吉田 滋

The chemical composition of ultra-high energy cosmic rays (UHECRs; > 10(19) eV) is a key parameter to understand their nature and origin. In the Akeno Giant Air Shower Array (AGASA) experiment, we measured the muon density at 1000 m from the cores for 159 UHECR events. The data were interpreted by the recent CORSIKA simulation (version 6.203) with QGSJET01 and SIBYLL2.1 interaction models. The data is consistent with light components for the QGSJET model. No positive signature has been found for the gamma ray dominance. In the presentation at the conference we will report detailed results and comparisons with the results from other experiments.

We are constructing 18 air Fluorescence Detectors (FD) in three stations for Telescope Array (TA) experiment (each station contains 6 detectors). The telescopes measure longitudinal developments of EAS by air fluorescence lights associated with extensive air shower EAS of ultra high energy cosmic rays. From the reconstructed EAS developments the primary energy of the cosmic rays and arrival direction can be determined. To observe the primary cosmic ray with high accuracy, we have to study air fluorescence yield, atmospheric monitoring, absolute calibration and performance monitoring of FD equipment carefully. For latter ones, we calibrate the components of FD: reflectance and curvature radius of segment mirrors; the characteristics of PMT and pre-amplifier (their gain, linearity, dynamic range); uniformity of the camera surface. Also we have plans of FD performance monitoring and adjusting: absolute PMT gain, mirror reflectance, and transmittance of the front window of camera etc. In this paper these calibrations and the plans of performance monitoring for FD of TA experiment are described.

With the Akeno Giant Air Shower Array (AGASA), we published the small-scale anisotropy of cosmic rays with energies above 10(19)eV. Among them, a broad cluster - BC1 around (20(h) 50(m), 32 degrees) - has interesting characteristics: The time distribution of members of the BC1 cluster is burst-like around MJD50000, and this cluster locates near the famous supernova remnant: the Cygnus Loop. From this BC1 direction, two observations from the Tibet and Milagro experiments have been reported. In this report, we summarize the results from the whole observation time of AGASA and the Akeno 1km(2) array experiment, and the relation with the observations from the Tibet and Milagro experiments is discussed.

The IceCube Neutrino Telescope, a huge Neutrino Telescope with 1 cubic km instrumented volume, starts construction in 2004. The project status and the expected sensitivity and performance for detecting high energy cosmic neutrinos are reported. Preliminary results based on AMANDA experience without taking into account IceCube's new digital technology indicate a sensitivity to a diffuse muon neutrino energy flux of 10(-8) GeV/cm(2) sr sec. Proposed models for GRB emission should be observable in less than one year. The capability of EHE neutrino detection is also briefly mentioned.

We present the results of numerical calculations on the propagation of extremely high energy (EHE) neutrinos and charged leptons in Earth for trajectories in the whole phase space of nadir angles. Our comprehensive calculation has shown that not only the secondary produced muons but also taus survive without decaying in the energy range of 10-100 PeV with an intensity approximately three orders of magnitude lower than the neutrino flux regardless of the EHE neutrino production model. They form detectable horizontal or downgoing events in a 1 km(3) underground neutrino telescope such as the IceCube detector. The event rate and the resulting detectability of EHE signals in comparison with the atmospheric muon background are also evaluated. The 90% C.L. upper limit of EHE neutrino fluxes by a km(2) detection area would be placed at E(2)dF/dEsimilar or equal to3.7x10(-8) GeV/cm(2) sec sr for nu(mu) and 4.6x10(-8) for nu(tau) with energies of 10(9) GeV in the absence of signals with an energy loss in a detection volume of 10 PeV or greater.

This article gives a summary of the primary energy estimation by observing ultra-high energy cosmic ray induced extensive air showers (down to the EeV energies - the energy range of the Japanese AGASA experiment). (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.

The IceCube neutrino telescope, to be constructed near the Antarctic South Pole, represents the next generation of neutrino telescope. Its large 1 km(3) size will make it uniquely sensitive to the detection of neutrinos from astrophysical sources. The current design of the detector is presented. The basic performance of the detector and its ability to search for neutrinos from various astrophysical sources has been studied using detailed simulations and is discussed. (C) 2003 Published by Elsevier B.V.

We present results of a Monte Carlo study of the sensitivity of the planned IceCube detector to predicted fluxes of muon neutrinos at TeV to PeV energies. A complete simulation of the detector and data analysis is used to study the detector's capability to search for muon neutrinos from potential sources such as active galaxies and gamma-ray bursts (GRBs). We study the effective area and the angular resolution of the detector as a function of muon energy and angle of incidence. We present detailed calculations of the sensitivity of the detector to both diffuse and pointlike neutrino fluxes, including an assessment of the sensitivity to neutrinos detected in coincidence with GRB observations. After three years of data taking, lceCube will be able to detect a point-source flux of E-v(2) x dN(v)/dE(v) = 7 x 10(-9) cm(-2) s(-1) GeV at a 5sigma significance, or, in the absence of a signal, place a 90% c.l. limit at a level of E-v(2) x dN(v)/dE(v) = 2 x 10(-9) cm(-2) s(-1) GeV. A diffuse E-2 flux would be detectable at a minimum strength of E-v(2) x dN(v)/dE(v) = 10(-8) cm(-2) s(-1) sr(-1) GeV. A GRB model following the formulation of Waxman and Bahcall would result in a 5sigma effect after the observation of 200 bursts in coincidence with satellite observations of the gamma rays. (C) 2003 Elsevier B.V. All rights reserved.

Using data from more than 10 years of observations with the Akeno Giant Air Shower Array (AGASA), we published a result that the energy spectrum of ultra-high energy cosmic rays extends beyond the cutoff energy predicted by Greisen [Rhys. Rev. Lett. 16 (1966) 748] and Zatsepin and Kuzmin [Zh. Eksp. Teor. Fiz. 4 (1966) 114]. In this paper, we reevaluate the energy determination method used for AGASA events with respect to the lateral distribution of shower particles, their attenuation with zenith angle, shower front structure, delayed particles observed far from the core and other factors. The currently assigned energies of AGASA events have an accuracy of 25% in event-reconstruction resolution and 18% in systematic errors around 10(20) eV. This systematic uncertainty is independent of primary energy above 10(19) eV. Based on the energy spectrum from 10(14.5) eV to a few times 10(20) eV determined at Akeno, there are surely events above 10(20) eV and the energy spectrum extends up to a few times 10(20) eV without a GZK cutoff. (C) 2002 Published by Elsevier Science B.V.

The origin of the highest energy cosmic rays (greater than or equal to10(20) eV) is not well understood. Interesting models called "top-down" scenarios have been proposed to explain the origin. The gamma-ray flux in ultra-high-energy cosmic rays is a key parameter for giving constraints on such models. To study the properties of gamma-ray showers, we carry out simulation studies that take into account both the Landau-Pomeranchuk-Migdal effect and electromagnetic interactions in the geomagnetic field. Based on an analysis of muons in air showers observed by the Akeno Giant Air Shower Array, the upper limits on the gamma-ray flux are estimated to be 28% above 10(19) eV and 67% above 10(19.5) eV in the observed air showers at a confidence level of 95%. Above 10(20) eV, the primary composition is in agreement with an extrapolation from lower energies, and there is no indication that the observed events are mostly gamma-ray showers. These results provide observational constraints for origin models up to the highest energies.

The average extensive air shower longitudinal development profile is measured. Events between 10(17) and 10(18) eV recorded by the HiRes/MIA hybrid experiment are used for the average profile. Several functional forms are examined using this average profile. The best-fit parameters for the above functions are determined. (C) 2001 Elsevier Science B.V. All rights reserved.

We study the spectrum and average mass composition of cosmic rays with primary energies between 10(17) and 10(18) eV using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the Michigan Muon Array (MIA). Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic-ray intensity is changing from a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Fly's Eye measurements and supports the previously found steepening near 4 x 10(17) eV.

With the Akeno Giant Air Shower Array, 581 cosmic rays above 10(19) eV, 47 above 4 x 10(19) eV, and seven above 10(20) eV were observed until 1998 August. The arrival direction distribution of these extremely high energy cosmic rays has been studied. While no significant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 x 10(19) eV, there are one triplet and three doublets within a separation angle of 2.degrees 5, and the probability of observing these clusters by a chance coincidence under an isotropic distribution is smaller than 1%. The triplet is especially observed against expected 0.05 events. The cos (theta(GC)) distribution expected from the dark matter halo model fits the data as well as an isotropic distribution above 2 x 10(19) and 4 x 10(19) eV, but the fit with the dark matter halo model is poorer than the isotropic distribution above 10(19) eV. The arrival direction distribution of seven 10(20) eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of the seven are members of doublets above about 4 x 10(19) eV.

We have observed flares of TeV-gamma rays from Mrk501 in 1997 using three telescopes of the Utah Seven Telescope Array at Dugway, Utah. Determination of the energy spectrum from such Active Galactic Nuclei (AGN) is very important, because the gamma-ray spectrum is expected to steepen around 10 TeV from objects like Mrk501 by the interaction of the infrared photons. We have developed the method to estimate energies of the gamma rays by stereoscopic analysis using multiple telescopes. The differential index of the energy spectrum obtained is well expressed by -2.5 between 700 GeV and 3 TeV. This spectrum seems to become steeper above several TeV. (C) 1999 Elsevier Science B.V. All rights reserved.

Anisotropy in the arrival directions of cosmic rays with energies above 10(17) eV is studied using data from the Akeno 20 km(2) array and the Akeno Giant Air Shower Array (AGASA), using a total of about 114 000 showers observed over 11 years. In the first harmonic analysis, we have found a strong anisotropy of similar to 4% around 10(18) eV, corresponding to a chance probability of similar to 0.2% after taking the number of independent trials into account. With two-dimensional analysis in right ascension and declination, this anisotropy is interpreted as an excess of showers near the directions of the Galactic Center and the Cygnus region. (C) 1999 Elsevier Science B.V.

Extremely high energy (similar to 10(22) eV) cosmic neutrino beams initiate high energy particle cascades in the background of relic neutrinos from the big bang. We perform numerical calculations to show that such cascades could contribute more than 10% to the observed cosmic ray flux above 3 X 10(19) eV if neutrinos have similar to eV masses. The required intensity of primary neutrinos could be consistent with astrophysical models far their production if the maximum neutrino energy reaches to similar to 10(22) eV and the massive neutrino dark matter is locally clustered. Future observations of ultrahigh energy cosmic rays will lead to an indirect but practical search for neutrino dark matter.

Experimental results from Haverah Park, Yakutsk, AGASA and Fly's Eye are reviewed. All these experiments work in the energy range above 10(17) eV. The 'dip' structure around 10(18.5) eV in the energy spectrum is well established by all the experiments, though the exact position differs slightly. Fly's Eye and Yakutsk results on the chemical composition indicate that the cosmic rays are getting lighter over the energy range from 10(17) eV to 10(19) eV, but the exact fraction is hadronic interaction model dependent, as indicated by the AGASA analysis. The arrival directions of cosmic rays are largely isotropic, but interesting features may be starting to emerge. Most of the experimental results can best be explained with the scenario that an extragalactic component gradually takes over a galactic population as energy increases and cosmic rays at the highest energies are dominated by particles coming from extragalactic space. However, identification of the extragalactic sources has not yet been successful because of limited statistics and the resolution of the data.

What in the cosmos can possibly be accelerating protons to 10(20) electron volts and beyond? And how can they preserve such extreme energies while plowing through the cosmic microwave background on their way to us?

We discuss in some detail the production of extremely high energy (EHE) neutrinos with energies above 10(18) eV. The most certain process for producing such neutrinos results from photopion production by EHE cosmic rays in the cosmic background photon field. However, using assumptions for the EHE cosmic-ray source evolution that are consistent with results from the deep QSO survey in the radio and X-ray range, the resultant flux of neutrinos from this process is not strong enough for plausible detection. A measurable flux of EHE neutrinos may be present, however, if the highest energy cosmic rays that have recently been detected well beyond 10(20) eV are the result of the annihilation of topological defects which formed in the early universe. Neutrinos resulting from such decays reach energies of the grand unified theory scale, and collisions of superhigh-energy neutrinos with the cosmic background neutrinos initiate neutrino cascading, which enhances the EHE neutrino flux at Earth. We have calculated the neutrino flux including this cascading effect for either massless or massive neutrinos, and we find that these fluxes are conceivably detectable by air fluorescence detectors now in development. The neutrino-induced showers would be recognized by their starting deep in the atmosphere. We evaluate the feasibility of detecting EHE neutrinos this way using air fluorescence air shower detectors and derive the expected event rate. Other processes for producing deeply penetrating air showers constitute a negligible background.

The data acquisition system of the Akeno Giant Air Shower Array (AGASA) is described, The AGASA array covers an area of about 100 km(2) and has been operated since 1990 to study the origin of extremely high energy cosmic rays. In the early stage of our experiment, AGASA was divided into four sub-arrays called branches for topographical reasons so that air showers were observed independently at each branch. In December 1995, we have improved the data acquisition system and unified the four branches into a single detection system. By this unification, the effective detection area of the AGASA increases by about 1.7 times in the early stage.

We have used Monte Carlo simulations to investigate the capabilities of a giant air shower observatory designed to detect showers initiated by cosmic rays with energies exceeding 10(19) eV. The observatory is to consist of an array of detectors that will characterise the air shower at ground level, and optical detectors to measure the fluorescence light emitted by the shower in the atmosphere. Using these detectors together in a 'hybrid' configuration, we find that precise geometrical reconstruction of the shower axis is possible, leading to excellent resolution in energy. The technique is also shown to provide very good reconstruction below 10(19) eV, at energies where the ground array is not fully efficient.

Accumulated data of the Akeno Giant Air Shower Array (AGASA) indicate that arrival directions of a significant fraction of extremely high energy cosmic rays (EHECR) are uniformly distributed over the observable sky. However, three pairs of showers with angular separation of less than 2.5 degrees within the pair are observed among the 36 events above 40 EeV (4 x 10(19) eV), corresponding to a chance probability of 2.9% from uniform distribution. It should be noted that two pairs of them are observed to be within 2.0 degrees of the supergalactic plane.

The properties of muons (greater than or equal to 1 GeV) in giant air showers between 10(16.5) eV, and 10(19.5) eV are measured by the Akeno 1 km(2), 20 km(2) and 100 km(2) air shower arrays. The lateral distribution of muons is well fitted by the formula given by Greisen with parameters beta = 2.52+/-0.02 and log(R(0)) = (0.58+/-0.04)(sec theta-1)+(2.39+/-0.05) between 10(16.5) eV and 10(19.0) eV within 800 m of the air shower axis, where theta is the zenith angle of the shower arrival direction. The relation between the total number of muons (N-mu) and the total number of electrons (N-e) derived using these parameters is expressed on average as N-mu = (2.6+/-1.3) x 10(5.0+a)(N-e/10(7))(b), where a = (1.07+/-0.13)(sec theta - 1) and b = (0.77+/-0.02) - (0.17+/-0.02)(sec theta - 1). The lateral distribution of muons becomes steeper than expected from extrapolation of the above formula for core distances > 800 m and is expressed as rho(mu) = N mu(C'(mu)/R(0)(2))r(-0.75)(1+r)(-beta){1+(R/800 m)(3)}(-delta) (r = R/R(0), where R is a core distance). No deviation from this formula has been observed for sec theta < 2.0 up to 10(19.0) eV. The relation between the muon density (rho mu (600)) and the charged-particle density on the ground at 600 m from the core (S(600)) is studied between 10(16.5) eV and 10(19.0) eV. A systematic change in the chemical composition of cosmicrays from a predominantly heavy to a predominantly light composition above 10(17.5) eV claimed by the Fly's Eye group has not been detected beyond the present experimental uncertainties. The present experiment suggests a much smaller rate of change of composition between 10(17.5) eV and 10(18.5) eV than that from the Fly's Eye experiment Fly's Eye experiment.

We discuss the propagation of ultrahigh energy (UHE) neutrinos with energies ranging up to the grand unification (GUT) scale. In this energy range, the Universe becomes opaque for UHE neutrinos due to interactions with black-body neutrinos (nu(bb)) and the UHE neutrinos propagate cascading if the sources produce neutrinos at high redshift epochs (z >> 10). The absorption due to the energy loss and the cascade development in the black-body neutrino field may lead to a pile-up and a cut-off in the energy spectrum of UHE neutrinos emitted at an early epoch. However, these modifications will be smeared if many sources distributed universally from early epochs to the present contribute to the bulk of the UHE neutrinos. In this case, the main effect of the cosmic neutrino background on the UHE neutrino spectrum is to change the spectral slope and knowledge of the primary energy distribution of UHE neutrinos and the source evolution in the Universe is indispensable for reliable inferences of the presence of the cosmic neutrino background from observations of the UHE neutrino spectrum.

We have studied the lateral distribution of charged particles associated with giant air showers and the attenuation of the local particle density at 600 m from the core, S600, with atmospheric depth using data collected with the Akeno Giant Air Shower Array (AGASA). The lateral distribution at distances of more than 1 km from the core has been observed to be much steeper than that suggested by some of the earlier measurements. The shape of the lateral distribution function has been observed to depend on the zenith angle of showers but there is no significant evidence for the dependence on the primary energy, within the resolution of the array. The systematic errors in energy estimation due to the uncertainties in the lateral distribution and the attenuation length of S600 are smaller than statistical errors. These errors have been estimated to be approximately 5% and approximately 3% respectively for near vertical showers with sec theta = 1.1, and approximately 10% and approximately 12% respectively for showers with sec theta = 1.4.

Based on an analysis of the extensive air shower data accumulated over the last ten years at Akeno Cosmic Ray Observatory, the value of the proton-air nuclei inelastic cross section (sigma(in)p-air) has been determined assuming the validity of quasi-Feynman scaling of particle production in the fragmentation region. The energy dependence of sigma(in)p-air can be represented as 290(E/1 TeV)0.052 mb in the energy interval 10(16.2)-10(17.6) eV, where E is the incident proton energy. The total p-p cross section (sigma(tot)p-p), derived using the nuclear distribution function obtained from the shell model, increases with energy as 38.5+1.37ln2(square-root s/10 GeV) mb.

The energy spectrum of primary cosmic rays above 10(17.0) eV has been updated from data collected with two extensive air shower arrays operating at Akeno, one with area 1 km2 and the other with area 20 km2. Along with our previous results in the lower energy region, the energy spectrum has been determined over about five decades of energy from 10(14.5) eV to 10(19.8) eV. A change in the index of the power-law energy spectrum is observed around 10(17.8) eV, as well as the usual features, namely the knee around 10(15.7) eV and the ankle around 10(19.0) eV. The indices of the differential power-law energy spectrum are: (2.62 +/- 0.12) below 10(15.7) eV, (3.02 +/- 0.03) for 10(15.7) approximately 10(17.8) eV and (3.16 +/- 0.08) for 10(17.8) approximately 10(18.8) eV. There is an indication of a flattening of the spectrum above approximately 10(18.8) eV with an index of (2.8 +/- 0.3). The flux above 10(18) eV is (1.5 approximately 2.4) x 10(-12) m-2 s-1 sr-1 and is in good agreement with other experiments. The number of showers above 10(19.5) eV is seven for an exposure of 80 km2 yr sr and further investigation by a new Akeno giant air shower array (AGASA), whose operation has started, is necessary to determine a cutoff energy in the spectrum, if any.

For advancement of the studies of cosmic rays, a telescope array project is now under consideration. © 1992.

A very large surface array has been constructed recently at Akeno, 120 km west of Tokyo, to study the spectral features of the primary cosmic ray energy spectrum at the highest energies and to search for discrete sources emitting cosmic rays at energies above 10(17) eV. The new array, AGASA (Akeno Giant Air Shower Array) spread over an area of about 100 km2, consists of 111 scintillation detectors, each 2.2 m2 in area and 27 muon detectors of six different sizes. The distance between the detectors is about 1 km. The data acquisition network developed for AGASA links detectors with each other and with the four branch controllers with two optical-fiber cables for all communications. We present here the salient design features of AGASA and discuss the estimates of the accuracy in the determination of arrival direction and primary energy of showers expected to be achieved with AGASA.