[en] The active galactic nucleus Markarian 421 (Mrk 421) is studied through the EAS-TOP extensive air shower array in the γ-ray energy ranges 20-50 TeV and 80-120 TeV. No evidence of d.c. emission is found in both datasets: the 90% flux limits are PHI (>25 TeV) < 1.3 · 10^-13 cm^-2 s^-1 and PHI(>90 TeV) < 2.5 · 10^-14 cm^-2 s^-1. A straight power law extrapolation up to E₀ ∼ 10¹⁵ eV of the spectrum obtained from the observations by the EGRET and Whipple experiments - above 100 MeV and 0.5 TeV respectively - is excluded with a confidence level >99.99%, as expected from the absorption of VHE γ-rays by the 2.7K cosmic microwave background radiation. An indication (at ∼ 90% c.l.) of a steepening of the spectrum, with respect to such extrapolation, at γ-ray energies lower than expected from the 2.7K cutoff is derived. This could be due either to a break in the source spectrum or to the absorption of VHE γ-rays by intergalactic infrared photons of wavelength ∼ 100 μm

[en] A numerical method is described which allows one to obtain the solutions of the linearized Navier-Stokes equation for the case of wave motion at the air-water interface in the presence of both soluble and insoluble films. The results are shown, and evidence of two vibration modes, i.e. transversal and longitudinal waves have been discussed. Their behaviour appears to be closely linked, especially in the case of insoluble films

[en] Extensive Air Showers are the product of interactions of cosmic rays with the earth's atmosphere. Through their detection by ground-based experiments, it is possible to study cosmic ray physics at energies not attainable by balloon or satellites techniques. We discuss the basic characteristics of atmospheric cascades and the experimental techniques, and as application examples, we show some recent results in two of the main field of research: the study of the CR primary spectrum and Very High-Ultra High Energy γ-ray astronomy. (author)

[en] The combined use of muon production height along the shower axis evaluated from muon direction and from muon arrival time selects high energy muons neither significantly scattered in the atmosphere nor deviated in the geomagnetic field. This allows the reconstruction of Extensive Air Shower (EAS) longitudinal development in the knee energy region, which provides informations about the nature of primary particle and its interaction with the air nuclei. We describe the possibility to reconstruct the muon height of production by the combination of tracking and timing measurements as provided by a large area tracking and timing telescope now available in the EAS-TOP experiment. 40 m² of bakelite RPCs have been installed under the existing muon-hadron detector in order to obtain accurate measurement of muon arrival time with respect to the arrival of the shower core

[en] We summarize the session 'Exploring the range from TeV to ZeV' that highlighted high energy neutrino astronomy as well the connection to the physics of cosmic rays and gamma-rays.

[en] Muons decaying in the water volume of a Cherenkov detector of the Pierre Auger Observatory provide a useful calibration point at low energy. Using the digitized waveform continuously recorded by the electronics of each tank, we have devised a simple method to extract the charge spectrum of the Michel electrons, whose typical signal is about 1/8 of a crossing vertical muon. This procedure, moreover, allows continuous monitoring of the detector operation and of its water level. We have checked the procedure with high statistics on a test tank at the Observatory base and applied with success on the whole array

[en] Two test detectors similar to the Pierre Auger Observatory Water Cherenkov Detectors have been installed at the Observatory site and at the Institut de Physique Nucleaire d'Orsay. The signals from the tanks are read out using three 9-inch photomultipliers and analyzed by both a digital oscilloscope with high sampling frequency and the Auger surface detector electronics. Additionally, the detectors are equipped with plastic scintillators serving as muon telescopes. The trigger is provided either by the muon telescope or by the coincidence of the three PMTs. The scintillators are movable allowing the study of the detector response to atmospheric muons arriving with different incident angles. In this paper, the results of measurements for vertical and inclined background muons are presented. These results are compared to simulations and important calibration parameters are extracted. The influence of the direct light detected by the PMTs, particularly important for inclined showers, is discussed

[en] In the last years a general consensus has emerged that, contrary to intuition, quantum-gravity effects may have relevant consequences for the propagation and interaction of high energy particles. This has given birth to the field of Phenomenological Quantum Gravity. We review some of the aspects of this new, very exciting frontier of Physics

[en] On February 23, 1989, an excess of counts of>100 TeV Extensive Air Showers has been recorded from the direction of the Crab Nebula by three independent detectors: Baksan (North Caucasus, USSR), KGF (India) and EAS-TOP (Gran Sasso, Italy). The chance probability of the combined detection is ∼ 10^-7. We discuss the EAS-TOP observations from the statistical point of view (chance probability ≤10^-2) and present some phenomenological characteristics of the events

[en] The ground array of the Pierre Auger Observatory will consist of 1600 water Cherenkov detectors, deployed over 3000 km². The remoteness and large number of detectors required a simple, automatic remote calibration procedure. The primary physics calibration is based on the average charge deposited by a vertical and central throughgoing muon, determined with good precision at the detector via a novel rate-based technique and later with higher precision via charge histograms. This value is named the vertical-equivalent muon (VEM). The VEM and the other parameters needed to maintain this calibration over the full energy range and to assess the quality of the detector are measured every minute. This allows an accurate determination of the energy deposited in each detector when an atmospheric cosmic ray shower occurs