a more efficient minimisation
strategy, and validate the FF fitting procedure by means of closure tests. We
then present the NNFF1.0 sets, and discuss their fit quality, their
perturbative convergence, and their stability upon variations of the kinematic
cuts and the fitted dataset. We find that the systematic inclusion of
higher-order QCD corrections significantly improves the description of the
data, especially in the small- region. We compare the NNFF1.0 sets to other
recent sets of FFs, finding in general a reasonable agreement, but also
important differences. Together with existing sets of unpolarised and polarised
parton distribution functions (PDFs), FFs and PDFs are now available from a
common fitting framework for the first time.Comment: 50 pages, 22 figures, 5 table
Statistical properties of the GALEX spectroscopic stellar sample
The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained
from slitless grism observations, for a total of more than 60,000 ultraviolet
spectra. We have determined statistical properties of the sample of GALEX
stars. We have defined a suitable system of spectroscopic indices, which
measure the main mid-UV features at the GALEX low spectral resolution and we
have employed it to determine the atmospheric parameters of of stars in the
range 4500<Teff<9000 K. Our preliminary results indicate that the sample is
formed by a majority of main sequence F- and G-type stars, with metallicity
[M/H]>-1 dex.Comment: 9 pages, 9 figures, accepted for publication in Astrophysics & Space
Science, UV universe special issu
Identification of the missing pluripotency mediator downstream of leukaemia inhibitory factor
Self-renewal of pluripotent mouse embryonic stem (ES) cells is sustained by the cytokine leukaemia inhibitory factor (LIF) acting through the transcription factor Stat3. Several targets of Stat3 have previously been identified, most notably the reprogramming factor Klf4. However, such factors are neither required nor sufficient for the potent effect of LIF. We took advantage of Stat3 null ES cells to confirm that Stat3 mediates the self-renewal response to LIF. Through comparative transcriptome analysis intersected with genome location data, we arrived at a set of candidate transcription factor effectors. Among these, Tfcp2l1 (also known as Crtr-1) was most abundant. Constitutive expression of Tfcp2l1 at levels similar to those induced by LIF effectively substituted for LIF or Stat3 in sustaining clonal self-renewal and pluripotency. Conversely, knockdown of Tfcp2l1 profoundly compromised responsiveness to LIF. We further found that Tfcp2l1 is both necessary and sufficient to direct molecular reprogramming of post-implantation epiblast stem cells to naĂŻve pluripotency. These results establish Tfcp2l1 as the principal bridge between LIF/Stat3 input and the transcription factor core of naĂŻve pluripotency
Probing the Nature of the Weakest Intergalactic Magnetic Fields with the High Energy Emission of Gamma-Ray Bursts
We investigate the delayed, secondary GeV-TeV emission of gamma-ray bursts
and its potential to probe the nature of intergalactic magnetic fields.
Geometrical effects are properly taken into account for the time delay between
primary high energy photons and secondary inverse Compton photons from
electron-positron pairs, which are produced in - interactions
with background radiation fields and deflected by intervening magnetic fields.
The time-dependent spectra of the delayed emission are evaluated for a wide
range of magnetic field strengths and redshifts. The typical flux and delay
time of secondary photons from bursts at are respectively GeV cm s and s if the field strengths are
G, as might be the case in intergalactic void regions. We find
crucial differences between the cases of coherent and tangled magnetic fields,
as well as dependences on the field coherence length.Comment: 19 pages, 9 figures, formulation revised, accepted for publication in
Ap
Systematic uncertainties in the determination of the local dark matter density
A precise determination of the local dark matter density and an accurate
control over the corresponding uncertainties are of paramount importance for
Dark Matter (DM) searches. Using very recent high-resolution numerical
simulations of a Milky Way like object, we study the systematic uncertainties
that affect the determination of the local dark matter density based on
dynamical measurements in the Galaxy. In particular, extracting from the
simulation with baryons the orientation of the Galactic stellar disk with
respect to the DM distribution, we study the DM density for an observer located
at 8 kpc from the Galactic center {\it on the stellar disk}, .
This quantity is found to be always larger than the average density in a
spherical shell of same radius , which is the quantity inferred
from dynamical measurements in the Galaxy, and to vary in the range
. This suggests that the actual dark matter
density in the solar neighbourhood is on average 21\% larger than the value
inferred from most dynamical measurements, and that the associated systematic
errors are larger than the statistical errors recently discussed in the
literature.Comment: 6 pages, 3 figures, matches published versio
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