Cosmic-ray electron flux measured by the PAMELA experiment between 1 and 625 GeV

Precision measurements of the electron component in the cosmic radiation provide important information about the origin and propagation of cosmic rays in the Galaxy. Here we present new results regarding negatively charged electrons between 1 and 625 GeV performed by the satellite-borne experiment PAMELA. This is the first time that cosmic-ray e? have been identified above 50 GeV. The electron spectrum can be described with a single power-law energy dependence with spectral index -3.18 ± 0.05 above the energy region influenced by the solar wind (> 30 GeV). No significant spectral features are observed and the data can be interpreted in terms of conventional diffusive propagation models. However, the data are also consistent with models including new cosmic-ray sources that could explain the rise in the positron fraction.     

New measurement of the cosmic-ray positron fraction from 5 to 15 GeV

We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e(+/-) instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.     

Precision measurement of cosmic-Ray antiproton spectrum

The energy spectrum of cosmic-ray antiprotons ( &pmacr;'s) has been measured in the range 0.18-3.56 GeV, based on 458 &pmacr;'s collected by BESS in a recent solar-minimum period. We have detected for the first time a characteristic peak at 2 GeV of &pmacr;'s originating from cosmic-ray interactions with the interstellar gas. The peak spectrum is reproduced by theoretical calculations, implying that the propagation models are basically correct and that different cosmic-ray species undergo a universal propagation. Future BESS data with still higher statistics will allow us to study the solar modulation and the propagation in detail and to search for primary &pmacr; components.     

The HERO project for the study of high-energy primary cosmic radiation

Proposals for the High-Energy Ray Observatory (HERO) comprising scientific equipment with increased power availability are presented. Under the long exposure (>7 years), it is proposed to investigate the spectrum and charge composition of cosmic-ray nuclei up to E ₀ ～ 10¹⁶ eV and to determine the behavior of the energy spectrum in the energy regions >100 GeV for cosmic-ray electrons and >50 GeV for γ radiation. The geometrical factor of the apparatus is 6.0–9.0 m² sr depending on the type of particles.     

An Alternative Approach to Understanding the Observed Positron Fraction

Space-based observations by PAMELA (Adriani et al., Nature 458, 607, 2009), Fermi-LAT (Ackerman et al., Phys. Rev. Lett. 105, 01103, 2012), and AMS (Aguilar et al., Phys. Rev. Lett. 110, 141102, 2013) have demonstrated that the positron fraction (e+/total-e) increases with increasing energy above about 10 GeV. According to the propagation model for Galactic cosmic rays in widespread use (Moskalenko & Strong, Astrophys. J. 493, 693, 1998), the production of secondary positrons from interaction of cosmic-ray protons and heavier nuclei with the interstellar medium gives a generally falling positron fraction between 10 and 100 GeV, with secondary positrons accounting for only ～20 % of the observed positron fraction at 100 GeV; so some other physical phenomena have been proposed to explain the data. An alternative approach to interpreting the positron observations is to consider these data as presenting an opportunity for re-examining models of Galactic cosmic-ray propagation. Following release of the PAMELA data, three groups published propagation models (Shaviv, et al., Phys. Rev. Lett. 103, 111302, 2009, Cowsik and Burch, Phys. Rev. D. 82, 023009, 2010, Katz et al., Mon. Not. R. Aston. Soc. 405, 1458 2010) in which the observed positron fraction is explained entirely by secondary positrons produced in the interstellar medium. In May of this year, stimulated by the AMS extension of the positron data to higher energy with excellent statistics, two of those groups presented further development of their calculations (Cowsik et al. 2013, Blum et al. 2013), again concluding that the observed positrons can be understood as secondaries. None of the authors of these five papers was registered for the 33rd International Cosmic Ray Conference (ICRC). Although I am not an author of any of these papers, I have some close familiarity with one of these recent papers, so the conference organizers invited me to bring this alternative approach to the attention of the conference. The present paper is a summary of the material I presented, along with a brief comment about reaction at the conference to this approach.     

Geminga contribution to the cosmic-ray positron excess according to the gamma-ray observations

We attempt to interpret the cosmic-ray positron excess by injection from the nearby pulsar Geminga, assuming a two-zone diffusion scenario and an injection spectrum with a low energy cutoff. Since the high energy positrons and electrons from Geminga can induce γ rays via inverse Compton scattering, we take into account the extended γ-ray observations around Geminga from HAWC for ∼10 TeV and from Fermi-LAT for ${ \mathcal O }(10)$ GeV. According to the extended γ-ray observation claimed by an analysis of Fermi-LAT data, we find that Geminga could explain the positron excess for a 30% energy conversion efficiency into positrons and electrons. However, based on the constraint on the extended γ rays given by another Fermi-LAT analysis, positrons from Geminga would be insufficient to account for the positron excess. Further robust analysis of Fermi-LAT data for the extended γ rays would be crucial to determine whether Geminga can explain the positron excess or not.     

PAMELA,FGST and sub-TeV dark matter

PAMELA's observation that the cosmic ray positron fraction increases rapidly with energy implies the presence of primary sources of energetic electron–positron pairs. Of particular interest is the possibility that dark matter annihilations in the halo of the Milky Way provide this anomalous flux of antimatter. The recent measurement of the cosmic ray electron spectrum by the Fermi Gamma Ray Space Telescope, however, can be used to constrain the nature of any such dark matter particle. In particular, it has been argued that in order to accommodate the observations of Fermi and provide the PAMELA positron excess, annihilating dark matter particles must be as massive as ∼1 TeV or heavier. In this Letter, we revisit Fermi's electron spectrum measurement within the context of annihilating dark matter, focusing on masses in the range of 100–1000 GeV, and considering effects such as variations in the astrophysical backgrounds from the presence of local cosmic ray accelerators, and the finite energy resolution of the Fermi Gamma Ray Space Telescope. When these factors are taken into account, we find that dark matter particles as light as ∼300 GeV can be capable of generating the positron fraction observed by PAMELA.     

Measurement of the deflection of cosmic-ray electrons in the energy range 75–250 GeV by the Earth’s magnetic field with the PAMELA experiment

The deflection of electrons in the Earth’s magnetic field in the energy range 75–250 GeV (the so-called east-west effect) has been measured with the PAMELA satellite-borne experiment. The results are presented for various L-shells. The data obtained can be used to construct mathematical models that describe the structure of the Earth’s magnetic field and to refine the already existing models. These data can also be directly applied to estimate the positron fraction in cosmic-ray electron fluxes both in the PAMELA experiment and in other satellite-borne experiments.     

Cosmic ray electron spectrum due to the dispersion of injection spectrum

The cosmic-ray total electron spectrum(electrons plus positrons) has been measured precisely up to Te V energies,with more interesting features found.Exhaustive analyses of the electron spectrum strongly support a spectral hardening above 100 GeV,rather than a featureless single power-law,which is confirmed by the most recent observations.Meanwhile,the measurements of the DAMPE satellite have verified the presence of a knee-like structure around 1 TeV in the electron spectrum,resembling the cosmic-ray knee.In this paper,we establish a physical model in which the observed electron spectrum is composed of a superposition of CR sources with various spectral indices and high-energy cutoffs.The dispersion of the power index is assumed to be Gaussian,while the cutoff energy Ec follows a power-law distribution.These simple ideas can account naturally for both the hundred-GeV excess and the TeV spectral break.     

Results from PAMELA,ATIC and FERMI: Pulsars or dark matter?

It is well known that dark matter dominates the dynamics of galaxies and clusters of galaxies. Its constituents remain a mystery despite an assiduous search for them over the past three decades. Recent results from the satellite-based PAMELA experiment show an excess in the positron fraction at energies between 10 and 100 GeV in the secondary cosmic ray spectrum. Other experiments, namely ATIC, HESS and FERMI, show an excess in the total electron (e ⁺ + e ⁻) spectrum for energies greater than 100 GeV. These excesses in the positron fraction as well as the electron spectrum can arise in local astrophysical processes like pulsars, or can be attributed to the annihilation of the dark matter particles. The latter possibility gives clues to the possible candidates for the dark matter in galaxies and other astrophysical systems. In this article, we give a report of these exciting developments.     

Ten years of CR physics with PAMELA

The satellite borne Pamela instrument is dedicated to the precise and high statistics study of CR fluxes on a four decades energy range. Pamela experiment is the last step of the “Russian-Italian Mission” (RIM) program established in 1992 between several Italian and Russian institutes and with the participation of Sweden and Germany. Designed as a cosmic ray observatory at 1 AU, it extensive program is made possible thanks to the outstanding performance of the instrument, the low energy threshold, the quasi-polar orbit and the 10 years duration of the observation. The physics program pays particular attention to the study of particles and antiparticles fluxes and includes search for dark matter, primordial antimatter, new matter in the Universe, study of cosmic-ray propagation, solar physics and solar modulation, and terrestrial magnetosphere. Very important is the discovery of the anomalous increase of the positron flux at energies higher that 50 GeV (the so called “Pamela anomaly”), and the abrupt spectral hardening of H and He, challenging the current paradigm of cosmic-ray acceleration and propagation in the Galaxy.     

Leptoquarks in lepton-quark collisions

Low-energy experiments permit the existence of leptoquarks with masses of order 100 GeV and couplings to quark-lepton pairs as large as gauge couplings. We study systematically the signatures of all possible scalar and vector leptoquarks in electron (positron)-proton collisions. Clear evidence for leptoquarks would be narrow peaks in the x-distributions of inclusive neutral and charged current processes. At HERA one will be able to explore the mass range up to 300 GeV through direct production, and even somewhat beyond the CM energy of 314 GeV through virtual effects. Conversely, leptoquarks with masses of 200 GeV can be discovered for couplings as small as 10⁻³ α_em.     

On the cosmic-ray spectra of three-body lepton-flavor-violating dark matter decays

We consider possible leptonic three-body decays of spin-1/2, charge-asymmetric dark matter. Assuming a general Dirac structure for the four-fermion contact interactions of interest, we study the cosmic-ray electron and positron spectra and show that good fits to the current data can be obtained for both charged-lepton-flavor-conserving and flavor-violating decay channels. We find that different choices for the Dirac structure of the underlying decay operator can be significantly compensated by different choices for the dark matter mass and lifetime. The decay modes we consider provide differing predictions for the cosmic-ray positron fraction at energies higher than those currently probed at the PAMELA experiment; these predictions might be tested at cosmic-ray detectors like AMS-02.     

Primary and secondary effects in cosmic-ray variations at solar proton events

The variations of the cosmic-ray rigidity spectrum in the energy range from 0.8 MeV to several dozen GeV at solar proton events in January 2005 and December 2006 have been analyzed. A comparison of the observed and model spectra revealed the power range of direct detection of solar cosmic rays and moments of their observations.     

May heavy neutrinos solve underground and cosmic-ray puzzles?

Primordial heavy neutrinos of the fourth generation might explain different astrophysical puzzles. The simplest fourth-neutrino scenario is consistent with known fourth-neutrino physics, cosmic ray antimatter, cosmic gamma fluxes, and positive signals in underground detectors for a very narrow neutrino mass window (46–47 GeV). However, accounting for the constraint of underground experiment CDMS prohibits solution of cosmic-ray puzzles in this scenario. We have analyzed extended heavy-neutrino models related to the clumpiness of neutrino density, new interactions in heavy-neutrino annihilation, neutrino asymmetry, and neutrino decay. We found that, in these models, the cosmic-ray imprint may fit the positive underground signals in DAMA/Nal experiment in the entire mass range 46–70 GeV allowed from uncertainties of electroweak parameters, while satisfaction of the CDMS constraint reduces the mass range to around 50 GeV, where all data can come to consent in the framework of the considered hypothesis. The text was submitted by the authors in English.     

Spatial dependent diffusion of cosmic rays and the excess of primary electrons derived from high precision measurements by AMS-02

The precise spectra of Cosmic Ray(CR) electrons and positrons have been published by the measurement of AMS-02. It is reasonable to regard the difference between the electron and positron spectra(?Φ = Φ_(e-)-Φ_(e+)) as being dominated by primary electrons. The resulting electron spectrum shows no sign of spectral softening above 20 GeV, which is in contrast with the prediction of the standard model of CR propagation. In this work, we generalize the analytic one-dimensional two-halo model of diffusion to a three-dimensional realistic calculation by implementing spatial variant diffusion coefficients in the DRAGON package. As a result, we can reproduce the spectral hardening of protons observed by several experiments, and predict an excess of high energy primary electrons which agrees with the measurement reasonably well. Unlike the break spectrum obtained for protons, the model calculation predicts a smooth electron excess and thus slightly over-predicts the flux from tens of GeV to 100 GeV. To understand this issue, further experimental and theoretical studies are necessary.     

Quark-gluon state of matter and positive excess of cosmic-ray muons at high energies

The problem of the relationship between the numbers of positively and negatively charged particles in the flux of cosmic-ray muons arriving at sea level with energies in excess of 0.1 TeV (up to 100 TeV) is discussed. It is shown that the formation of quark—gluon matter as the result of high-energy nuclear interactions leads to a reduction of the positive excess in cosmic-ray muons at the above energies. At the present time, the quark-gluon state of matter is studied in accelerator experiments at colliding-particle energies of up to √s = 200 GeV per nucleon. Estimates presented in this article for the positive excess of muons having energies of up to 3 or 4 TeV are based on available data from accelerator experiments; at higher muon energies, the respective estimates are based on extrapolating these data.     

Analysis of proton-carbon inelastic cross sections measured in satellite experiment and upper limit on primary cosmic ray deuteron flux in the energy range 20 to 600 GeV

An analysis has been made of the experimental results of Akimovet al on the inelastic cross sections of proton on proton and carbon targets in the energy range 20 to 600 GeV obtained from artificial earth satellites. It is found that an upper limit of 4% at 95% confidence level can be set on the fraction of deuterons relative to the flux of protons in the primary cosmic radiation at energies in the range 20 to 60 GeV. There is an indication for a rise of (29±7) mb in the inelastic cross section of proton against carbon in the energy range of 200 to 600 GeV over and above what is expected from Glauber’s theory. If this rise has to be interpreted as due to contamination from cosmic ray deuterons, the fraction of deuterons relative to protons needed is (15±4)% in this energy region.     

Dark matter,neutrino mass,cutoff for cosmic-ray neutrino,and the Higgs boson invisible decay from a neutrino portal interaction

We study an effective theory beyond the standard model(SM) where either of the two additional gauge singlets, a Majorana fermion and a real scalar, constitutes all or some fraction of dark matter. In particular, we focus on the masses of the two singlets in the range of O(10) MeV-O(10) GeV with a neutrino portal interaction, which plays an important role not only in particle physics but also in cosmology and astronomy. We point out that the thermal dark matter abundance can be explained by(co-)annihilation, where the dark matter with a mass greater than 2 GeV can be tested in future lepton colliders, CEPC, ILC, FCC-ee and CLIC, in the light of the Higgs boson invisible decay. When the gauge singlets are lighter than O(100) MeV, the interaction can affect the neutrino propagation in the universe due to its annihilation with cosmic background neutrino into the gauge singlets. Although in this case it can not be the dominant dark matter, the singlets are produced by the invisible decay of the Higgs boson at such a rate which is fully within reach of future lepton colliders. In particular, a high energy cutoff of cosmic-ray neutrino,which may account for the non-detection of Greisen-Zatsepin-Kuzmin(GZK) neutrino or the non-observation of the Glashow resonance, can be set. Interestingly, given the cutoff and the mass(range) of WIMPs, a neutrino mass can be"measured" kinematically.