Based on general considerations rather than model-dependent fits to specific scenarios, we argue that an increase with energy of the positron fraction in cosmic rays, suggested by several experiments at E>~7 GeV, most likely requires a primary source of electron-positron pairs. We discuss the possible alternatives, and find none of them plausible on astrophysical or particle physics grounds. Further observational ways to test different scenarios are discussed.
The rapid rise in the cosmic ray positron fraction above 10 GeV, as measured by PAMELA and AMS, suggests the existence of nearby primary sources of high energy positrons, such as pulsars or annihilating/decaying dark matter. In contrast, the spectrum of secondary positrons produced through the collisions of cosmic rays in the interstellar medium is predicted to fall rapidly with energy, and thus is unable to account for the observed rise. It has been proposed, however, that secondary positrons could be produced and then accelerated in nearby supernova remnants, potentially explaining the obser…
We provide a scenario in which a hidden U(1) gauge boson constitutes dark matter of the Universe and decays into the standard-model particles through a kinetic mixing with an $U(1)_{B-L}$ gauge boson. Interestingly, our model can naturally account for the steep rise in the positron fraction recently reported by PAMELA. Moreover, we find that due to the charge assignment of $U(1)_{B-L}$, only a small amount of antiprotons are produced in the decay, which is also consistent with the PAMELA and other observational data.
The AMS experiment onboard the International Space Station has recently provided cosmic ray electron and positron data with unprecedented precision in the range from 0.5 to 350 GeV. The observed rise in the positron fraction at energies above 10 GeV remains unexplained, with proposed solutions ranging from local pulsars to TeV-scale dark matter. Here, we make use of this high quality data to place stringent limits on dark matter with masses below ~300 GeV, annihilating or decaying to leptonic final states, essentially independent of the origin of this rise. We significantly improve on existing…
Astrophysical and cosmological observations do not require the dark matter particles to be absolutely stable. If they are indeed unstable, their decay into positrons might occur at a sufficiently large rate to allow the indirect detection of dark matter through an anomalous contribution to the cosmic positron flux. In this paper we discuss the implications of the excess in the positron fraction recently reported by the PAMELA collaboration for the scenario of decaying dark matter. To this end, we have performed a model-independent analysis of possible signatures by studying various decay chann…
Recent data on Galactic cosmic-ray (CR) leptons and hadrons gave rise to two exciting problems: on the lepton side, the origin of the rise of the CR positron fraction e+/(e- + e+) at ~10 - 300 GeV of energy; on the hadron side, the nature of the spectral hardening observed in CR protons and nuclei at ~TeV energies. The lepton anomaly indicates the existence of a nearby e+/- source. It has been proposed that high-energy positrons can be produced inside nearby supernova remnants (SNRs) via interactions of CR hadrons with the ambient medium. A distinctive prediction of this mechanism is a high-en…
Over the last three years, several satellite and balloon observatories have suggested intriguing features in the cosmic ray lepton spectra. Most notably, the PAMELA satellite has suggested an "anomalous" rise with energy of the cosmic ray positron fraction. In this article, we summarize the global picture emerging from the data and recapitulate the main features of different types of explanations proposed. The perspectives in testing different scenarios as well as inferring some astrophysical diagnostics from current/near future experiments are also discussed.
The anomalies observed in recent cosmic ray experiments seem to strongly constrain the nature of the dark matter. In this letter, we investigate a possibility of the fermionic dark matter with a minimal extension of the standard model. We found that the dark matter decays caused by the dimension six operators can naturally explain the anomalies.
Dark Matter annihilation (DMA) may yield an excess of gamma rays and antimatter particles, like antiprotons and positrons, above the background from cosmic ray interactions. The excess of diffuse Galactic Gamma Rays from EGRET shows all the features expected from DMA. The new precise measurements of the antiproton and positron fractions from PAMELA are compared with the EGRET excess. It is shown that the charged particles are strongly dependent on the propagation model used. The usual propagation models with isotropic propagation models are incompatible with the recently observed convection in…
The AMS-02 collaboration has just published a high precision measurement of the cosmic positron fraction $e^+/(e^- + e^+)$, which rises with energy from $\sim 5$ GeV to $\sim 350$ GeV. The result indicates the existence of primary electron/positron sources to account for the positron excess. In this work, we investigate the possibility that the nearby mature pulsars are the primary positron sources. By fitting the data we find that the positrons from a single nearby pulsar, such as Geminga or Monogem, with the spectral index $α\sim 2$ can interpret the AMS-02 result. We also investigate the po…
The rise in the cosmic-ray positron fraction measured by the PAMELA satellite is likely due to the presence of astrophysical sources of positrons, e.g. pulsars, on the kpc scale around the Earth. Nevertheless, assessing the properties of these sources from the positron data requires a good knowledge of the secondary positron component generated by the interaction of cosmic rays with the interstellar gas. In this paper, we investigate the impact of the spectral hardening in the cosmic-ray proton and helium fluxes recently reported by the ATIC2 and CREAM balloon experiments, on the predictions o…
Cosmic-ray positrons have long been considered a powerful probe of dark matter annihilation. In particular, myriad studies of the unexpected rise in the positron fraction have debated its dark matter or pulsar origins. In this paper, we instead examine the potential for extremely precise positron measurements by AMS-02 to probe hard leptophilic dark matter candidates that do not have spectral features similar to the bulk of the observed positron excess. Utilizing a detailed cosmic-ray propagation model that includes a primary positron flux generated by Galactic pulsars in addition to a seconda…
We study the spectrum of cosmic ray positrons produced by a scaling distribution of non-superconducting cosmic strings. In this scenario, the positrons are produced from the jets which form from the cosmic string cusp annihilation process. The spectral shape is a robust feature of our scenario, and is in good agreement with the results from the recent PAMELA and ATIC experiments. In particular, the model predicts a sharp upper cutoff in the spectrum, and a flux which rises as the upper cutoff is approached. The energy at which the flux peaks is determined by the initial jet energy. The amplitu…
The annihilation of light dark matter was recently advocated as a possible explanation of the large positron injection rate at the Galactic center deduced from observations by the SPI spectrometer aboard INTEGRAL. The modelling of internal Bremsstrahlung and in-flight annihilation radiations associated to this process drastically reduced the mass range of this light dark matter particle. We estimate critically the various energy losses and radiations involved in the propagation of the positron before its annihilation --in- flight or at rest. Using a simple model with mono-energetic positrons i…
We show that the anomaly of the positron fraction observed by the PAMELA experiment can be attributed to recent supernova explosion(s) in a dense gas cloud (DC) near the Earth. Protons are accelerated around the supernova remnant (SNR). Electrons and positrons are created through hadronic interactions inside the DC. Their spectrum is harder than that of the background because the SNR spends much time in a radiative phase. Our scenario predicts that the anti-proton flux dominates that of the background for >~100 GeV. We compare the results with observations (Fermi, HESS, PPB-BETS, and ATIC).
The rise of the cosmic ray positron fraction with energy, as first observed with high confidence by PAMELA, implies that a large flux of high energy positrons has been recently (or is being currently) injected into the local volume of the Milky Way. With the new and much more precise measurement of the positron fraction recently provided by AMS, we revisit the question of the origin of these high energy positrons. We find that while some dark matter models (annihilating directly to electrons or muons) no longer appear to be capable of accommodating these data, other models in which ~1-3 TeV da…
Recent observations by PAMELA, Fermi-LAT, and AMS-02 have conclusively indicated a rise in the cosmic-ray positron fraction above 10 GeV, a feature which is impossible to mimic under the paradigm of secondary positron production with self-consistent Galactic cosmic-ray propagation models. A leading explanation for the rising positron fraction is an additional source of electron-positron pairs, for example one or more mature, energetic, and relatively nearby pulsars. We point out that any one of two well-known nearby pulsars, Geminga and Monogem, can satisfactorily provide enough positrons to r…
The AMS-02 experiment has recently released data which confirms a rise in the cosmic-ray positron fraction as a function of energy up to approximately 350 GeV. Over the past decade, attempts to interpret this positron excess in terms of dark-matter decays have become increasingly complex and have led to a number of general expectations about the decaying dark-matter particles: such particles cannot undergo simple two-body decays to leptons, for example, and they must have rather heavy TeV-scale masses. In this paper, by contrast, we show that Dynamical Dark Matter (DDM) can not only match exis…
We consider indirect detection of meta-stable dark matter particles decaying into a stable neutral particle and a pair of standard model fermions. Due to the softer energy spectra from the three-body decay, such models could potentially explain the AMS-02 positron excess without being constrained by the Fermi-LAT gamma-ray data and the cosmic ray anti-proton measurements. We scrutinize over different final state fermions, paying special attention to handling of the cosmic ray background and including various contributions from cosmic ray propagation with the help of the \textsc{LikeDM} package…
The unexpected energy spectrum of the positron/electron ratio is interpreted astrophysically, with a possible exception of the 100-300 GeV range. The data indicate that this ratio, after a decline between $0.5-8$ GeV, rises steadily with a trend towards saturation at 200-400GeV. These observations (except for the trend) appear to be in conflict with the diffusive shock acceleration (DSA) mechanism, operating in a \emph{single} supernova remnant (SNR) shock. We argue that $e^{+}/e^{-}$ ratio can still be explained by the DSA if positrons are accelerated in a \emph{subset} of SNR shocks which: (…
Recently, dark matter indirect searches have gained a lot of attention, mostly due to the possibility of recent anomalies in cosmic rays and microwave sky maps being due to the annihilation or decay of dark matter. In this thesis, we argue however that these signals are plagued by irreducible astrophysical backgrounds and show how plausible conventional physics can mimic the alleged dark matter signals. In particular, we consider the possibility that the rise in the positron fraction observed by the PAMELA satellite is due to the production through (hadronic) cosmic ray spallation and subseque…
It has long been suggested that nearby pulsars within $\sim 1 \,{\rm kpc}$ are the leading candidate of the 10-500 GeV cosmic-ray positron excess measured by PAMELA and other experiments. The recent measurement of surface brightness profile of TeV nebulae surrounding Geminga and PSR~B0656+14 by the High-Altitude Water Cherenkov Observatory (HAWC) suggests inefficient diffusion of particles from the sources, giving rise to a debate on the pulsar interpretation of the cosmic-ray positron excess. Here we argue that GeV observations provide more direct constraints on the positron density in the Te…
Recent positron flux fraction measurements in cosmic-rays (CR) made by the AMS-02 satellite confirm and extends the evidence on the existence of a new (yet unknown) source of high energy positrons. To explain this excess, we use the gravitino of bilinear R-parity violating SUSY models as a decaying Dark Matter candidate, as the source of those high energy particles. Being a long lived weak-interacting and spin 3/2 particle, it offers several particularities which makes it an attractive candidate. We compute the electron, positron and $γ$-ray\ fluxes produced by each gravitino decay channel at …
We discuss recent observations of high energy cosmic ray positrons and electrons in the context of hadronic interactions in supernova remnants, the suspected accelerators of galactic cosmic rays. Diffusive shock acceleration can harden the energy spectrum of secondary positrons relative to that of the primary protons (and electrons) and thus explain the rise in the positron fraction observed by PAMELA above 10 GeV. We normalize the hadronic interaction rate by holding pion decay to be responsible for the gamma-rays detected by HESS from some SNRs. By simulating the spatial and temporal distrib…
Electron-positron pair creation in the superposition of two oscillating electric field pulses with largely different frequency, duration and relative positioning
We revisit the model proposed earlier to account for the observed increase in the positron fraction in cosmic rays with increasing energy, in the light of new data from the Alpha Magnetic Spectrometer (AMS-02) experiment. The model accounts for the production and acceleration of secondary electrons and positrons in nearby supernova remnants which results in an additional, harder component that becomes dominant at high energies. By fitting this to AMS-02 data we can calculate the expected concomitant rise of the boron-to-carbon ratio, as well as of the fraction of antiprotons. If these predicti…
We study a two component dark matter candidate inspired by the Minimal Walking Technicolor model. Dark matter consists of a dominant SIMP-like dark atom component made of bound states between primordial helium nuclei and a doubly charged technilepton, and a small WIMP-like component made of another dark atom bound state between a doubly charged technibaryon and a technilepton. This scenario is consistent with direct search experimental findings because the dominant SIMP component interacts too strongly to reach the depths of current detectors with sufficient energy to recoil and the WIMP-like …
We consider interaction of a high-energy electron beam with two counterpropagating femtosecond laser pulses. Nonlinear Compton scattering and electron-positron pair production by the emitted photons result in development of an electromagnetic "shower-type" cascade, which however collapses rather quickly due to energy losses by secondary particles. Nevertheless, the laser field accelerates the low-energy electrons and positrons trapped in the focal region, thus giving rise to development of electromagnetic cascade of another type ("avalanche-type"). This effect of cascade collapse and revival c…
If interpreted in terms of decaying dark matter, the steep rise in the positron fraction of cosmic rays above 10 GeV, as observed by the PAMELA experiment, suggests an underlying production mechanism that favors leptonic channels. We consider a scenario where a portion of the dark matter is made of the gauginos of an unbroken hidden-sector U(1), which interact with the visible sector only through a tiny kinetic mixing. The second component of the dark matter is made of neutralinos, and depending on the mass spectrum, the lightest neutralino or the hidden gaugino becomes unstable and subject to…
A dramatic increase in the accuracy and statistics of space-borne cosmic ray (CR) measurements has yielded several breakthroughs over the last several years. The most puzzling is the rise in the positron fraction above ~10 GeV over the predictions of the propagation models assuming pure secondary production. The accuracy of the antiproton production cross section is critical for astrophysical applications and searches for new physics since antiprotons in CRs seem to hold the keys to many puzzles including the origin of those excess positrons. However, model calculations of antiproton productio…
We calculate the energy spectra of cosmic rays (CR) and their secondaries produced in a supernova remnant (SNR), taking into account the time-dependence of the SNR shock. We model the trajectories of charged particles as a random walk with a prescribed diffusioncoefficient, accelerating the particles at each shock crossing. Secondary production by CRs colliding with gas is included as a Monte Carlo process. We find that SNRs produce less antimatter than suggested previously: The positron/electron ratio and the antiproton/proton ratio are a few percent and few $\times 10^{-5}$, respectively. Bo…
The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and positrons on the cosmic optical and infrared backgrounds. Electrons and positrons cool in the intergalactic medium while gyrating in its magnetic fields, which could cause either its global heating or the production of lower-energy secondary gamma-rays. The energy distribution of gamma-rays surviving the cosmological journey carries …
