Abstract
Primary cosmic rays of energy greater than ∼10 14 eV must be studied by indirect experiments measuring the particles generated in the EAS (Extensive Air Shower) development in atmosphere. These experiments are mainly limited by the systematic errors due to their energy calibration. I will discuss the main sources of these errors: the choice of the hadronic interaction model and of the mass of the primary particle (that cannot be measured on a event by event basis). I will then summarize some recent measurements of the all particle spectrum, and I will show that, keeping into account the differences due to the energy calibration, they all agree on the spectral shape. Then I will describe the measurements of the light and heavy primaries mass groups spectra, discussing the claimed features. Using a simple calculation of the elemental spectra (based on the hypothesis that the knee energies follow a Peter's cycle) I will try to discuss if all these results can be interpreted in a common picture. • Surface arrays: sampling the EAS at fixed atmospheric depth. Almost all of these arrays are able to simultaneously detect more than one EAS components: usually the electromagnetic and the muonic ones. Detecting the particle density and arrival times at different distances from the shower core these arrays derive the arrival direction of the primary cosmic ray, the number of charged particles (Nch) and the number of muons (N) in the EAS at observation level. Both Nch and N are derived as normalization of the lateral distribution of the particle density and can be defined either as the total number of particles at observation level or as the number of particles at a fixed distance from the shower core (distance that has to be fixed by every single experiment and depends both on the detector layout and on the primary energy range studied by the array). These detectors operate with a 100% duty cycle. • Cherenkov arrays: detecting the cherenkov light emitted by particles during EAS development. The big advantage of these arrays is that they perform an almost
Highlights
Primary cosmic rays with energy greater than 1014 eV cannot be studied by direct experiments operating on balloon or on satellites and their detection is only possible by means of indirect Extensive Air Showers (EAS) experiments
Detecting the particle density and arrival times at different distances from the shower core these arrays derive the arrival direction of the primary cosmic ray, the number of charged particles (Nch) and the number of muons (N ) in the EAS at observation level. Both Nch and N are derived as normalization of the lateral distribution of the particle density and can be defined either as the total number of particles at observation level or as the number of particles at a fixed distance from the shower core
Having discussed the main sources of systematic errors affecting the energy and mass measurements performed by indirect experiments, I will discuss some of the more recent results obtained in the 1014–1018 eV energy range
Summary
Primary cosmic rays with energy greater than 1014 eV cannot be studied by direct experiments operating on balloon or on satellites and their detection is only possible by means of indirect EAS experiments. Detecting the particle density and arrival times at different distances from the shower core these arrays derive the arrival direction of the primary cosmic ray, the number of charged particles (Nch) and the number of muons (N ) in the EAS at observation level Both Nch and N are derived as normalization of the lateral distribution of the particle density and can be defined either as the total number of particles at observation level or as the number of particles at a fixed distance from the shower core (distance that has to be fixed by every single experiment and depends both on the detector layout and on the primary energy range studied by the array). The atmospheric depth of the shower maximum (a parameter indicating the mass of the primary particle) can be estimated comparing the cherenkov photon densities measured at two different distances from the shower core These arrays operate only during clear moonless nights, their duty cycle is ∼10−15%. I will show that recent results can, at least qualitatively, be described by elemental spectra with change of slope at energies scaling with Z, even if the recent ARGO-YBJ results [1] cannot be included in a simple framework and an additional galactic component dominating the spectrum above 1016 eV must be introduced [2, 3]
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