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.     

Calculation of the TeV prompt muon component in very high energy cosmic ray showers

HEMAS-DPM is a Monte Carlo for the simulation of very high energy cosmic ray showers, which includes the DPMJET-II code based on the two component Dual Parton Model. DPMJET-II provides also charm production in agreement with data and, for p exceeding 5 GeV/c, with perturbative QCD results in proton-proton interactions. In this respect, a new scheme has been considered for the inclusive production of D mesons at large p in hadronic collisions in the framework of perturbative fragmentation functions, allowing an analysis at the Next to Leading Order (NLO) level which goes beyond the fixed I(_s³) perturbative theory of open charm production. We have applied HEMAS-DPM to the calculation of the prompt muon component for E_μ ≥ 1 TeV in air showers considering the two extreme cases of primary protons and Fe nuclei.     

The cosmic ray primary composition in the “knee” region through the EAS electromagnetic and muon measurements at EAS-TOP

The evolution of the cosmic ray primary composition in the energy range 10⁶–10⁷ GeV (i.e. the “knee” region) is studied by means of the e.m. and muon data of the Extensive Air Shower EAS-TOP array (Campo Imperatore, National Gran Sasso Laboratories). The measurement is performed through: (a) the correlated muon number (N_μ) and shower size (N_e) spectra, and (b) the evolution of the average muon numbers and their distributions as a function of the shower size. From analysis (a) the dominance of helium primaries at the knee, and therefore the possibility that the knee itself is due to a break in their energy spectrum (at E_k^He=(3.5±0.3)×10⁶ GeV) are deduced. Concerning analysis (b), the measurement accuracies allow the classification in terms of three mass groups: light (p,He), intermediate (CNO), and heavy (Fe). At primary energies E₀≈10⁶ GeV the results are consistent with the extrapolations of the data from direct experiments. In the knee region the obtained evolution of the energy spectra leads to: (i) an average steep spectrum of the light mass group (γ_p,He>3.1), (ii) a spectrum of the intermediate mass group harder than the one of the light component (γ_CNO2.75, possibly bending at E_k^CNO≈(6–7)×10⁶ GeV), (iii) a constant slope for the spectrum of the heavy primaries (γ_Fe2.3–2.7) consistent with the direct measurements. In the investigated energy range, the average primary mass increases from lnA=1.6–1.9 at E₀1.5×10⁶ GeV to lnA=2.8–3.1 at E₀1.5×10⁷ GeV. The result supports the standard acceleration and propagation models of galactic cosmic rays that predict rigidity dependent cut-offs for the primary spectra of the different nuclei. The uncertainties connected to the hadronic interaction model (QGSJET in CORSIKA) used for the interpretation are discussed.     

Distance measurements as a probe of cosmic acceleration

A major recent development in observational cosmology has been an accurate measurement of the luminosity distance–redshift relation out to redshifts z =0.8 from Type Ia supernova standard candles. The results have been argued as evidence for cosmic acceleration. It is well known that this assertion depends on the assumption that we know the equation of state for all mass–energy other than normal pressureless matter; popular models are based either on the cosmological constant or on the more general quintessence formulation. However, this assertion also depends on a number of other assumptions, implicit in the derivation of the standard cosmological field equations: large-scale isotropy and homogeneity, the flatness of the Universe, and the validity of general relativity on cosmological scales (where it has not been tested). A detailed examination of the effects of these assumptions on the interplay between the luminosity distance–redshift relation and the acceleration of the Universe is not possible unless one can define the precise nature of the failure of any particular assumption. However a simple quantitative investigation is possible and reveals a number of considerations about the relative importance of the different assumptions. In this paper we present such an investigation. We find that the relationship between the distant-redshift relation and the sign of the deceleration parameter is fairly robust and is unaffected if only one of the assumptions that we investigate is invalid so long as the deceleration parameter is not close to zero (it would not be close to zero in the currently favoured Ω_Λ=1−Ω_matter=0.7 or 0.8 Universe, for example). Failures of two or more assumptions in concordance may have stronger effects.     

Surface and underground measurements of long-term changes in the cosmic ray solar diurnal variation

North/south directional telescopes operating at the surface and vertical and inclined telescopes operating at a depth of 60 m.w.e. underground in London have been employed to study changes in the cosmic ray solar diurnal variation over the past few years. In order to extend the study to the low rigidity end of the spectrum, results obtained by the NM64 neutron monitors operating at Deep River and Goose Bay in Canada have also been examined. The surface telescope data require that the full corotation amplitude of 0.59 per cent should have been observed during almost the entire solar cycle with the possible exception of the year 1965 when cosmic ray intensity was a maximum. However, the effective amplitude observed by neutron monitors during most of the solar cycle was only about 0.52 per cent and this reduction has been ascribed to the lower value of the exponent of the energy spectrum which prevails amongst the latitude sensitive primaries. Nevertheless, the upper limiting rigidity was varying during the course of the solar cycle, its value being high when solar activity was high and low when solar activity decreased. During 1965, even though the upper limiting rigidity assumed its lowest value, the free space amplitude was also diminished by a little over 10 per cent. Even though the theory of rigid corotation invoking a purely azimuthal streaming of the cosmic ray gas successfully predicts the free space amplitude, it fails to explain the phase changes observed by both types of monitor and which are quite significant. The underground data require that the component due to atmospheric temperature effects is negligibly small and that throughout the rigidity range covered by the recorder response, there is present an apparent anisotropy due to the orbital motion of the Earth around the Sun. Also the underground data roughly confirm the changes in upper limiting rigidity which were observed by the surface instruments.     

Gamma-ray probe of cosmic ray pressure in galaxy clusters and cosmological implications

Cosmic rays produced in cluster accretion and merger shocks provide pressure to the intracluster medium (ICM) and affect the mass estimates of galaxy clusters. Although direct evidence for cosmic ray ions in the ICM is still lacking, they produce γ-ray emission through the decay of neutral pions produced in their collisions with ICM nucleons. We investigate the capability of the Gamma-ray Large Area Space Telescope ( GLAST ) and imaging atmospheric Čerenkov telescopes (IACTs) for constraining the cosmic ray pressure contribution to the ICM. We show that GLAST can be used to place stringent upper limits, a few per cent for individual nearby rich clusters, on the ratio of pressures of the cosmic rays and thermal gas. We further show that it is possible to place tight (≲10 per cent) constraints for distant  ( z ≲ 0.25)  clusters in the case of hard spectrum, by stacking signals from samples of known clusters. The GLAST limits could be made more precise with the constraint on the cosmic ray spectrum potentially provided by IACTs. Future γ-ray observations of clusters can constrain the evolution of cosmic ray energy density, which would have important implications for cosmological tests with upcoming X-ray and Sunyaev–Zel'dovich effect cluster surveys.     

The size of antimatter bodies and the primary very high energy cosmic ray flux

The consequences of antimatter bodies on the very high energy primary cosmic ray flux are considered. The effects of various models of cosmic ray origin and properties of astrophysical parameters are discussed. A simple expression for the production of antiprotons inN–N collisions as a function of energy of the incident proton is obtained by utilizing characteristics of particles produced in high energy collisions. It is assumed that sufficient time will have elapsed for all antibaryons to decay to antiprotons. It is shown that the measurement of antinuclei in the primary cosmic ray spectrum above 10¹⁷ eV could help to establish the size of antimatter bodies.This research was supported by A.F.O.S.R. Grant No. F-44620-69-C-0019.     

Prediction of the diffuse neutrino flux from cosmic ray interactions near supernova remnants

In this paper, we present high-energy neutrino spectra from 21 Galactic supernova remnants (SNRs), derived from gamma-ray measurements in the GeV–TeV range. We find that only the strongest sources, i.e. G40.5-0.5 in the north and Vela Junior in the south could be detected as single point sources by IceCube or KM3NeT, respectively. For the first time, it is also possible to derive a diffuse signal by applying the observed correlation between gamma-ray emission and radio signal. Radio data from 234 supernova remnants listed in Green’s catalog are used to show that the total diffuse neutrino flux is approximately a factor of 2.5 higher compared to the sources that are resolved so far. We show that the signal at above 10 TeV energies can actually become comparable to the diffuse neutrino flux component from interactions in the interstellar medium. Recently, the IceCube collaboration announced the detection of a first diffuse signal of astrophysical high-energy neutrinos. Directional information cannot unambiguously reveal the nature of the sources at this point due to low statistics. A number of events come from close to the Galactic center and one of the main questions is whether at least a part of the signal can be of Galactic nature. In this paper, we show that the diffuse flux from well-resolved SNRs is at least a factor of 20 below the observed flux.     

The solar coronal green line as an index of cosmic ray modulation

In recent years several papers have indicated that the solar coronal green line emission (λ=5303Å) is the ‘best’ indicator of cosmic ray modulation. This paper questions this result for the following reasons: (a) the problems of green line measurement obscure the true meaning of the resulting observations; (b) there is a lack of any direct physical mechanism relating green line intensity and modulation; (c) a negative result came from a careful attempt to relate the cosmic ray diurnal variation and green line emission.     

The energy spectrum of primary cosmic ray electrons from 2 GeV to 200 GeV

A balloon borne counter telescope with a gas Cerenkov counter is used to measure the energy of primary cosmic ray electrons between 2 and 200 GeV. Electrons are identified by the characteristic electron-photon shower which they produce in a 15 radiation length deep stack of high-Z material interleaved with scintillation counters. Calibrations with monoenergetic electrons up to 14 GeV and monoenergetic protons up to 28 GeV from accelerators are used to develop criteria to statistically separate electrons from proton induced events. The results from six balloon flights (total exposure time 63 hours) are combined to obtain the electron energy spectrum. Up to about 30 GeV the spectrum measured in this experiment can be directly checked with calibrations and agrees well with results from other experiments. Above this energy the flux reported here is somewhat higher than the determinations reported by most other authors. We do not attach significance to an apparent flattening of the energy spectrum above 50 GeV. There is no evidence for a steepening of the spectrum at energies below 200 GeV.Work supported in part by the National Aeronautics and Space Administration grant NGL 14-001-005.Also Department of Physics.NSF trainee (1965–1969).     

The fermi mechanism and the source spectrum of cosmic ray nuclei

It is shown that the velocity term, occurring in the expression for the rate of energy gain by the Fermi mechanism of acceleration, is to be taken into account in case of acceleration of non-relativistic particles. A spectral form of accelerated particles is derived on this basis and is called the ‘Fermi Spectrum’. At non-relativistic energies this spectral form is significantly different from the currently used forms of power law in total energy per nucleon and in rigidity, and lies about midway between them. It is shown that using this form of source spectrum of cosmic ray nuclei, satisfactory agreement can be obtained between the calculated values and the observed ones of the ratios of H²/He⁴ and He³/He⁴, and the energy spectra of protons and helium nuclei near the Earth.     

The expected pulsations of the heliosphere relevant to cosmic ray variations

The possible natural large-scale pulsations of the solar wind cavity are examined. The period of the pulsationsT2R/a (R is the size of the cavity,a is the sound velocity in the interstellar space near the solar system) may vary from a year to tens of years. The relevant new type of cosmic ray variations is predicted.     

A measurement of the flux of cosmic ray iron at 5×10 eV

We present results from the initial flight of our Balloon Air CHerenkov (BACH) payload. BACH detects air Cherenkov radiation from cosmic ray nuclei as coincident flashes in two optical modules. The flight (dubbed PDQ–BACH) took place on April 22, 1998 from Ft. Sumner, New Mexico. During an exposure of 2.75 h, with a typical threshold energy for iron nuclei of 2.2×10¹³ eV, we observed several events cleanly identifiable as iron group nuclei. Analysis of the data yields a new flux measurement that is fully consistent with that reported by other investigations.     

A composition dependent energy scale and the determination of the cosmic ray primary mass in the ankle region

At present there are still several open questions about the origin of the ultra high energy cosmic rays. However, great progress in this area has been made in recent years due to the data collected by the present generation of ground based detectors like the Pierre Auger Observatory and Telescope Array. In particular, it is believed that the study of the composition of the cosmic rays as a function of energy can play a fundamental role for the understanding of the origin of the cosmic rays.The observatories belonging to this generation are composed of arrays of surface detectors and fluorescence telescopes. The duty cycle of the fluorescence telescopes is ∼10% in contrast with the ∼100% of the surface detectors. Therefore, the energy calibration of the events observed by the surface detectors is performed by using a calibration curve obtained from a set of high quality events observed in coincidence by both types of detectors. The advantage of this method is that the reconstructed energy of the events observed by the surface detectors becomes almost independent of simulations of the showers because just a small part of the reconstructed energy (the missing energy), obtained from the fluorescence telescopes, comes from simulations. However, the calibration curve obtained in this way depends on the composition of the cosmic rays, which can introduce biases in composition analyses when parameters with a strong dependence on primary energy are considered. In this work we develop an analytical method to study these effects. We consider AMIGA (Auger Muons and Infill for the Ground Array), the low energy extension of the Pierre Auger Observatory corresponding to the surface detectors, to illustrate the use of the method. In particular, we study the biases introduced by an energy calibration dependent on composition on the determination of the mean value of the number of muons, at a given distance to the showers axis, which is one of the parameters most sensitive to primary mass and has an almost linear dependence with primary energy.