M. Bertaina

EVOLUTION OF THE COSMIC-RAY ANISOTROPY ABOVE 10 14 eV

The amplitude and phase of the cosmic-ray anisotropy are well established experimentally between 1011 eV and 1014 eV. The study of their evolution in the energy region 1014-1016 eV can provide a significant tool for the understanding of the steepening (knee) of the primary spectrum. In this Letter, we extend the EAS-TOP measurement performed at E 0 ≈ 1014 eV to higher energies by using the full data set (eight years of data taking). Results derived at about 1014 and 4 × 1014 eV are compared and discussed. Hints of increasing amplitude and change of phase above 1014 eV are reported. The significance of the observation for the understanding of cosmic-ray propagation is discussed.

Measurement of the cosmic ray hadron spectrum up to 30-TeV at mountain altitude: The Primary proton spectrum

Abstract The flux of cosmic ray hadrons at the atmospheric depth of 820 gxa0cm−2 has been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore, National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well described by a single power law: S h (E h )=(2.25±0.21±0.34 sys )×10 −7 E h 1000 (−2.79±0.05) m −2 s −1 sr −1 GeV −1 overthe energy range 30 GeV–30 TeV. The procedure and the accuracy of the measurement are discussed. The primary proton spectrum is derived from the data by using the CORSIKA/QGSJET code to compute the local hadron flux as a function of the primary proton spectrum and to calculate and subtract the heavy nuclei contribution (basing on direct measurements). Over a wide energy range E0=0.5–50 TeV its best fit is given by a single power law: S(E 0 )=(9.8±1.1±1.6 sys )×10 −5 E 0 1000 (−2.80±0.06) m −2 s −1 sr −1 GeV −1 The validity of the CORSIKA/QGSJET code for such application has been checked using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the measured hadron fluxes at the two experimental depths (820 and 1030 gxa0cm−2 respectively) at better than 10% in the considered energy range.The flux of cosmic ray hadrons at the atmospheric depth of 820 g/cm^2 has been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore, National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well described by a single power law : S(E_h) = (2.25 +- 0.21 +- 0.34(sys)) 10^(-7)(E_h/1000)^(-2.79 +- 0.05) m^(-2) s^(-1) sr^(-1) GeV^(-1) over the energy range 30 GeV-30 TeV. The procedure and the accuracy of the measurement are discussed. The primary proton spectrum is derived from the data by using the CORSIKA/QGSJET code to compute the local hadron flux as a function of the primary proton spectrum and to calculate and subtract the heavy nuclei contribution (basing on direct measurements). Over a wide energy range E_0 = 0.5-50 TeV its best fit is given by a single power law : S(E_0) = (9.8 +- 1.1 +- 1.6(sys)) 10^(-5) (E_0/1000)^(-2.80 +- 0.06) m^(-2) s^(-1) sr^(-1) GeV^(-1). The validity of the CORSIKA/QGSJET code for such application has been checked using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the measured hadron fluxes at the two experimental depths (820 and 1030 g/cm^2 respectively) at better than 10% in the considered energy range.

A limit to the rate of ultra high energy γ-rays in the primary cosmic radiation

Abstract An upper limit to the flux of Ultra High Energy (UHE) γ-rays in the primary cosmic radiation is obtained through the data of the electromagnetic and the muon detectors of the EAS-TOP Extensive Air Shower array (Campo Imperatore, National Gran Sasso Laboratories, atmospheric depth 810g cm−2). The search is performed by selecting Extensive Air Showers (EAS) with low muon content. For EAS electron sizes Ne > 6.3 · 105, no showers are observed with the core located inside a fiducial area and no muons recorded in the 140 m2 muon detector, during a live time of 8440 h. The 90% c.l. upper limit to the relative intensity of γ-ray with respect to cosmic ray (c.r.) primaries is I γ I c.r. −5 , at primary energy E0 ≥ 1015 eV: this limit is lower than reported in previous measurements.

The hadron calorimeter of EAS-TOP: operation, calibration and resolution

Abstract We describe and discuss the operation, calibration and stability of the EAS-TOP calorimeter (Campo Imperatore, National Gran Sasso Laboratories), a large area hadron and muon detector devoted to cosmic-ray physics. It consists of iron slabs (for a total thickness of 818xa0gxa0cm −2 ) and Iarocci tubes as sensitive layers, operating in the streamer mode and the “quasi proportional” regime. Using a model describing the operation of the “quasi proportional” chambers, we derive a calibration curve in the energy range 50–5000xa0GeV, whose reliability has been indirectly checked through on-site measurements, by means of an accelerator beam run (up to≃600–700xa0GeV) and by comparing the model predictions on hadron shower transition curves with the data.

Study of the cosmic ray primary spectrum at 1015 < E0 < 1016 eV with the EAS-TOP array

Abstract The break observed in the electron shower size power law spectrum of Extensive Air Showers (EAS) at corresponding primary energy E0 ∼ (3–5)1015 eV (“knee”) is studied different EAS components (electromagnetic and muonic) and at different atmospheric depths. A consistent description is obtained. The interpretation of data in terms of primary composition, and following the most accepted high energy interaction models, leads to an increasing average primary mass in this energy range. The study of such behaviour is expected to provide a crucial information for the understanding of the physical parameters that characterize the break for the different primaries.

Study of horizontal air showers from EAS-TOP: a possible tool for UHE neutrino detection?

Abstract Horizontal Air Showers, i.e. events observed at “quasi” horizontal incidence, are studied from EAS-TOP at Campo Imperatore (National Gran Sasso Laboratories). The “reality” of such events as due to deeply penetrating particles is discussed from the experimental point of view. We deduce: • -the measurement of the HE muon flux at E μ = 20 TeV : F μ (> 20 TeV ) = (1.3 ± 0.4) × 10 −11 cm −2 s −1 sr −1 , • -the upper limit to the extraterrestrial HE neutrino flux: I ν (> 10 5 GeV ) −9 cm −2 s −1 sr −1 . • The possibility to develop the technique for UHE ν detection is discussed.

The LVD experiment at Gran Sasso

SummaryThe Large-Volume Detector (LVD) in the Gran Sasso underground Laboratory is a multipurpose detector consisting of a large volume of liquid scintillator (at present 562 tons are in data taking) interleaved with limited-streamer tubes. Several physical problems are investigated with LVD, the major being the search for neutrino bursts from gravitational stellar collapses in our Galaxy. In this paper we discuss some results on cosmic neutrinos and cosmic-ray muons obtained with the first of the five towers of LVD (operational since June 1992) and part of the second tower (operational since June 1994). The results of the search for supernovae neutrinos show that LVD is a neutrino observatory able to detect neutrinos of different flavours from gravitational stellar collapses in all our Galaxy, over a wide range of burst durations. Indeed, the carbon-based liquid-scintillator target gives a unique possibility to directly detect neutral- and charged-currents neutrino interactions with a very good signature. This characteristic of LVD allows us to make an indirect estimate of the neutrino rest mass and of neutrino oscillations from supernovae in our Galaxy. No evidence for burst candidates has been found in the data recorded from June 1992 to March 1995, for a total live time of 682 days and a total exposure of 613 tons per year. We present the results of a time coincidence analysis between low-energy signals, eventually due to neutrinos of different flavours, and γ-ray bursts (GRBs) detected by the BATSE experiment. This search covers the period from June 1993 to March 1995, during which 41 GRBs have been selected from the BATSE data. Since no excess of events in LVD has been found, upper limits on the neutrino fluxes are reported for (νe, p), and for neutral- and charged-currents neutrino interactions of different flavours with the C-nuclei of the scintillator. The muon intensity as a function of slant depth is presented. These measurements, obtained during a live time period of 11.556 hours, cover a slant depths range from about 3000 to about 20 000 hg/cm2 of standard rock and extend over five decades of intensity. An interesting result is that the muon flux is independent of slant depth beyond a depth of about 14 000 hg/cm2 of standard rock, and corresponds to near horizontal muons. This is direct evidence that this flux is due to atmospheric neutrinos interacting in the rock surrounding LVD.

Studies of the knee in the electron and muon components of extensive air showers at EAS-TOP

The region of the “knee” of the cosmic ray primary spectrum (10 15 0 16 eV) is studied in the electromagnetic and muon components of Extensive Air Showers by means of the EAS-TOP array. Independent and correlated analysis of the two measurements are presented as a function of zenith angle (i.e. atmospheric depth).

The proton attenuation length and the p-air inelastic cross section at √s2 TeV from EAS-TOP

The attenuation length of proton primaries and the proton-air inelastic cross section are studied at energies E 0 ≈ 10 15 eV by using the EAS-TOP array data. Proton initiated Extensive Air Showers, in the energy range (2 ÷ 4) 10 15 eV and near maximum development, are selected from their N μ ( E μ > 1 GeV) and N e sizes. The experimental attenuation length A obs is compared with those obtained from simulations, including the full detector response. based on different interaction models (HDPM, VENUS, DPMJET, QGSJET and SIBYLL) in the frame of CORSIKA code. The proton-air inelastic cross section is also inferred by using the factor k = Λ obs sim /λ p —air sim obtained from each interaction model.

The shapes of the atmospheric Cherenkov light images from extensive air showers

Experimental results on the shapes of atmospheric Cherenkov light images from Extensive Air Showers at E0 = 1013–1014 eV are presented. The results are based on data obtained by the first element of the Cherenkov light array of the EAS-TOP experiment at Campo Imperatore (2005 m a.s.l., National Gran Sasso Laboratories, Italy). Images are obtained with pixel field of view ω = 1.5 × 10−5 sr and full field of view Ω ≈ 10−3 sr. The shapes of the images agree with the expectations and their fluctuations are large and significant. Experimental data on the dependence of the image shapes on the detection geometry are reported. The images are well reproduced by two independent side by side detectors, within measured accuracies. Such accuracies (≈ 15% for large photoelectron content) define the level at which each pixel content is related to the shower structure. The presence of events characterized by patterns with multiple structures is proved. It is shown that such features are related to physical fluctuations of the shower structure.