Richard Taillet

Cosmic Rays below Z = 30 in a Diffusion Model: New Constraints on Propagation Parameters

Cosmic-ray nuclei fluxes are expected to be measured with high precision in the near future. For instance, high-quality data on the antiproton component could give important clues about the nature of the astronomical dark matter. A very good understanding of the different aspects of cosmic-ray propagation is therefore necessary. In this paper, we use cosmic-ray nuclei data to give constraints on the diffusion parameters. Propagation is studied with semianalytical solutions of a diffusion model, and we give new analytical solutions for radioactively produced species. Our model includes convection and reacceleration, as well as the standard energy losses. We perform a ?2 analysis over B/C data for a large number of configurations obtained by varying the relevant parameters of the diffusion model. A very good agreement with B/C data arises for a number of configurations, all of which are compatible with sub-Fe/Fe data. Different source spectra Q(E) and diffusion coefficients K(E) have been tried, but for both parameters only one form gives a good fit. Another important result is that models without convection or without reacceleration are excluded. We find that the various parameters, i.e., the diffusion coefficient normalization K0 and spectral index ?, the halo thickness L, the Alfv?n velocity VA, and the convection velocity Vc are strongly correlated. We obtain limits on the spectral index ? of the diffusion coefficient, and in particular we exclude a Kolmogorov spectrum (? = ).

Antiprotons from Spallations of Cosmic Rays on Interstellar Matter

Cosmic-ray antiprotons provide an important probe for the study of Galactic dark matter, as they could be produced by neutralino annihilations, primordial black holes evaporations, or other exotic sources. On the other hand, antiprotons are anyway produced by standard nuclear reactions of cosmic-ray nuclei on interstellar matter (spallations), which are known to occur in the Galaxy. This process is responsible for a background flux that must be carefully determined to estimate the detectability of an hypothetical exotic signal. In this paper we provide a new evaluation of the interstellar cosmic antiproton flux that is fully consistent with cosmic-ray nuclei in the framework of a two-zone diffusion model. We also study and conservatively quantify all possible sources of uncertainty that may affect that antiproton flux. In particular, the primary cosmic rays are by now so well measured that the corresponding error is removed. Uncertainties related to propagation are shown to range between 10% and 25%, depending on which part of the spectrum is considered.Cosmic ray antiprotons provide an important probe for the study of the galactic Dark Matter, as they could be produced by exotic sources. On the other hand, antiprotons are anyway produced by standard nuclear reactions of cosmic ray nuclei on interstellar matter. This process is responsible for a background flux that must be carefully determined to estimate the detectability of an hypothetical exotic signal. Estimates of this background suffer from potential uncertainties of various origins. The propagation of cosmic antiprotons depends on several physical characteristics of the Galaxy which are poorly known. Antiprotons are created from cosmic protons and helium nuclei whose fluxes were not measured with great accuracy until very recently. Calculations of antiproton fluxes make use of nuclear physics models with their own shortcomings and uncertainties. The goal of this paper is to give a new evaluation of the cosmic antiproton flux along with the associated uncertainties. The propagation parameters were tightly constrained in Maurin et al. 2001 by an analysis of cosmic ray nuclei data in the framework of a two-zone diffusion model and we apply these parameters to the propagation of antiprotons. We use the recently published data on proton and helion fluxes, and we find that this particular source of uncertainty has become negligible. The Monte Carlo program DTUNUC was used to carefully examine nuclear reactions. We find that all the cosmic antiproton fluxes naturally coming out of the calculation are fully compatible with experimental data. Uncertainties in this flux have been strongly reduced. Those related to propagation are less than 25%. All other possible sources of uncertainty have also been studied.

Galactic secondary positron flux at the Earth

Context. Secondary positrons are produced by spallation of cosmic rays within the interstellar gas. Measurements have been typically expressed in terms of the positron fraction, which exhibits an increase above 10 GeV. Many scenarios have been proposed to explain this feature, among them some additional primary positrons originating from dark matter annihilation in the Galaxy. Aims. The PAMELA satellite has provided high quality data that has enabled high accuracy statistical analyses to be made, showing that the increase in the positron fraction extends up to about 100 GeV. It is therefore of paramount importance to constrain theoretically the expected secondary positron flux to interpret the observations in an accurate way. Methods. We focus on calculating the secondary positron flux by using and comparing different up-to-date nuclear cross-sections and by considering an independent model of cosmic ray propagation. We carefully study the origins of the theoretical uncertainties in the positron flux. Results. We find the secondary positron flux to be reproduced well by the available observations, and to have theoretical uncertainties that we quantify to be as large as about one order of magnitude. We also discuss the positron fraction issue and find that our predictions may be consistent with the data taken before PAMELA. For PAMELA data, we find that an excess is probably present after considering uncertainties in the positron flux, although its amplitude depends strongly on the assumptions made in relation to the electron flux. By fitting the current electron data, we show that when considering a soft electron spectrum, the amplitude of the excess might be far lower than usually claimed. Conclusions. We provide fresh insights that may help to explain the positron data with or without new physical model ingredients. PAMELA observations and the forthcoming AMS-02 mission will allow stronger constraints to be aplaced on the cosmic-ray transport parameters, and are likely to reduce drastically the theoretical uncertainties.

Detection of neutralino annihilation photons from external galaxies

We consider neutralino annihilation in dense extragalactic systems known to be dominated by dark matter, in particular M87 and several local dwarf spheroidal galaxies. These annihilations can produce energetic gamma rays which may be visible to atmospheric Cerenkov telescopes. We explore the supersymmetric parameter space, and compute the expected flux of gamma--rays coming from these objects. It is shown that some parts of the parameter space lead to a signal observable with the next generation of Cerenkov telescopes, provided the supersymmetric dark matter has a clumpy structure, as may be expected in a hierarchical scenario for structure formation.

New results on source and diffusion spectral features of galactic cosmic rays: I- B/C ratio

In a previous study (Maurin et al. 2001), we explored the set of parameters describing diusive propagation of cosmic rays (galactic convection, reacceleration, halo thickness, spectral index and normalization of the diusion coecient), and we identified those giving a good fit to the measured B=C ratio. This study is now extended to take into account a sixth free parameter, namely the spectral index of sources. We use an updated version of our code where the reacceleration term comes from standard minimal reacceleration models. The goal of this paper is to present a general view of the evolution of the goodness of fit to B=C data with the propagation parameters. In particular, we find that, unlike the well accepted picture, and in accordance with our previous study, a Kolmogorov-like power spectrum for diusion is strongly disfavored. Rather, the 2 analysis points towards & 0:7 along with source spectra index .2.0. Two distinct energy dependences are used for the source spectra: the usual power-law in rigidity and a law modified at low energy, the second choice being only slightly preferred. We also show that the results are not much aected by a dierent choice for the diusion scheme. Finally, we compare our findings to recent works, using other propagation models. This study will be further refined in a companion paper, focusing on the fluxes of cosmic ray nuclei.

A database of charged cosmic rays

Aims. This paper gives a description of a new online database and associated online tools (data selection, data export, plots, etc.) for charged cosmic-ray measurements. The experimental setups (type, flight dates, techniques) from which the data originate are included in the database, along with the references to all relevant publications. Methods. The database relies on the MySQL5 engine. The web pages and queries are based on PHP, AJAX and the jquery, jquery.cluetip, jquery-ui, and table-sorter third-party libraries. Results. In this first release, we restrict ourselves to Galactic cosmic rays with Z 30 and a kinetic energy per nucleon up to a few tens of TeV/n. This corresponds to more than 200 di erent sub-experiments (i.e., di erent experiments, or data from the same experiment flying at di erent times) in as many publications. Conclusions. We set up a cosmic-ray database (CRDB) and provide tools to sort and visualise the data. New data can be submitted, providing the community with a collaborative tool to archive past and future cosmic-ray measurements.

Clumpiness of dark matter and the positron annihilation signal

The small-scale distribution of dark matter in Galactic halos is poorly known. Several studies suggest that it could be very clumpy, which turns out to be of paramount importance when investigating the annihilation signal from exotic particles (e.g. supersymmetric or Kaluza-Klein). In this paper we focus on the annihilation signal in positrons. We estimate the associated uncertainty, due to the fact that we do not know exactly how the clumps are distributed in the Galactic halo. To this aim, we perform a statistical study based on analytical computations, as well as numerical simulations. In particular, we study the average and variance of the annihilation signal over many Galactic halos having the same statistical properties. We find that the so-called boost factor used by many authors should be handled with care, as i) it depends on energy and ii) it may be different for positrons, antiprotons and gamma rays, a fact which has not received any attention before. As an illustration, we use our results to discuss the positron spectrum measurements by the HEAT experiment.The small-scale distribution of dark matter in Galactic halos is poorly known. Several studies suggest that it could be very clumpy, which turns out to be of paramount importance when investigating the annihilation signal from exotic particles (e.g. supersymmetric or Kaluza-Klein). In this paper we focus on the annihilation signal in positrons. We estimate the associated uncertainty, due to the fact that we do not know exactly how the clumps are distributed in the Galactic halo. To this aim, we perform a statistical study based on analytical computations, as well as numerical simulations. In particular, we study the average and variance of the annihilation signal over many Galactic halos having the same statistical properties. We find that the so-called boost factor used by many authors should be handled with care, as i) it depends on energy and ii) it may be different for positrons, antiprotons and gamma rays, a fact which has not received any attention before. As an illustration, we use our results to discuss the positron spectrum measurements by the HEAT experiment.

Antiprotons from primordial black holes

Primordial black holes (pbhs) have motivated many studies since it was shown that they should evap- orate and produce all kinds of particles (Hawking 1974). Recent experimental measurements of cosmic rays with great accuracy, theoretical investigations on the possible formation mechanisms and detailed evaporation pro- cesses have revived the interest in such astrophysical objects. This article aims to use the latest developments in antiproton propagation models (Maurin et al. 2001; Donato et al. 2001) together with new data from BESS, CAPRICE and AMS experiments to constrain the local amount of pbh dark matter. Depending on the diusion halo parameters and on the details of the emission mechanisms, we derive an average upper limit of the order of PBH 1:7 10 33 gc m 3 .

Kaluza-Klein dark matter and Galactic antiprotons

Extra dimensions offer new ways to address long-standing problems in beyond-the-standard-model particle physics. In some classes of extra-dimensional models, the lightest Kaluza-Klein particle is a viable dark matter candidate. In this work, we study indirect detection of Kaluza-Klein dark matter via its annihilation into antiprotons. We use a sophisticated galactic cosmic ray diffusion model whose parameters are fully constrained by an extensive set of experimental data. We find that models with universal extra dimensions remain unconstrained by cosmic ray antiprotons while low-mass weakly interacting massive particle candidates of a few tens of GeV that arise in Randall-Sundrum geometries can be probed.

β–radioactive cosmic rays in a diffusion model: Test for a local bubble?

In a previous analysis, Maurin et al. (2001) have constrained several parameters of the cosmic ray diusive propagation (the diusion coecient normalization K0 and its spectral index , the halo half-thickness L ,t he Alfv en velocity Va, and the galactic wind Vc) using stable nuclei. In a second paper (Donato et al. 2002), these parameters were shown to reproduce the observed antiproton spectrum with no further adjustment. In the present paper, we extend the analysis to the -radioactive nuclei 10 Be, 26 Al and 36 Cl. These species will be shown to be particularly sensitive to the properties of the local interstellar medium (lism). As studies of the lism suggest that we live in an underdense bubble of extentrhole 50{200 pc, this local feature must be taken into account. We present a modied version of our diusion model which describes the underdensity as a hole in the galactic disc; we believe some of the formul presented here are new. It is found that the presence of the bubble leads to a decrease in the radioactive fluxes which can be approximated by a simple factor exp( rhole=lrad )w herelrad = p K0 is the typical distance travelled by a radioactive nucleus before it decays. We nd that each of the radioactive nuclei independently points towards a bubble of radius 100 pc. If these nuclei are considered simultaneously, only models with a bubble radius rhole 60{100 pc are marginally consistent with data. In particular, the standard caserhole = 0 pc is disfavored. Our main concern is about the consistency of the currently available data, especially 26 Al/ 27 Al.