Energy spectrum and mass composition of primary cosmic rays at energies 10<Superscript>15</Superscript>–10<Superscript>18</Superscript> eV

Data on muon and electron components of extensive air showers (EAS) (obtained with the EAS MSU array) were used to derive the primary cosmic ray (PCR) mass composition. It is shown that for energies beyond the knee at energy 3 × 10¹⁵ eV the abundance of heavy nuclei increases with energy. But at energies above 10¹⁷ eV the abundance of light nuclei starts to grow. The primary cosmic ray spectrum in the range 10¹⁵–10¹⁸ eV is analyzed. It is shown that at energies above 10¹⁷ eV the additional component appears and it differs from the bulk of Galactic cosmic rays generated by shocks in SN remnants.     

Rising Inelastic Cross Section and its Contribution to Steepening of the Cosmic Ray Energy Spectrum

Calculations have been carried out to investigate the contribution of an increasing total cross section to steepening of the cosmic ray energy spectrum at very high energies (E ? 10¹² eV). Our results suggest that increasing total cross section contributes only a part to the steepening of primary energy spectrum at very high energies even if the composition is pure primary iron flux.     

Energy spectrum of primary cosmic rays at energies of 2 × 10<Superscript>13</Superscript> to 5 × 10<Superscript>17</Superscript> eV,according to Tien Shan data

The primary cosmic ray energy spectrum at energies of 10¹⁵ to 5 × 10¹⁷ eV is presented using the results from observations by the Tien Shan HADRON array. The spectrum was obtained from the spectrum of showers according to the number of electrons using a new way of determining the parameter of spatial distribution function S of electrons. The energy spectrum can be extended to low energies up to 2 × 10¹³ eV using data from separate experiments at the former Tien Shan array. Conclusions are drawn regarding changes in the form of the spectrum and its chemical composition at energies over 10¹⁶ eV. The spectrum is compared to the results from the TUNKA installation.     

The extensive air shower muon number spectrum beyond the knee in the cosmic-ray energy spectrum

The extensive air shower (EAS) muon number spectrum is obtained with increased statistics using the central muon detector of the EAS MSU array, which records muons with energies above 10 GeV. The dependence of the mass composition of primary cosmic rays on the energy is considered. The conclusion is confirmed that for energies from 3 × 10¹⁵ eV (the primary energy spectrum knee) up to 10¹⁷ eV a change in the composition associated with an increase in the proportion of heavy nuclei occurs; however, after the energy of 10¹⁷ eV, the proportion of heavy nuclei begins to decrease and the composition becomes lighter. A comparison with similar data from other experiments is conducted. The existence of an additional component of cosmic rays is confirmed; earlier an indication of its presence was derived from data on the EAS electron number spectrum.     

Estimating the fraction of gamma quanta in the primary cosmic ray flux with energies of ∼1017 eV from the MSU EAS array data

Using data from the MSU EAS array and model calculations, we search for events with abnormally small fractions of muons with energies above 10 GeV in showers with particle numbers of >2 × 10⁷ and zenith angles of <30 degrees. We confirm with good statistical accuracy that the content of gamma quanta in the primary cosmic ray flux can be as high as 2% at energies of ～10¹⁷ eV.     

Search for anisotropy in arrival directions of ultrahigh-energy cosmic rays by using the marr wavelet on the equatorial sphere

The arrival direction distribution of cosmic rays for energies E>10¹⁸ eV detected at the Yakutsk EAS array is analyzed using the Marr wavelet. Deviation from the isotropic distribution of 361 showers above 10¹⁹ eV is found at a significance level of 0.007, which corroborates previous evidence of the cosmic ray anisotropy in this energy range. In contrast to the previous methods, the wavelet analysis makes it possible to localize both a celestial-sphere domain containing a supposed source of excess cosmic ray flux and the scale parameter of the two-dimensional wavelet.     

A DEDUCTION ON PRIMARY COSMIC PAY COMPOSITION AND SPECTRA ABOVE 1014eV

The spectra of primary cosmic ray nuclei are fitted from experimental data under 10¹⁴eV and extrapolated to their bending energies. The primary cosmic ray composition above 10¹⁴eV is deduced according to the rigidity cut off model by selecting an appropriate bending energy for proton so that the sum of various nucleus components is consistent with the experimental total particle spectrum. The method used is independent on the behaviour of hadronic interaction.     

Ultrahigh-energy cosmic rays: Possible origin and spectrum

The complicated shape of the cosmic ray spectrum recorded by giant arrays in the energy range 10¹⁷–10²⁰ eV is analyzed. It is shown that in the energy region ∼10¹⁸–10¹⁹ eV the spectrum probably coincides with the injection spectrum whose exponent is equal approximately to 3.2–3.3. The flatter component in the energy region (3.2–5.0)×10¹⁹ eV is due to braking of extragalactic protons on primordial photons (the cosmic background radiation). At energies exceeding 3.2×10¹⁹ eV the spectrum does not have a blackbody cutoff. The possibility of determining the distances at which cosmic rays originate and investigating the evolution of their sources on the basis of ultrahigh-energy cosmic ray data is discussed. Zh. éksp. Teor. Fiz. 113, 12–20 (January 1998)     

Primary cosmic ray composition at energies from 10<Superscript>17</Superscript> to 10<Superscript>18</Superscript> eV according to the EAS MSU array data

EAS MSU array data on the composition of primary cosmic rays at energies above 10₁₇ eV are analyzed. The problem of existence of a cosmic ray component that is not related to the conventional mechanism of formation of galactic cosmic rays is considered and the fraction of γ rays in primary cosmic rays is estimated.     

Modeling Source Spectra and Composition of Ultrahigh Energy Cosmic Rays

The inverse problem of cosmic ray transport of ultra-high energy cosmic rays is considered. The source spectrum and composition are derived based on the recent Auger data on energy spectrum, energy dependence of mean logarithm of atomic mass number and its variance. The dependence of results on the extrapolation of observable spectrum beyond energies 10²⁰ eV is investigated.

     

Change of primary cosmic ray mass composition at superhigh energies

The EAS MSU array experimental data are analyzed in relation to the primary cosmic ray composition in the energy range above 10¹⁷–10¹⁸ eV. The problem of the existence of an additional cosmic ray component, which cannot be explained in the framework of traditional mechanism of Galactic cosmic ray production, is considered. The fraction of gamma-quanta in the primary cosmic radiation is evaluated as well.     

Modernization of the Carpet-2 array of the Baksan Neutrino Observatory

The state of the art and the project of modernization of the extensive-air-shower array Carpet-2 of the Baksan Neutrino Observatory of the Institute for Nuclear Research, Russian Academy of Sciences are described. The modernized array will allow the performance of detailed study of variations in the cosmic ray intensity, the energy spectra and composition of primary cosmic rays in the energy range 10¹³–10¹⁶ eV, and the anisotropy of primary cosmic rays with energies above 10¹³ eV.     

Mass Composition of Cosmic Rays with Energies above 1017 eV According to the Data from the Muon Detectors of the Yakutsk EAS Array

The lateral distribution of muons in extensive air showers with energies above 1017 eV detected by underground scintillation detectors with a threshold of 1.0 GeV at the Yakutsk array in 1986–2016 has been analyzed. The experimental data on the muon flux density at a distance of 300 m from the shower axis have been compared to the calculations within various models of hadron interactions at ultrahigh energies. The experimental data are in the best agreement with the QGSJet01 and QGSJet II-04 models. The mass composition of cosmic rays in the energy range of (1–30) × 10¹⁷ eV changes from middle nuclei to a purely proton composition.

     

Arrival directions and chemical composition of ultrahigh-energy cosmic rays

Chemical composition of ultrahigh-energy cosmic rays is estimated through the reliably determined (both experimentally and theoretically) distribution of the number of showers in the galactic latitude. Experimental data at energies of ～10¹⁹ eV agree with the theoretical calculations, provided that cosmic rays involve predominantly heavy nuclei. An enhanced flux of cosmic rays from the galactic plane is detected at energies of ～10¹⁹ eV.     

Cosmic ray energy spectrum from measurements of air showers

This review focuses on high-energy cosmic rays in the PeV energy range and above. Of particular interest is the knee of the spectrum around 3 PeV and the transition from cosmic rays of Galactic origin to particles from extra-galactic sources. Our goal is to establish a baseline spectrum from 1014 to 10^20 eV by combining the results of many measurements at different energies. In combination with measurements of the nuclear composition of the primaries, the shape of the energy spectrum places constraints on the number and spectra of sources that may contribute to the observed spectrum.     

New physics, the cosmic ray spectrum knee, and pp cross section measurements

We explore the possibility that a new-physics interaction can provide an explanation for the knee just above 10⁶ GeV in the cosmic ray spectrum. We model the new-physics modifications to the total proton–proton cross section with an incoherent term that allows for missing energy above the scale of new physics. We add the constraint that the new physics must also be consistent with published pp cross section measurements, using cosmic ray observations, an order of magnitude and more above the knee. We find that the rise in cross section required at energies above the knee is radical. The increase in cross section suggests that it may be more appropriate to treat the scattering process in the black disc limit at such high energies. In this case there may be no clean separation between the standard model and new-physics contributions to the total cross section. We model the missing energy in this limit and find a good fit to the Tibet III cosmic ray flux data. We comment on testing the new-physics proposal for the cosmic ray knee at the Large Hadron Collider.     

Study of the energy spectrum and mass composition of primary cosmic rays in the energy range of 10<Superscript>18</Superscript>–10<Superscript>20</Superscript> eV using a balloon setup in antarctica (SPHERE-antarctica project)

An Antarctic balloon experiment for measuring the energy spectrum and elemental composition of cosmic rays in the ultrahigh-energy range (10¹⁸–10²⁰) eV is proposed. Scientific equipment will measure fluorescence caused by an extensive air shower formed in the atmosphere by an ultrahigh energy particle and Cherenkov light of this shower reflected from a snow surface. It is assumed that the balloon will fly in the circumpolar orbit in Antarctica at a height of ~25 km for (2–3) winter (in the Southern Hemisphere)months. For this time, ~3000 events caused by particles with energies above 10¹⁸ eV and (200–300) events caused by particles with energies above 10¹⁹ eV will be detected.     

Some recent developments in cosmic rays

Recent work on the energy spectrum, composition and anisotropy of the primary cosmic ray flux from about 10⁹ eV to 10²⁰ eV is summarized, together with related information on phonons between 1 MeV and 200 MeV. Solar particles are not discussed, the emphasis being on topics bearing on the origin of the radiation, which is still an unsolved problem, although the probabilities are strongly in favour of an entirely galactic origin for the particles. Mechanisms of acceleration are not discussed.