J. Nishimura

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.

Precision Measurement of Cosmic-Ray Antiproton Spectrum

The energy spectrum of cosmic-ray antiprotons ( &pmacr;s) has been measured in the range 0.18-3.56 GeV, based on 458 &pmacr;s collected by BESS in a recent solar-minimum period. We have detected for the first time a characteristic peak at 2 GeV of &pmacr;s originating from cosmic-ray interactions with the interstellar gas. The peak spectrum is reproduced by theoretical calculations, implying that the propagation models are basically correct and that different cosmic-ray species undergo a universal propagation. Future BESS data with still higher statistics will allow us to study the solar modulation and the propagation in detail and to search for primary &pmacr; components.

The Lateral and the Angular Structure Functions of Electron Showers

Lateral and angular distribution functions of electron showers are derived analytically with and without the Landau approximation including ionization loss. Tables and the numerical results of these functions are presented, and their results are applied to the analysis of high energy cosmic ray phenomena. Relations between the present theories and others are discussed critically, and it is shown that other theories can be regarded as special cases of the theory presented. (auth)

Measurements of cosmic-ray low-energy antiproton and proton spectra in a transient period of solar field reversal.

The energy spectra of cosmic-ray low-energy antiprotons ( *ps) and protons ( ps) have been measured by BESS in 1999 and 2000, during a period covering reversal at the solar magnetic field. Based on these measurements, a sudden increase of the *p/p flux ratio following the solar magnetic field reversal was observed, and it generally agrees with a drift model of the solar modulation.

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.

Measurement of the cosmic-ray low-energy antiproton spectrum with the first BESS-Polar Antarctic flight

Abstract The BESS-Polar spectrometer had its first successful balloon flight over Antarctica in December 2004. During the 8.5-day long-duration flight, almost 0.9 billion events were recorded and 1,520 antiprotons were detected in the energy range 0.1–4.2 GeV. In this Letter, we report the antiproton spectrum obtained, discuss the origin of cosmic-ray antiprotons, and use antiproton data to probe the effect of charge-sign-dependent drift in the solar modulation.

Measurement of the cosmic-ray antiproton spectrum at solar minimum with a long-duration balloon flight over Antarctica

The energy spectrum of cosmic-ray antiprotons (ps) from 0.17 to 3.5 GeV has been measured using 7886 ps detected by BESS-Polar II during a long-duration flight over Antarctica near solar minimum in December 2007 and January 2008. This shows good consistency with secondary p calculations. Cosmologically primary ps have been investigated by comparing measured and calculated p spectra. BESS-Polar II data show no evidence of primary ps from the evaporation of primordial black holes.

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.

Search for Antihelium with the BESS-Polar Spectrometer

In two long-duration balloon flights over Antarctica, the Balloon-borne Experiment with a Superconducting Spectrometer (BESS) collaboration has searched for antihelium in the cosmic radiation with the highest sensitivity reported. BESS-Polar I flew in 2004, observing for 8.5 days. BESS-Polar II flew in 2007-2008, observing for 24.5 days. No antihelium candidate was found in BESS-Polar I data among 8.4×10(6) |Z|=2 nuclei from 1.0 to 20 GV or in BESS-Polar II data among 4.0×10(7) |Z|=2 nuclei from 1.0 to 14 GV. Assuming antihelium to have the same spectral shape as helium, a 95% confidence upper limit to the possible abundance of antihelium relative to helium of 6.9×10(-8)} was determined combining all BESS data, including the two BESS-Polar flights. With no assumed antihelium spectrum and a weighted average of the lowest antihelium efficiencies for each flight, an upper limit of 1.0×10(-7) from 1.6 to 14 GV was determined for the combined BESS-Polar data. Under both antihelium spectral assumptions, these are the lowest limits obtained to date.