Direct constraints on diffusion models from cosmic-ray positron data: Excluding the Minimal model for dark matter searches

Galactic Cosmic-ray (CR) transport parameters are usually constrained by the boron-to-carbon ratio. This procedure is generically plagued with degeneracies between the diffusion coefficient and the vertical extent of the Galactic magnetic halo. The latter is of paramount importance for indirect dark matter (DM) searches, because it fixes the amount of DM annihilation or decay that contributes to the local antimatter CR flux. These degeneracies could be broken by using secondary radioactive species, but the current data still have large error bars, and this method is extremely sensitive to the very local interstellar medium (ISM) properties. Here, we propose to use the low-energy CR positrons in the GeV range as another direct constraint on diffusion models. We show that the PAMELA data disfavor small diffusion halo ($L\lesssim 3$ kpc) and large diffusion slope models, and exclude the minimal ({\em min}) configuration (Maurin et al. 2001, Donato et al. 2004) widely used in the literature to bracket the uncertainties in the DM signal predictions. This is complementary to indirect constraints (diffuse radio and gamma-ray emissions) and has strong impact on DM searches. Indeed this makes the antiproton constraints more robust while enhancing the discovery/exclusion potential of current and future experiments, like AMS-02 and GAPS, especially in the antiproton and antideuteron channels.Comment: 7 pages, 3 figures. V2: minor changes to match to the published version; misprints in Eqs.(4) fixe

A global fit of the MSSM with GAMBIT

We study the seven-dimensional Minimal Supersymmetric Standard Model (MSSM7) with the new GAMBIT software framework, with all parameters defined at the weak scale. Our analysis significantly extends previous weak-scale, phenomenological MSSM fits, by adding more and newer experimental analyses, improving the accuracy and detail of theoretical predictions, including dominant uncertainties from the Standard Model, the Galactic dark matter halo and the quark content of the nucleon, and employing novel and highly-efficient statistical sampling methods to scan the parameter space. We find regions of the MSSM7 that exhibit co-annihilation of neutralinos with charginos, stops and sbottoms, as well as models that undergo resonant annihilation via both light and heavy Higgs funnels. We find high-likelihood models with light charginos, stops and sbottoms that have the potential to be within the future reach of the LHC. Large parts of our preferred parameter regions will also be accessible to the next generation of direct and indirect dark matter searches, making prospects for discovery in the near future rather good

Phénoménologie et détection du rayonnement cosmique nucléaire

One century after the discovery of cosmic rays – a flux of energetic charged particles which bombards the upper layers of Earth's atmosphere –, many questions remain still open on its origin, nature and transport. The precise measurement of the cosmic-ray ion flux aims to study the acceleration and propagation processes. In particular, the measurement of secondary-to-primary ratios allows to constrain propagation models very effectively due to its direct dependency to the grammage seen by the particles during their transport. The knowledge and the characterisation of the processes related to the propagation make it possible to reconstruct the cosmic-ray source spectrum and thus to constrain the acceleration processes, but also to test the existence of exotic contributions such as the annihilation of dark-matter particles. This thesis treats two aspects of cosmic-ray physics: the phenomenology and the detection. Concerning the phenomenological aspect, the work presented here consists in evaluating and studying the constraints on galactic cosmic-ray propagation models provided by current measurements using a Markov Chain Monte Carlo. The experimental aspect of this work concerns the participation in the construction, the validation and the data analysis of the CherCam subdetector – a Cherenkov imager measuring the charge of cosmic-ray ions for the CREAM experiment – whose preliminary results are presented.Un siècle après la découverte du rayonnement cosmique – un flux de particules énergétiques chargées qui bombarde les couches supérieures de l'atmosphère terrestre –, beaucoup de questions restent encore ouvertes sur son origine, sa nature et son transport. La mesure précise du flux des ions du rayonnement cosmique a pour objectif d'étudier les processus d'accélération et de propagation. En particulier la mesure des rapports secondaire sur primaire permet de contraindre très efficacement les modèles de propagation car elle est directement liée au grammage vu par les particules durant leur transport. La connaissance et la caractérisation des processus liés à la propagation permet de reconstruire le spectre source du rayonnement cosmique et donc de contraindre les processus d'accélération, mais aussi de tester l'existence dans le rayonnement cosmique de contributions exotiques comme l'annihilation de particules de matière noire. Cette thèse traite deux aspects de la physique du rayonnement cosmique: la phénoménologie et la détection. Concernant l'aspect phénoménologique, le travail présenté consiste à évaluer et à étudier les contraintes que les mesures actuelles permettent d'apporter sur les modèles de propagation du rayonnement dans la Galaxie à l'aide d'un Monte Carlo par chaînes de Markov. L'aspect expérimental de ce travail porte sur la participation à la construction, la validation et l'analyse des données du sous-détecteur CherCam – un imageur Cherenkov mesurant la charge des ions du rayonnement cosmique pour l'expérience CREAM –, dont les résultats préliminaires sont présentés

Phénoménologie et détection du rayonnement cosmique nucléaire

Un siècle après la découverte du rayonnement cosmique un flux de particules énergétiques chargées qui bombarde les couches supérieures de l'atmosphère terrestre , beaucoup de questions restent encore ouvertes sur son origine, sa nature et son transport. La mesure précise du flux des ions du rayonnement cosmique a pour objectif d'étudier les processus d'accélération et de propagation. En particulier la mesure des rapports secondaire sur primaire permet de contraindre très efficacement les modèles de propagation car elle est directement liée au grammage vu par les particules durant leur transport. La connaissance et la caractérisation des processus liés à la propagation permet de reconstruire le spectre source du rayonnement cosmique et donc de contraindre les processus d'accélération, mais aussi de tester l'existence dans le rayonnement cosmique de contributions exotiques comme l'annihilation de particules de matière noire. Cette thèse traite deux aspects de la physique du rayonnement cosmique: la phénoménologie et la détection. Concernant l'aspect phénoménologique, le travail présenté consiste à évaluer et à étudier les contraintes que les mesures actuelles permettent d'apporter sur les modèles de propagation du rayonnement dans la Galaxie à l'aide d'un Monte Carlo par chaînes de Markov. L'aspect expérimental de ce travail porte sur la participation à la construction, la validation et l'analyse des données du sous-détecteur CherCam un imageur Cherenkov mesurant la charge des ions du rayonnement cosmique pour l'expérience CREAM , dont les résultats préliminaires sont présentés.One century after the discovery of cosmic rays a flux of energetic charged particles which bombards the upper layers of Earth's atmosphere , many questions remain still open on its origin, nature and transport. The precise measurement of the cosmic-ray ion flux aims to study the acceleration and propagation processes. In particular, the measurement of secondary-to-primary ratios allows to constrain propagation models very effectively due to its direct dependency to the grammage seen by the particles during their transport. The knowledge and the characterisation of the processes related to the propagation make it possible to reconstruct the cosmic-ray source spectrum and thus to constrain the acceleration processes, but also to test the existence of exotic contributions such as the annihilation of dark-matter particles. This thesis treats two aspects of cosmic-ray physics: the phenomenology and the detection. Concerning the phenomenological aspect, the work presented here consists in evaluating and studying the constraints on galactic cosmic-ray propagation models provided by current measurements using a Markov Chain Monte Carlo. The experimental aspect of this work concerns the participation in the construction, the validation and the data analysis of the CherCam subdetector a Cherenkov imager measuring the charge of cosmic-ray ions for the CREAM experiment whose preliminary results are presented.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Constraints on cosmic-ray propagation and acceleration models from recent data

We are studying the constraints obtained on transport and acceleration mechanisms of galactic cosmic rays by using statistical tools in combination with the propagation package GALPROP and recent PAMELA data. Using only PAMELA data allows us to avoid inconsistencies between data sets from different experiments, minimise uncertainties on solar modulation parameters, and have a complementary and precise data set on (anti-)matter as well as primary and secondary nuclei over 3 orders of magnitude in energy. This allows us to simultaneously place strong constraints on cosmic-ray propagation and acceleration models. We describe our methodology and present some preliminary results in this paper.QC 20120214</p

Comparison of statistical sampling methods with ScannerBit, the GAMBIT scanning module

We introduce ScannerBit, the statistics and sampling module of the public, open-source global fitting framework GAMBIT. ScannerBit provides a standardised interface to different sampling algorithms, enabling the use and comparison of multiple computational methods for inferring profile likelihoods, Bayesian posteriors, and other statistical quantities. The current version offers random, grid, raster, nested sampling, differential evolution, Markov Chain Monte Carlo (MCMC) and ensemble Monte Carlo samplers. We also announce the release of a new standalone differential evolution sampler, Diver, and describe its design, usage and interface to ScannerBit. We subject Diver and three other samplers (the nested sampler MultiNest, the MCMC GreAT, and the native ScannerBit implementation of the ensemble Monte Carlo algorithm T-Walk) to a battery of statistical tests. For this we use a realistic physical likelihood function, based on the scalar singlet model of dark matter. We examine the performance of each sampler as a function of its adjustable settings, and the dimensionality of the sampling problem. We evaluate performance on four metrics: optimality of the best fit found, completeness in exploring the best-fit region, number of likelihood evaluations, and total runtime. For Bayesian posterior estimation at high resolution, T-Walk provides the most accurate and timely mapping of the full parameter space. For profile likelihood analysis in less than about ten dimensions, we find that Diver and MultiNest score similarly in terms of best fit and speed, outperforming GreAT and T-Walk; in ten or more dimensions, Diver substantially outperforms the other three samplers on all metrics

What do Galactic electrons and positrons tell us about dark matter?

International audienceWe devise a new semi-analytical method dedicated to the propagation of Galactic electrons and positrons from MeV to TeV energies: the pinching method. 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. This new tool is fast and allows to carry out extensive scans over parameters. We strongly constrain the cosmic ray propagation parameters by requiring that the secondary component of positrons does not overshoot the AMS-02 measurements. We find that only models with a large diffusion coefficient and a large magnetic halo size are selected by this test. Therefore, we find that the positron excess appears from 1 GeV. We then explore the possibility to explain the positron excess with a component coming from the annihilation of dark matter particles. We show that the pure dark matter interpretation of the AMS-02 positron data is strongly disfavoured. This conclusion is based solely on the positron data, and no other observation, such as the antiproton and gamma ray fluxes or the CMB anisotropies, needs to be invoked. MeV dark matter particles annihilating or decaying to electron-positron pairs cannot, in principle, be observed via local cosmic ray measurements because of the shielding solar magnetic field. We take advantage of spacecraft Voyager-I's capacity for detecting interstellar cosmic rays since it crossed the heliopause in 2012. This opens up a new avenue to probe dark matter particles in the sub-GeV energy/mass range that we exploit here for the first time

Uncertainties on propagation parameters: impact on the interpretation of the positron fraction

International audienceThe positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 GeV, and it was found to be a steadily increasing function of energy above 10 GeV. This behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their 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, the so-called weakly interacting massive particles (WIMPs). Alternatively, it could be produced by nearby sources, such as pulsars. These hypotheses are probed in [1], in light of the latest AMS-02 positron fraction measurements. The cosmic ray positron transport in the Galaxy is described using a semi-analytic two-zone model. For consistency, the secondary and primary components of the positron flux are calculated together with the same propagation model. We show that the results inferred for both hypotheses crucially depend on the propagation parameters, estimated with the Boron-to-Carbon ratio. Their uncertainties turn out to be very significant, and overshadow even the statical errors from the positron data