101 research outputs found
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&
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
Spin physics and TMD studies at A Fixed-Target ExpeRiment at the LHC (AFTER@LHC)
We report on the opportunities for spin physics and Transverse-Momentum
Dependent distribution (TMD) studies at a future multi-purpose fixed-target
experiment using the proton or lead ion LHC beams extracted by a bent crystal.
The LHC multi-TeV beams allow for the most energetic fixed-target experiments
ever performed, opening new domains of particle and nuclear physics and
complementing that of collider physics, in particular that of RHIC and the EIC
projects. The luminosity achievable with AFTER@LHC using typical targets would
surpass that of RHIC by more that 3 orders of magnitude in a similar energy
region. In unpolarised proton-proton collisions, AFTER@LHC allows for
measurements of TMDs such as the Boer-Mulders quark distributions, the
distribution of unpolarised and linearly polarised gluons in unpolarised
protons. Using the polarisation of hydrogen and nuclear targets, one can
measure transverse single-spin asymmetries of quark and gluon sensitive probes,
such as, respectively, Drell-Yan pair and quarkonium production. The
fixed-target mode has the advantage to allow for measurements in the
target-rapidity region, namely at large x^uparrow in the polarised nucleon.
Overall, this allows for an ambitious spin program which we outline here.Comment: 6 pages, 4 figures, 1 table, LaTeX. Proceedings of the Fourth
International Workshop on Transverse Polarisation Phenomena in Hard Processes
(Transversity 2014), 9-13 June, 2013, Chia, Ital
The High-Acceptance Dielectron Spectrometer HADES
HADES is a versatile magnetic spectrometer aimed at studying dielectron
production in pion, proton and heavy-ion induced collisions. Its main features
include a ring imaging gas Cherenkov detector for electron-hadron
discrimination, a tracking system consisting of a set of 6 superconducting
coils producing a toroidal field and drift chambers and a multiplicity and
electron trigger array for additional electron-hadron discrimination and event
characterization. A two-stage trigger system enhances events containing
electrons. The physics program is focused on the investigation of hadron
properties in nuclei and in the hot and dense hadronic matter. The detector
system is characterized by an 85% azimuthal coverage over a polar angle
interval from 18 to 85 degree, a single electron efficiency of 50% and a vector
meson mass resolution of 2.5%. Identification of pions, kaons and protons is
achieved combining time-of-flight and energy loss measurements over a large
momentum range. This paper describes the main features and the performance of
the detector system
Stable laws and cosmic ray physics
International audienceIn the new precision era for cosmic ray astrophysics, theoretical predictionscannot content themselves with average trends, but need to correctly take intoaccount intrinsic uncertainties. The space-time discreteness of the cosmic raysources, joined with a substantial ignorance of their precise epochs andlocations (with the possible exception of the most recent and close ones) playsan important role in this sense. We elaborate a statistical theory to deal withthis problem, relating the composite probability P({\Psi}) to obtain a flux{\Psi} at the Earth to the single-source probability p({\psi}) to contributewith a flux {\psi}. The main difficulty arises since p({\psi}) is a fat taildistribution, characterized by power-law or broken power-law behaviour up tovery large fluxes for which central limit theorem does not hold, and leading towell-known stable laws as opposed to Gaussian distributions. We find thatrelatively simple recipes provide a satisfactory description of the probabilityP({\Psi}). We also find that a naive Gaussian fit to simulation results wouldunderestimate the probability of very large fluxes, i.e. several times abovethe average, while overestimating the probability of relatively milderexcursions. At large energies, large flux fluctuations are prevented by causalconsiderations, while at low energies a partial knowledge on the recent andnearby population of sources plays an important role. A few proposal have beendiscussed in the literature to account for spectral breaks recently reported incosmic ray data in terms of local contributions. We apply our newly developedtheory to assess their probabilities, finding that they are relatively small
Galactic halo size in the light of recent AMS-02 data
The vertical diffusive halo size of the Galaxy, , is a key parameter for
dark matter indirect searches. It can be better determined thanks to recent
AMS-02 data. We set constraints on from Be/B and Be/Be data, and we
performed a consistency check with positron data. We detail the dependence of
Be/B and Be/Be on and forecast on which energy range better data
would be helpful for future improvements. We used USINE v3.5 for the
propagation of nuclei, and were calculated with the pinching method of
Boudaud et al. (2017). The current AMS-02 Be/B ( precision) and
ACE-CRIS Be/Be ( precision) data bring similar and consistent
constraints on . The AMS-02 Be/B data alone constrain ~kpc at
a 68\% confidence level (spanning different benchmark transport
configurations), a range for which most models do not overproduce positrons.
Future experiments need to deliver percent-level accuracy on Be/Be
anywhere below 10 GV to further constrain . Forthcoming AMS-02, HELIX, and
PAMELA Be/Be results will further test and possibly tighten the
limits derived here. Elemental ratios involving radioactive species with
different lifetimes (e.g. Al/Mg and Cl/Ar) are also awaited to provide
complementary and robuster constraints.Comment: 15 pages, 10 figures, 6 tables. Additional references and few
clarifications (matches A&A accepted version
The pinching method for Galactic cosmic ray positrons: Implications in the light of precision measurements
18p, 11 figures, comments welcome - Astronomy & Astrophysics manuscript no. draft_crac3_bodyInternational audienceContext. Two years ago, the Ams-02 collaboration released the most precise measurement of the cosmic ray positron flux. In the conventional approach, in which positrons are considered as purely secondary particles, the theoretical predictions fall way below the data above 10 GeV. One suggested explanation for this anomaly is the annihilation of dark matter particles, the so-called weakly interactive massive particles (WIMPs), into standard model particles. Most analyses have focused on the high-energy part of the positron spectrum, where the anomaly lies, disregarding the complicated GeV low-energy region where Galactic cosmic ray transport is more difficult to model and solar modulation comes into play.Aims. Given the high quality of the latest measurements by Ams-02, it is now possible to systematically re-examine the positron anomaly over the entire energy range, this time taking into account transport processes so far neglected, such as Galactic convection or diffusive re-acceleration. These might impact somewhat on the high-energy positron flux so that a complete and systematic estimate of the secondary component must be performed and compared to the Ams-02 measurements. The flux yielded by WIMPs also needs to be re-calculated more accurately to explore how dark matter might source the positron excess. Methods. We devise a new semi-analytical method to take into account transport processes thus far neglected, but important below a few GeV. It is essentially based on the pinching of inverse Compton and synchrotron energy losses from the magnetic halo, where they take place, inside the Galactic disc. The corresponding energy loss rate is artificially enhanced by the so-called pinching factor, which needs to be calculated at each energy. We have checked that this approach reproduces the results of the Green function method at the per mille level. This new tool is fast and allows one to carry out extensive scans over the cosmic ray propagation parameters.Results. We derive the positron flux from sub-GeV to TeV energies for both gas spallation and dark matter annihilation. We carry out a scan over the cosmic ray propagation parameters, which we strongly constrain by requiring that the secondary component does not overshoot the Ams-02 measurements. We find that only models with large diffusion coefficients are selected by this test. We then add to the secondary component the positron flux yielded by dark matter annihilation. We carry out a scan over WIMP mass to fit the annihilation cross-section and branching ratios, successively exploring the cases of a typical beyond-the-standard-model WIMP and an annihilation through light mediators. In the former case, the best fit yields a p-value of 0.4% for a WIMP mass of 264 GeV, a value that does not allow to reproduce the highest energy data points. If we require the mass to be larger than 500 GeV, the best-fit χ2 per degree of freedom always exceeds a value of 3. The case of light mediators is even worse, with a best-fit χ2 per degree of freedom always larger than 15.Conclusions. We explicitly show that the cosmic ray positron flux is a powerful and independent probe of Galactic cosmic ray propagation. It should be used as a complementary observable to other tracers such as the boron-to-carbon ratio. This analysis shows also that the pure dark matter interpretation of the positron excess is strongly disfavoured. This conclusion is based solely on the positron data, and no other observation, such as the antiproton flux or the CMB anisotropies, needs to be invoked
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