N. Thakur

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.

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.

The BESS-Polar Ultra-Thin Superconducting Solenoid Magnet and Its Operational Characteristics During Long-Duration Scientific Ballooning Over Antarctica

An ultra-thin superconducting solenoid has been developed for a cosmic-ray spectrometer ballooning over Antarctica, which is named BESS-Polar II. The coil with a diameter of 0.9 m, a length of 1.4 m and a thickness of 3.5 mm, is wound with high-strength aluminum stabilized superconductor and provides 0.8 T in the spectrometer. Based on the experience at the BESS-Polar-I solenoid flight for nine days in 2004, the BESS-Polar-II solenoid, which was cryogenically improved, realized a persistent current mode operation for 25 days in the second flight campaign in December 2007 though January 2008. It has contributed to accumulate the cosmic-ray observation data with 4700 million events and 16 terabyte in a hard disk unit. This report will describe the second solenoid performance during the flight.

Measurements of galactic cosmic-ray hydrogen and helium isotopes with the BESS-polar II instrument

Nicolas Picot-Clémente∗†, K. Abe2, H. Fuke3, S. Haino4, T. Hams5, M. Hasegawa4, A. Horikoshi4, A. Itazaki2, K.C. Kim1, T. Kumazawa4, A. Kusumoto2, M.H. Lee1, Y. Makida4, S. Matsuda4, Y. Matsukawa2, K. Matsumoto4, J.W. Mitchell5, A.A. Moiseev5, J. Nishimura1, M. Nozaki4, R. Orito2, J.F. Ormes6, K. Sakai5, M. Sasaki5, E.S. Seo1, Y. Shikaze2, R. Shinoda7, R.E. Streitmatter5, J. Suzuki4, Y. Takasugi2, K. Takeuchi2, K. Tanaka4, N. Thakur6, T. Yamagami4, A. Yamamoto4, T. Yoshida3, K. Yoshimura4 1 Institute for Physical Science and Technology, University of Maryland, College Park MD 20742, USA 2 Kobe University, Kobe, Hyogo 657-8501, Japan 3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA), Sagamihara, Kanagawa 252-5210, Japan 4 High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 5 National Aeronautics and Space Administration, Goddard Space Flight Center (NASA/GFSC), Greenbelt, MD 20771, USA 6 University of Denver, Denver, CO 80208, USA 7 The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan

Search for cosmic-ray antideuterons with BESS-Polar

Although cosmic-ray antideuterons can be produced in primary cosmic-ray interactions with the interstellar medium in the same way as cosmic-ray antiprotons, the probability is much smaller, especially at low energies, because of the very low production cross-section and strict kinematic requirement compared to secondary antiproton production. The lack of significant astrophysical background indicates that a search for low-energy antideuterons could be a good probe for a novel production mechanisms such as pair-annihilation of neutralino dark matter or evaporation of primordial black holes. The BESS-Polar program has accumulated cosmic-ray data in near solar minimum conditions with more than ten times the statistics of those obtained by BESS flights during the previous solar minimum period. Based on these data, we perform a new antideuteron search with unprecedented sensitivity.

Precise Measurements of Hydrogen and Helium Isotopes with BESS-Polar II

A precise knowledge of cosmic-ray hydrogen and helium isotopes provides important information to better understand Galactic cosmic-ray propagation. Deuteron and helium 3 species are mainly secondary particles created by the spallation of primary proton and helium 4 particles during their propagation in the Galaxy. Secondary-to-primary ratios thus bring direct information on the average amount of material traversed by cosmic rays in the interstellar medium. The Balloon-borne Experiment with Superconducting Spectrometer BESS-Polar II flew over Antarctica for 24.5 days from December 2007 through January 2008, during the 23rd solar cycle minimum. The instrument is made of complementary particle detectors which allow to precisely measure the charge, velocity and rigidity of incident cosmic rays. It can accurately separate and precisely measure cosmic-ray hydrogen and helium isotopes between 0.2 and 1.5 GeV/nucleon. These data, which are the most precise to date, will be reported and their implications will be discussed.

BESS-Polar: Search for Antihelium

We have searched for antihelium in cosmic-rays since 1993 using a series of nine conventional BESS northern latitude balloon flights and two long-duration BESS-Polar Antarctic balloon flights. The BESS-Polar spectrometer is an evolutionary development of the previous BESS instruments, adapted to long duration flight. No antihelium candidate was found in the rigidity ranges of 0.6-20 GV among 8 × 10 6 helium nuclei events for BESS-Polar I and in the rigidity range of 0.6-14 GV among 4 × 10 7 events for BESS-Polar II, respectively. A resultant upper limit of 6.9 × 10 −8 for the abundance ratio of antihelium/helium at the top of the atmosphere in the rigidity range of 1-14 GV was set by combining all the BESS and BESS-Polar flight data. This is the most stringent limit obtained to date.

Transient variations in cosmic ray proton fluxes from BESS-Polar I

BESS (Balloon-borne Experiment with a Superconducting Spectrometer ) had its first circumpolar flight from Williams Field near McMurdo Station, Antarctica from Dec. 13 to 21, 2004. Ou r sub-1% precision reveals BESS-Polar I proton fluxes exhibit transient variations at the few1% level. The time prog ression of proton flux has three main features; a rising flux at the beginning of the flight, a transition region around Dec. 17, followed by quasi-periodic variation. Neutron monitor data show that the BESS-Polar I flight occurred during the recov ery phase of a small Forbush decrease. The solar wind plasma and particle data show that this flight took place during the tail end of a high-energy, multiple-eruption solar energetic particle (SEP) event. A high speed solar wind stream arrived n ear the Earth around Dec. 17, 2004. We present the flux progression as a function of energy between 0.1 100.0 GeV a nd suggest possible physical interpretations.

BESS-Polar Experiment –Progress and Future Prospect–

The aims of the BESS-Polar experiment are precise measurements of the low-energy antiproton spectrum and search for cosmologically significant antimatter. After its first flight (BESS-Polar I), we had developed a new spectrometer based on the feedback from the results. Most of the detector components had been redesigned and upgraded to improve their performance and to increase the data taking period and capacity. The second flight (BESS-Polar II) was successfully carried out in December 2007 – January 2008. We performed 24.5 days scientific observation just at the solar minimum. In this paper, BESS-Polar II instrument and flight summary will be presented.