190 research outputs found
A Markov Chain Monte Carlo technique to sample transport and source parameters of Galactic cosmic rays: II. Results for the diffusion model combining B/C and radioactive nuclei
On-going measurements of the cosmic radiation (nuclear, electronic, and
gamma-ray) are shedding new light on cosmic-ray physics. A comprehensive
picture of these data relies on an accurate determination of the transport and
source parameters of propagation models. A Markov Chain Monte Carlo is used to
obtain these parameters in a diffusion model. From the measurement of the B/C
ratio and radioactive cosmic-ray clocks, we calculate their probability density
functions, with a special emphasis on the halo size L of the Galaxy and the
local underdense bubble of size r_h. The analysis relies on the USINE code for
propagation and on a Markov Chain Monte Carlo technique (Putze et al. 2009,
paper I of this series) for the parameter determination. As found in previous
studies, the B/C best-fit model favours diffusion/convection/reacceleration
(Model III) over diffusion/reacceleration (Model II). A combined fit on B/C and
the isotopic ratios (10Be/9Be, 26Al/27Al, 36Cl/Cl) leads to L ~ 8 kpc and r_h ~
120 pc for the best-fit Model III. This value for r_h is consistent with direct
measurements of the local interstallar medium. For Model II, L ~ 4 kpc and r_h
is consistent with zero. We showed the potential and usefulness of the Markov
Chain Monte Carlo technique in the analysis of cosmic-ray measurements in
diffusion models. The size of the diffusive halo depends crucially on the value
of the diffusion slope delta, and also on the presence/absence of the local
underdensity damping effect on radioactive nuclei. More precise data from
on-going experiments are expected to clarify this issue.Comment: 20 pages, 14 figures, minor language corrections to match the A&A
accepted versio
Nuclear Cosmic Rays propagation in the Atmosphere
The transport of the nuclear cosmic ray flux in the atmosphere is studied and
the atmospheric corrections to be applied to the measurements are calculated.
The contribution of the calculated corrections to the accuracy of the
experimental results are discussed and evaluated over the kinetic energy range
10-10 GeV/n. The Boron (B) and Carbon (C) elements system is used as a
test case. It is shown that the required corrections become largely dominant at
the highest energies investigated. The results are discussed.Comment: Proc. of 30th International Cosmic Ray Conference, Merida, Mexico; 4
page
Theoretical uncertainties in extracting cosmic-ray diffusion parameters: the boron-to-carbon ratio
PAMELA and, more recently, AMS-02, are ushering us into a new era of greatly
reduced statistical uncertainties in experimental measurements of cosmic-ray
fluxes. In particular, new determinations of traditional diagnostic tools such
as the boron-to-carbon ratio (B/C) are expected to significantly reduce errors
on cosmic-ray diffusion parameters, with important implications for
astroparticle physics, ranging from inferring primary source spectra to
indirect dark matter searches. It is timely to stress, however, that the
conclusions obtained crucially depend on the framework in which the data are
interpreted as well as from some nuclear input parameters. We aim at assessing
the theoretical uncertainties affecting the outcome, with models as simple as
possible while still retaining the key dependencies. We compare different
semi-analytical, two-zone model descriptions of cosmic-ray transport in the
Galaxy. We test for the effect of a primary source contamination in the boron
flux by parametrically altering its flux, as well as for nuclear cross section
uncertainties. Our study on preliminary results from AMS-02 suggests that,
differently for instance from the leptonic case, realistic modelling of the
geometry of the Galaxy and of the source distribution are of minor importance
to correctly reproduce B/C data at high energies and thus, to a large extent,
for the extraction of diffusion parameters. The Ansatz on the lack of primary
injection of boron represents the most serious bias, and requires
multi-messenger studies to be addressed. If this uncertainty could be lifted,
nuclear uncertainties would still represent a serious concern, which degrade
the systematic error on the inferred parameters to the 20% level, or three
times the estimated experimental sensitivity. In order to reduce this, a new
nuclear cross section measurement campaign is probably required.Comment: 14 pages, 11 figures, 4 tables, published in A&
Systematic uncertainties on the cosmic-ray transport parameters: Is it possible to reconcile B/C data with delta = 1/3 or delta = 1/2?
The B/C ratio is used in cosmic-ray physics to constrain the transport
parameters. However, from the same set of data, the various published values
show a puzzling large scatter of these parameters. We investigate the impact of
using different inputs (gas density and hydrogen fraction in the Galactic disc,
source spectral shape, low-energy dependence of the diffusion coefficient, and
nuclear fragmentation cross-sections) on the best-fit values of the transport
parameters. We quantify the systematics produced when varying these inputs, and
compare them to statistical uncertainties. We discuss the consequences for the
slope of the diffusion coefficient delta. The analysis relies on the
propagation code USINE interfaced with the Minuit minimisation routines. We
find the typical systematic uncertainties to be larger than the statistical
ones. The several published values of delta (from 0.3 to 0.8) can be recovered
when varying the low-energy shape of the diffusion coefficient and the
convective wind strength. Models including a convective wind are characterised
by delta > 0.6, which cannot be reconcile with the expected theoretical values
(1/3 and 1/2). However, from a statistical point of view (chi^2 analysis),
models with both reacceleration and convection-hence large delta-are favoured.
The next favoured models in line yield delta that can be accommodated with 1/3
and 1/2, but require a strong upturn of the diffusion coefficient at low energy
(and no convection). To date, using the best statistical tools, the transport
parameter determination is still plagued by many unknowns at low energy (~
GeV/n). To disentangle between all these configurations, measurements of the
B/C ratio at TeV/n energies and/or combination with other secondary-to-primary
ratios is necessary.Comment: 12 pages, 7 figures, minor language corrections to match the A&A
accepted versio
p, He, and C to Fe cosmic-ray primary fluxes in diffusion models: Source and transport signatures on fluxes and ratios
The propagated fluxes of proton, helium, and heavier primary cosmic-ray
species (up to Fe) are a means to indirectly access the source spectrum of
cosmic rays. We check the compatibility of the primary fluxes with the
transport parameters derived from the B/C analysis, but also if they bring
further constraints. Proton data are well described in the simplest model
defined by a power-law source spectrum and plain diffusion. They can also be
accommodated by models with, e.g., convection and/or reacceleration. There is
no need for breaks in the source spectral indices below TeV/n. Fits on
the primary fluxes alone do not provide physical constraints on the transport
parameters. If we let free the source spectrum and fix the diffusion coefficient such as to reproduce the B/C ratio, the MCMC analysis constrains
the source spectral index to be in the range for all primary
species up to Fe, regardless of the value of the diffusion slope . The
low-energy shape of the source spectrum is degenerate with the
low-energy shape of the diffusion coefficient: we find
for p and He data, but for C
to Fe primary species. This is consistent with the toy-model calculation in
which the shape of the p/He and C/O to Fe/O data is reproduced if
(no need for different slopes ). When
plotted as a function of the kinetic energy per nucleon, the low-energy p/He
ratio is shaped mostly by the modulation effect, whereas primary/O ratios are
mostly shaped by their destruction rate.Comment: 18 pages, 14 figures: accepted in A&A (1 table added
Pippi - painless parsing, post-processing and plotting of posterior and likelihood samples
Interpreting samples from likelihood or posterior probability density
functions is rarely as straightforward as it seems it should be. Producing
publication-quality graphics of these distributions is often similarly painful.
In this short note I describe pippi, a simple, publicly-available package for
parsing and post-processing such samples, as well as generating high-quality
PDF graphics of the results. Pippi is easily and extensively configurable and
customisable, both in its options for parsing and post-processing samples, and
in the visual aspects of the figures it produces. I illustrate some of these
using an existing supersymmetric global fit, performed in the context of a
gamma-ray search for dark matter. Pippi can be downloaded and followed at
http://github.com/patscott/pippi .Comment: 4 pages, 1 figure. v3: Updated for pippi 2.0. New features include
hdf5 support, out-of-core processing, inline post-processing with arbitrary
Python code in the input file, and observable-specific data cuts. Pippi can
be downloaded from http://github.com/patscott/pipp
A new look at the cosmic ray positron fraction
The positron fraction in cosmic rays was found to be a steadily increasing in
function of energy, above 10 GeV. This behaviour contradicts standard
astrophysical mechanisms, in which positrons are secondary particles, produced
in the interactions of primary cosmic rays during the propagation in the
interstellar medium. The observed anomaly in the positron fraction triggered a
lot of excitement, as it could be interpreted as an indirect signature of the
presence of dark matter species in the Galaxy. Alternatively, it could be
produced by nearby astrophysical sources, such as pulsars. Both hypotheses are
probed in this work in light of the latest AMS-02 positron fraction
measurements. The transport of the primary and secondary positrons in the
Galaxy is described using a semi-analytic two-zone model. MicrOMEGAs is used to
model the positron flux generated by dark matter species. The description of
the positron fraction from astrophysical sources is based on the pulsar
observations included in the ATNF catalogue. We find that the mass of the
favoured dark matter candidates is always larger than 500 GeV. The only dark
matter species that fulfils the numerous gamma ray and cosmic microwave
background bounds is a particle annihilating into four leptons through a light
scalar or vector mediator, with a mixture of tau (75%) and electron (25%)
channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also be
explained by a single astrophysical source and a list of five pulsars from the
ATNF catalogue is given. Those results are obtained with the cosmic ray
transport parameters that best fit the B/C ratio. Uncertainties in the
propagation parameters turn out to be very significant. In the WIMP
annihilation cross section to mass plane for instance, they overshadow the
error contours derived from the positron data.Comment: 20 pages, 16 figures, accepted for publication in A&A, corresponds to
published versio
Status of cosmic-ray antideuteron searches
The precise measurement of cosmic-ray antiparticles serves as important means
for identifying the nature of dark matter. Recent years showed that identifying
the nature of dark matter with cosmic-ray positrons and higher energy
antiprotons is difficult, and has lead to a significantly increased interest in
cosmic-ray antideuteron searches. Antideuterons may also be generated in dark
matter annihilations or decays, offering a potential breakthrough in unexplored
phase space for dark matter. Low-energy antideuterons are an important approach
because the flux from dark matter interactions exceeds the background flux by
more than two orders of magnitude in the low-energy range for a wide variety of
models. This review is based on the "dbar14 - dedicated cosmic-ray antideuteron
workshop", which brought together theorists and experimentalists in the field
to discuss the current status, perspectives, and challenges for cosmic-ray
antideuteron searches and discusses the motivation for antideuteron searches,
the theoretical and experimental uncertainties of antideuteron production and
propagation in our Galaxy, as well as give an experimental cosmic-ray
antideuteron search status update. This report is a condensed summary of the
article "Review of the theoretical and experimental status of dark matter
identification with cosmic-ray antideuteron" (arXiv:1505.07785).Comment: 9 pages, 4 figures, ICRC 2015 proceeding
Cosmic-ray antiproton constraints on light dark matter candidates
Some direct detection experiments have recently collected excess events that
could be interpreted as a dark matter (DM) signal, pointing to particles in the
10 GeV mass range. We show that scenarios in which DM can self-annihilate
with significant couplings to quarks are likely excluded by the cosmic-ray (CR)
antiproton data, provided the annihilation is S-wave dominated when DM
decouples in the early universe. These limits apply to most of supersymmetric
candidates, eg in the minimal supersymmetric standard model (MSSM) and in the
next-to-MSSM (NMSSM), and more generally to any thermal DM particle with
hadronizing annihilation final states.Comment: Contribution to the proceedings of TAUP-2011 (Munich, 5-9 IX 2011). 4
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