K. Kasahara

THE ENERGY SPECTRUM OF COSMIC-RAY ELECTRONS FROM 10 TO 100 GeV OBSERVED WITH A HIGHLY GRANULATED IMAGING CALORIMETER

Cosmic-ray electrons1 have been observed in the energy range from 12 to D100 GeV with a new balloon-borne payload, the Balloon-borne Electron Telescope with Scintillating Fibers (BETS). This is the —rst publication of the absolute energy spectrum of electrons measured with a highly granulated —ber calorimeter. The calorimeter makes it possible to select electrons against the background protons by detailed observation of both the longitudinal and the lateral shower development. The performance of the detector was calibrated by the CERN-SPS accelerator beams: electrons from 5 to 100 GeV, protons from 60 to 250 GeV. The balloon observations were carried out twice, in 1997 and 1998, at the Sanriku Balloon Center (Institute of Space and Astronautical Science) in Japan. The observation time was D13 hr in all at an altitude above 34 km. A total of 1349 electron candidates were collected, and the 628 events with energies above 12.5 GeV, well above the geomagnetic rigidity cutoU of D10 GV, have been used to compose a diUerential absolute energy spectrum at the top of the atmosphere. The energy spectrum is described by a power-law index of 3.00 ^ 0.09, and the absolute diUerential intensity at 10 GeV is 0.199 ^ 0.015 m~2 s~1 sr~1 GeV~1. The overall shape of the energy spectrum in 10 D 100 GeV can be explained by a diUusion model, in which we assume an energy-dependent diUusion coefficient (PE0.3) for an injection spectrum, E~2.4.

Large-scale sidereal anisotropy of Galactic cosmic-ray intensity observed by the Tibet air shower array

We present the large-scale sidereal anisotropy of Galactic cosmic-ray intensity in the multi-TeV region observed with the Tibet-III air shower array during the period from 1999 through 2003. The sidereal daily variation of cosmic rays observed in this experiment shows an excess of relative intensity around 4-7 hr local sidereal time as well as a deficit around 12 hr local sidereal time. While the amplitude of the excess is not significant when averaged over all declinations, the excess in individual declination bands becomes larger and clearer as the viewing direction moves toward the south. The maximum phase of the excess intensity changes from ~7 hr at the Northern Hemisphere to ~4 hr at the equatorial region. We also show that both the amplitude and the phase of the first harmonic vector of the daily variation are remarkably independent of primary energy in the multi-TeV region. This is the first result determining the energy and declination dependences of the full 24 hr profiles of the sidereal daily variation in the multi-TeV region with a single air shower experiment.

Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array

Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606

A Northern Sky Survey for Steady Tera-Electron Volt Gamma-Ray Point Sources Using the Tibet Air Shower Array

\mathrm{g}/{\mathrm{cm}}^{2})

Comparison of three-dimensional and one-dimensional schemes in the calculation of atmospheric neutrinos

in Tibet. This experiment can detect air-shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons in the energy range from 200 to 1000 TeV. The differential energy spectrum obtained in this energy region can be fitted by a power law with the index of

Observation of multi-TeV gamma rays from the Crab Nebula using the Tibet air shower array

\ensuremath{-}2.97\ifmmode\pm\else\textpm\fi{}0.06,

Measurement of air shower cores to study the cosmic ray composition in the knee energy region

which is steeper than that obtained by direct measurements at lower energies. We also obtained the energy spectrum of helium nuclei at particle energies around 1000 TeV.

Implication of the sidereal anisotropy of ∼5 TeV cosmic ray intensity observed with the Tibet III air shower array

Results of a steady TeV γ-ray point-source search using data taken from the Tibet HD (1997 February-1999 September) and Tibet III (1999 November-2001 October) arrays are presented. From 0° to 60° in declination, significant excesses from the well-known steady source Crab Nebula and the high state of the flare-type source Markarian 421 are observed. Because the levels of significance from other positions are not sufficiently high, 90% confidence level upper limits on the flux are set assuming different power-law spectra. To allow cross-checking, two independently developed analyses are used in this work.

Moon shadow by cosmic rays under the influence of geomagnetic field and search for antiprotons at multi-TeV energies

A 3-dimensional calculation of atmospheric neutrino flux is presented, and the results are compared with those of a 1-dimensional calculation. In this study, the interaction and propagation of particles is treated in a 3-dimensional way including the curvature of charged particles due to the geomagnetic field, which is assumed to be a dipole field. The purpose of this paper is limited to the comparison of calculation schemes. The updated flux value with a new interaction model and primary flux model will be reported in a separate paper. Except for nearly horizontal directions, the flux is very similar to the result of 1-dimensional calculations. However, for near-horizontal directions an enhancement of the neutrino flux is seen even at energies as high as 1 GeV. The production height of neutrinos is lower than the prediction of the 1-dimensional calculation for near-horizontal directions, and is a little higher for near-vertical directions. However, the difference is not evident except for near-horizontal directions.

Atmospheric gamma-ray observation with the BETS detector for calibrating atmospheric neutrino flux calculations

The Tibet experiment, operating at Yangbajing (4300 m above sea level), is the lowest energy air shower array, and the new high-density array constructed in 1996 is sensitive to gamma-ray air showers at energies as low as 3 TeV. With this new array, the Crab Nebula was observed in multi-TeV gamma-rays and a signal was detected at the 5.5 sigma level. We also obtained the energy spectrum of gamma-rays in the energy region above 3 TeV which partially overlaps those observed with imaging atmospheric Cerenkov telescopes. The Crab spectrum observed in this energy region can be represented by the power-law fit dJ&parl0;E&parr0;&solm0;dE=&parl0;4.61+/-0.90&parr0;x10-12&parl0;E&solm0;3 TeV&parr0;-2.62+/-0.17 cm-2 s-1 TeV-1. This is the first observation of gamma-ray signals from point sources with a conventional air shower array using scintillation detectors.