Cosmic-ray positrons: are there primary sources?

Galactic cosmic rays consist of primary and secondary particles. Primary cosmic rays are thought to be energized by first order Fermi acceleration processes at supernova shock fronts within our Galaxy. The cosmic rays that eventually reach the Earth from this source are mainly protons and atomic nuclei, but also include electrons. Secondary cosmic rays are created in collisions of primary particles with the diffuse interstellar gas. They are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.     

Search for shadowing of primary cosmic radiation by the moon at TeV energies

The antimatter research through the earth moon ion spectrometer (ARTEMIS) experiment is described in terms of searching for the moon's shadow in the primary cosmic radiation at TeV energies. Observations were made using the Whipple 10 m Imaging Atmospheric erenkov Telescope, in conjunction with a special optical filter to suppress unwanted moonlight. Monte Carlo simulations were used to predict that the magnitude of shadowing based on a pure proton beam would be 1% of the cosmic ray background. Observations and event classification are discussed, and results of a search for shadowing based on both protons and antiprotons are presented. Atmospheric fluctuations giving rise to non-statistical event rates constituted a limiting factor to the sensitivity of the shadow search as they rivaled or exceeded statistical errors. It is demonstrated that the experiment does not have sufficient sensitivity to moon shadowing which would allow a search for an antimatter component in the primary cosmic ray flux at the percent level. Possible future improvements of the technique are discussed.     

A classification of the available astrophysica data of particularHii regions

We have calculated the energy spectra of cosmic ray secondary antiprotons and positrons using the latest available data on inclusive reactions. Using the measured positron spectrum, we have found that the amount of matter traversed by the cosmic rays in the few GeV region to bem≈4.7±1.5 g cm^?2 of interstellar hydrogen. The computed antiproton to proton ratio is about 4×10^?4 for energies 5–10 GeV. This is sufficient to make observations of antiprotons feasible from balloon flights. We have also pointed out the type of information that can be obtained if accurate information of the spectra of these two components becomes available.     

Secondary production of antiprotons in cosmic radiation

A reliable representation to the invariant cross-section for the production of antiprotons ( ) in inclusive reactions has been obtained, which fits the data extremely well from threshold to ISR energies. Using this, the production spectrum of by cosmic ray interaction with interstellar gas is calculated and is compared with other existing calculations. The equilibrium spectrum in the Galaxy has been derived from about 100 MeV to a few hundred GeV in kinetic energy using Leaky Box Model for the propagation of cosmic rays, by taking into account all energy loss processes. It is found that /P ratio calculated here is very much smaller than the observed ratio. In view of the fact that this excess of observed /P ratio is much larger than the measured upper limits on the fraction of antimatter in the form of antinuclei, the excess /P ratio has not been attributed to the existence of antimatter. Instead, it is shown that Closed Galaxy Model for the propagation of cosmic rays predicts more antiprotons than observed. Therefore, it is suggested that if cosmic rays contain about 50% new component of local origin, the resulting Closed Galaxy Model explains well the observations. The present calculations also predict too small a flux of below a few hundred MeV to make this energy region ideally suited to look for antiprotons of primordial origin.     

Heavy quark production in ultra high energy cosmic ray interactions

In this paper we present a comprehensive study of the heavy quark production in ultra high energy cosmic ray interactions in the atmosphere considering that the primary cosmic ray can be either a photon, neutrino or a proton. The analysis is performed using a unified framework – the dipole formalism – and the saturation effects, associated to the physical process of parton recombination, are taken into account. We demonstrate that the contribution of heavy quarks for cosmic ray interactions is in general non-negligible and can be dominant depending of the process considered. Moreover, our results indicate that new dynamical mechanisms should be included in order to obtain reliable predictions for the heavy quark production in pp collisions at ultra high cosmic ray energies.     

Antimatter bodies and closure of the Universe

One of the hypotheses to account for the cosmic gamma-ray bursts invokes collisions between antimatter bodies of asteroidal mass and stars. This paper points out that, should this idea be correct, the numbers suggest that the antimatter is cosmological, and that alocally measurable ratio might be used to determine the effective gain in mean mass density relevant to the possible closure of the Universe. The effective mass gain factor would have a minimum value of 2 and could be much larger.     

Suprathermal Grains: Origin of primary cosmic rays

Charged dust grains of radiia3×10^–6 cm could be a help in understanding the production of primary cosmic ray particles in extensive air showers (EAS). A two-stage acceleration mechanism is proposed in order to accelerate dust grains up to relativistic energy. In the first stage, dust grains acquire suprathermal energy (Suprathermal Grains) by the Fermi mechanism. In the second stage, suprathermal grains attain relativistic energy by the Alfvén magnetic pumping mechanism yielding the primary cosmic ray particles. Ionization loss has been considered to be a most important loss mechanism for charged dust grains in a fully ionized medium. It is suggested that graphite dust grains of intergalactic origin may be responsible for high energy (>10²⁰ eV) cosmic rays.     

On the secondary production of galactic cosmic ray electrons

The differential energy spectrum and charge ratio of primary cosmic ray electrons produced by collisions of primary cosmic ray particles with the interstellar medium is calculated by means of the two temperature statistical model of high-energy interactions. Two realistic models for the primary cosmic ray flux are considered. Contributions to the primary cosmic ray electron intensity from both the pion and kaon decay modes have been included. The distribution of matter in the galaxy and energy loss of produced secondaries and electrons are considered. The results are compared to recent experimental data.     

Production and propagation of cosmic ray H2 and He3 nuclei

The results of detailed calculations on the production of H² and He³ nuclei by cosmic ray protons and helium nuclei in interstellar medium are presented. The flux and energy spectra of these nuclei as well as those of cosmic ray H¹ and He⁴ nuclei in the vicinity of the Earth are calculated. For this purpose the source spectra are assumed to be in the form of a power law in total energy per nucleon with an additional velocity dependent term. This spectrum denoted as Fermi Spectrum, is about midway between the power law spectrum in rigidity and in total energy per nucleon. The fluxes are calculated taking into account: (1) energy dependent cross-sections of thirteen nuclear reactions of cosmic ray protons and helium nuclei with interstellar H¹ and He⁴ leading to the production of H² and He³ nuclei, (2) angular distributions and kinematics of these reactions, (3) ionization loss of the primary and secondary nuclei in interstellar medium, (4) elastic collisions of cosmic ray protons and helium nuclei, (5) distributions of cosmic ray path-lengths in in terstellar space as in gaussian and exponential forms, and (6) interplanetary modulation of cosmic rays from the numerical solution of the complete Fokker-Planck equation describing the diffusion, convection and adiabatic deceleration of cosmic ray nuclei in the solar system. On comparing the calculated values of H²/He⁴ and He³/(He³+He⁴) as a function of energy with the observed data of several investigators, it is found that agreement between the calculated values and most of the observed data is obtained on the basis of: (a) source spectrum in the form of Fermi Spectrum, (b) distribution of path-lengths as in the gaussian form with a mean value of 4 g cm^–2 of hydrogen or as in exponential form with leakage path length of 4 g cm^–2.     

Dust grain origin of cosmic ray air showers

It is suggested that cosmic rays of energies as high as 10²⁰ eV consist of dust grains of relativistic energies. Such dust grains as typical in interstellar space are accelerated first by a strong radiation pressure of luminous, compact galaxies and then by magnetic processes. A grain with the mass of about 10^–16 g and the Lorentz factor of about 10³ attains an energy as large as 10²⁰ eV and produces a huge extensive air shower. Such grains survive against the collisions with cosmic microwave photons. This would remove the serious difficulty, if both the cosmic microwave radiation and the huge extensive air showers, which were regarded as due to protons of energies greater than 10¹⁹ eV, existed in spite of that the protons should strongly attenuate by the collisions with the radiation.     

Local gamma ray events as tests of the antimatter theory of gamma ray bursts

Nearby examples of the antimatter chunks postulated by Sofia and Van Horn to explain the cosmic gamma ray bursts may produce detectable gamma ray events when struck by solar system meteoroids. These events would have a much shorter time scale and higher energy spectrum than the bursts already observed. In order to have a reasonably high event rate, the local meteoroid population must extend to a distance from the Sun of the order of 0.1 pc, but the required distance could become much lower if the instrumental threshold is improved. We also examine the expected gamma ray flux for interaction of the antimatter bodies with the solar wind, and find it far below present instrumental capabilities.     

Evolution and observational signature of diffused antiworld

The existence of macroscopic regions with antibaryon excess in the baryon asymmetric Universe with general baryon excess is the possible consequence of practically all models of baryosynthesis. Diffusion of matter and antimatter to the border of antimatter domains defines the minimal scale of the antimatter domains surviving to the present time. A model of diffused antiworld is considered, in which the density within the surviving antimatter domains is too low to form gravitationally bound objects. The possibility to test this model by measurements of cosmic gamma ray fluxes is discussed. The expected gamma ray flux is found to be acceptable for modern cosmic gamma ray detectors and for those planned for the near future.     

The relative abundance of the carbon isotopes12C and13C in primary cosmic radiation

Mass measurements have been performed on stopping cosmic ray carbon nuclei in a nuclear emulsion stack, which was exposed to the primary radiation in a high altitude balloon flight. The mass determinations are based on measurements of mean track width and residual range in the range intervals 0<R<0.75 mm and 1<R<12 mm. The mean track width measurements have been performed with nuclear track photometers of special construction. The mass measurements in the interval 0<R<0.75 mm have given a nearly symmetrical mass distribution. The width of the distribution is equal to that expected for a distribution which contains only one isotope. The result indicates that one of the stable isotopes is appreciably more abundant than the other. The measurements in the range interval 1<R<12 mm gave the isotopic ratio¹³C/¹²C+¹³C)=0.08. The ratio has been extrapolated to the cosmic ray source. It is found to be smaller in the source than at the point of measurement. Different assumptions about the origin of the cosmic radiation are discussed with regard to the results obtained in this investigation.     

Gamma ray signatures of ultra high energy cosmic ray accelerators: electromagnetic cascade versus synchrotron radiation of secondary electrons

We discuss the possibility of observing ultra high energy cosmic ray sources in high energy gamma rays. Protons propagating away from their accelerators produce secondary electrons during interactions with cosmic microwave background photons. These electrons start an electromagnetic cascade that results in a broad band gamma ray emission. We show that in a magnetized Universe (B≳10⁻¹² G) such emission is likely to be too extended to be detected above the diffuse background. A more promising possibility comes from the detection of synchrotron photons from the extremely energetic secondary electrons. Although this emission is produced in a rather extended region of size ∼10 Mpc, it is expected to be point-like and detectable at GeV energies if the intergalactic magnetic field is at the nanogauss level.      

Annihilation model of quasi-stellar objects

The possibility that annihilation is a major source of energy in cosmic physics is discussed. Since Klein suggested that the Universe might be matter-antimatter symmetric over two decades ago, there have been a significant number of papers developing the consequences of this view. These, however, have been largely ignored in the general literature. There have also been a number of papers claiming to prove that there cannot be antimatter anywhere in the observable Universe. In the first part of this paper an assessment of the differing views is given, and it is shown that none of the arguments against antimatter is convincing. The existence of antimatter is not in conflict with any observational fact. The reason for the negative attitude towards the existence of antimatter seems to be that this view is in conflict with a number of speculative but generally accepted theories. However, recent magnetospheric and heliospheric research, includingin situ measurements of cosmic plasmas, is now drastically changing cosmic plasma physics in a way that leads to growing scepticism about quite a few of the speculative theories.An attempt is made to develop a simple phenomenological model of QSOs based on star-antistar collisions. This model can account for such basic observational properties as the acceleration to very large (non-cosmological) velocities, the existence of broad emission lines, and at the same time narrow absorption lines with different redshifts. The absence of blueshifts is also explained. The model predicts that relatively young QSOs should be at cosmological distances whereas the old ones may very well be much closer to us than indicated by their redshift.     

Galactic cosmic rays and N2 dissociation on Titan

The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N₂, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N₂ excitation and dissociation rates and the ionization rates of the principal atmospheric constituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source.     

Search for correlations of GRB and cosmic rays

It is possible that violent processes resulting in Gamma Ray Bursts produce also high energy photons and cosmic rays. The possible correlations of very short GRB with, e.g., CMB, cosmic rays is briefly discussed. We have also begun preparation of the experiment correlating in real time data from Maze cosmic ray detector and Pi of the Sky robotic telescope.     

The cosmic ray primary composition between 10 and 10 eV from Extensive Air Showers electromagnetic and TeV muon data

The cosmic ray primary composition in the energy range between 10¹⁵ and 10¹⁶ eV, i.e., around the “knee” of the primary spectrum, has been studied through the combined measurements of the EAS-TOP air shower array (2005 m a.s.l., 10⁵ m² collecting area) and the MACRO underground detector (963 m a.s.l., 3100 m w.e. of minimum rock overburden, 920 m² effective area) at the National Gran Sasso Laboratories. The used observables are the air shower size (N_e) measured by EAS-TOP and the muon number (N_μ) recorded by MACRO. The two detectors are separated on average by 1200 m of rock, and located at a respective zenith angle of about 30°. The energy threshold at the surface for muons reaching the MACRO depth is approximately 1.3 TeV. Such muons are produced in the early stages of the shower development and in a kinematic region quite different from the one relevant for the usual N_μ−N_e studies. The measurement leads to a primary composition becoming heavier at the knee of the primary spectrum, the knee itself resulting from the steepening of the spectrum of a primary light component (p, He) of Δγ=0.7±0.4 at E₀4×10¹⁵ eV. The result confirms the ones reported from the observation of the low energy muons at the surface (typically in the GeV energy range), showing that the conclusions do not depend on the production region kinematics. Thus, the hadronic interaction model used (CORSIKA/QGSJET) provides consistent composition results from data related to secondaries produced in a rapidity region exceeding the central one. Such an evolution of the composition in the knee region supports the “standard” galactic acceleration/propagation models that imply rigidity dependent breaks of the different components, and therefore breaks occurring at lower energies in the spectra of the light nuclei.     

On the spectrum of ultrahigh energy cosmic rays and the γ-ray burst origin hypothesis

It has been suggested that cosmological γ-ray bursts (GRBs) can produce the observed flux of cosmic rays at the highest energies. However, recent studies of GRBs indicate that their redshift distribution likely follows the average star formation rate of the universe and that GRBs were more numerous at high redshifts. As a consequence, we show that photomeson production energy losses suffered by ultrahigh energy cosmic rays coming from GRBs would produce too sharp a spectral energy cutoff to be consistent with the air shower data. Futhermore, we show that cosmolgical GRBs fail to supply the energy input required to account for the cosmic ray flux above 10¹⁹ eV by a factor of 100–1000.     

“Dark energy” in the Local Void

The unexpected discovery of the accelerated cosmic expansion in 1998 has filled the Universe with the embarrassing presence of an unidentified “dark energy”, or cosmological constant, devoid of any physical meaning. While this standard cosmology seems to work well at the global level, improved knowledge of the kinematics and other properties of our extragalactic neighborhood indicates the need for a better theory. We investigate whether the recently suggested repulsive-gravity scenario can account for some of the features that are unexplained by the standard model. Through simple dynamical considerations, we find that the Local Void could host an amount of antimatter (∼5×10¹⁵  M _⊙) roughly equivalent to the mass of a typical supercluster, thus restoring the matter-antimatter symmetry. The antigravity field produced by this “dark repulsor” can explain the anomalous motion of the Local Sheet away from the Local Void, as well as several other properties of nearby galaxies that seem to require void evacuation and structure formation much faster than expected from the standard model. At the global cosmological level, gravitational repulsion from antimatter hidden in voids can provide more than enough potential energy to drive both the cosmic expansion and its acceleration, with no need for an initial “explosion” and dark energy. Moreover, the discrete distribution of these dark repulsors, in contrast to the uniformly permeating dark energy, can also explain dark flows and other recently observed excessive inhomogeneities and anisotropies of the Universe.