T. Maeno

Precise Measurement of Cosmic-Ray Proton and Helium Spectra with the BESS Spectrometer

We report cosmic-ray proton and helium spectra in energy ranges of 1-120 GeV nucleon-1 and 1-54 GeV nucleon-1, respectively, measured by a flight of the Balloon-borne Experiment with Superconducting Spectrometer (BESS) in 1998. The magnetic rigidity of the cosmic ray was reliably determined by highly precise measurement of the circular track in a uniform solenoidal magnetic field of 1 T. Those spectra were determined within overall uncertainties of ±5% for protons and ±10% for helium nuclei including statistical and systematic errors.

Successive measurements of cosmic-ray antiproton spectrum in a positive phase of the solar cycle

Abstract The energy spectrum of cosmic-ray antiprotons ( p s) has been measured by BESS successively in 1993, 1995, 1997 and 1998. In total, 848 p s were clearly identified in energy range 0.18–4.20 GeV. From these successive measurements of the p spectrum at various solar activity, we discuss about the effect of the solar modulation and the origin of cosmic-ray p s. The p / p ratios showed no distinctive year-to-year variation during the positive Suns polarity phase.

Search for Cosmic-Ray Antideuterons

We performed a search for cosmic-ray antideuterons using data collected during four BESS balloon flights from 1997 to 2000. No candidate was found. We derived, for the first time, an upper limit of 1.9 x 10(-4) (m2s sr GeV/nucleon)(-1) for the differential flux of cosmic-ray antideuterons, at the 95% confidence level, between 0.17 and 1.15 GeV/nucleon at the top of the atmosphere.

Precise measurements of atmospheric muon fluxes with the BESS spectrometer

The vertical absolute fluxes of atmospheric muons and muon charge ratio have been measured precisely at different geomagnetic locations by using the BESS spectrometer. The observations had been performed at sea level (30 m above sea level) in Tsukuba, Japan, and at 360 m above sea level in Lynn Lake, Canada. The vertical cutoff rigidities in Tsukuba (36.2°N, 140.1°E) and in Lynn Lake (56.5°N, 101.0°W) are 11.4 and 0.4 GV, respectively. We have obtained vertical fluxes of positive and negative muons in a momentum range from 0.6 to 20 GeV/c with systematic errors <3% in both measurements. By comparing the data collected at two different geomagnetic latitudes, we have seen an effect of cutoff rigidity. The dependence on the atmospheric pressure and temperature, and the solar modulation effect have been also clearly observed. We also clearly observed the decrease of charge ratio of muons at low momentum side with at higher cutoff rigidity region.

Measurements of atmospheric muon spectra at mountain altitude

We report new measurements of absolute fluxes of atmospheric muons at mountain altitude. The measurements were carried out with the BESS detector at the top of Mt. Norikura, 2 770 m above sea level, in Japan. The overall errors were less than 10%. The measured results are discussed in comparison with theoretical calculations.

Bess and its future prospect for polar long duration flights

Abstract The Balloon-borne Experiment with a Superconducting Spectrometer, BESS, aims to study elementary particle/antiparticle phenomena in the early history of the Universe. The instrument has a unique feature of a thin superconducting solenoid magnet enabling a large geometrical acceptance with a horizontally cylindrical configuration. Seven balloon flights have been successfully carried out since 1993. More than 10 3 comic-ray antiproton have been unambiguously detected, and the energy spectrum has been measured with the characteristic peak at 2 GeV. The search for cosmic-ray antihelium brought the upper-limit of the antihelium/helium ratio down to −6 . To extend the highly sensitive measurements, we are planning polar long duration flights in Antarctica focusing on the very low energy antiproton spectrum towards the solar-minimum in the next decade.

Measurement of cosmic-ray proton and antiproton spectra at mountain altitude

Abstract Energy spectra of atmospheric secondary cosmic-ray protons and antiprotons were measured at mountain altitude, 2770xa0m above sea level. We observed more than 10 5 protons and 10 2 antiprotons. Our proton spectrum is generally consistent with other previous measurements. The observed antiproton spectrum suggests that the energy loss due to non-annihilation processes is less significant than that assumed in previous model calculations.

Progress in Search for Antihelium with BESS

We have searched for antihelium nuclei in cosmic rays using the data obtained from balloon flights of the BESS magnetic spectrometer. The search was mainly based on track-quality selection, followed by rigidity analysis, and on the time-of-flight and dE/dx measurements by the scintillation counter hodoscope. We analysed all the data collected during 1993–2000 with a common analysis procedure. No antihelium nuclei events were found in the energy range from 1 to 14 GV. In order to determine a new upper limit, we have simulated the loss in the air and in the instrument of He (He) using the GEANT/GHEISHA code. Combined with the data collected in 1993 through 2000, a new 95 % confidence upper limit for the ratio of He/He at the top of the atmosphere of 6.8 × 10−7 has been obtained to be after correcting for the interactions in the air and in the instruments.

Measurement of low-energy antiproton detection efficiency in BESS below 1 GeV

Abstract An accelerator experiment was performed using a low-energy antiproton beam to measure antiproton detection efficiency of BESS, a balloon-borne spectrometer with a superconducting solenoid. Measured efficiencies showed good agreement with calculated ones derived from the BESS Monte Carlo simulation based on GEANT/GHEISHA . With detailed verification of the BESS simulation, the relative systematic error of detection efficiency derived from the BESS simulation has been determined to be ±5%, compared with the previous estimation of ±15% which was the dominant uncertainty for measurements of cosmic-ray antiproton flux.

Precise measurements of cosmic-ray hydrogen and helium spectra with BESS

Abstract A series of Balloon-borne Experiments with a Superconducting Spectrometer, BESS, has been made since 1993. We report here the absolute cosmic-ray proton and helium spectra in energy ranges of 1 to 120 GeV and 1 to 54 GeV / nucleon, respectively, as measured by the 1998 balloon flight of the BESS spectrometer. Those spectra were determined within overall uncertainties of ±5 % for protons and ±10 % for helium nuclei including statistical and systematic errors. The covered energy range is relevant to the atmospheric neutrinos observed as “fully contained events” in Super-Kamiokande. In order to extend the energy range up to around 1 TeV, we have started to design and develop new tracking detectors. Those energy spectra in a higher energy region will help to estimate the interstellar spectra of primary cosmic rays and the absolute fluxes of “up-going muons.”