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BESS-Polar Experiment PSB1-0046-02 1 BESS-Polar experiment T. Yoshida1, A. Yamamoto1, J. Mitchell2, K. Abe3, H. Fuke1,3, S. Haino1,3, T. Hams2, N. Ikeda4, A. Itazaki4, K. Izumi1,3, M. H. Lee5, T. Maeno4, Y. Makida1, S. Matsuda3, H. Matsumoto3, A. Moiseev2, J. Nishimura3, M. Nozaki4, H. Omiya1, J. F. Ormes2, M. Sasaki2, E. S. Seo5, Y. Shikaze4, A. Stephens2, R. Streitmatter2, J. Suzuki1, Y. Takasugi4, K. Tanaka1, K. Tanizaki4, T. Yamagami6, Y. Yamamoto3, K. Yamato4, and K. Yoshimura1 1 High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 2 Goddard Space Flight Center / NASA, Greenbelt, MD 20771, USA 3 The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 4 Kobe University, Kobe, Hyogo 657-8501, Japan 5 University of Maryland, College Park, MD 20742, USA 6 Institute of Space and Astronautical Science (ISAS), Sagamihara, Kanagawa 229-8510, Japan ABSTRACT For investigating elementary particle phenomena in the early Universe, the BESS-Polar experiment was proposed for highly sensitive studies of the low-energy antiprotons and extensive searches for antinuclei in the cosmic radiations. A new superconducting spectrometer is being developed for long-duration balloon flights. In order to extend detectable energy range of the antiprotons down to 100 MeV, materials along the trajectory of the incident particle is minimized. The spectrometer will be completed in 2003, and the first long-duration flight is planned in 2004. INTRODUCTION Observations of anomalous excess of antiproton ) flux (Bogolomov et al. 1979, Golden et al. 1979, 1 - -1 Buffington et al. 1981), compared to the predictions by V 10 e G the secondary production models, had suggested 1 - c existence of novel processes of the antiproton e s 1 production. To search for low-energy antiprotons of - r s cosmic origin, BESS (Balloon-borne Experiment with a 2 - Superconducting Spectrometer) experiment was m ( proposed in 1987 (Orito 1987). x u l Since 1993, seven successful flights were carried f 10-2 n o out at Lynn Lake, Canada. More than two thousand t o r BESS(98) antiprotons were definitely identified in a kinetic p o CAPRICE(98) t energy range between 150 MeV to 4.2 GeV by 2000 n BESS(97) A BESS(95) (Yoshimura et al. 1995, Moiseev et al. 1997, Secondary production Matsunaga et al. 1998, Orito et al. 2000, Maeno et al. &Propagation CAPRICE(94) 2001, Asaoka et al. 2002). As shown in Figure 1, a Bergstroem IMAX Mitsui BESS(93) characteristic peak of the antiproton spectrum around 10-3 2 GeV was clearly measured. The results show that the 10-1 110 cosmic antiprotons are dominantly produced by Kinetic Energy (GeV) collisions of high-energy primary cosmic-rays with Fig.1 Cosmic-ray antiproton spectra measured by BESS and other experiment. 2 interstellar matters. And propagation models in the galaxy are basically consistent. However the spectrum below 1 GeV in the solar minimum period seems to be softer than that predicted by the secondary production models. Of course we have still not had enough statistics and propagation models have large uncertainties, but we cannot rule out possible existence of some exotic processes of the antiproton production such as evaporation of primordial black holes (Turner 1982, Maki et al. 1996) or annihilation of supersymmetric dark matters (Stecker 1985, Bergström et al. 1999) in the Universe. The flux of the primary antiprotons, if they exist in the cosmic radiations, should be strongly affected by the solar modulation (Mitsui et al. 1996), because models of the primary antiproton production predicted flat spectra in low energy region. Thus it is very important to estimate the solar modulation effects on the low-energy antiprotons. Charge dependent solar modulation effects were pointed out (Bieber et al. 1999), i.e., negative charged particles might have different modulation effects from positive ones, and these effects used to be studied by the comparison between the modulation effects to protons and to electrons. The recent BESS result confirmed the charge dependence using high statistical samples of the protons and the antiprotons (Asaoka et al. 2002). Searches for cosmic antimatters have been continued to investigate matter/antimatter asymmetry in the Universe. Under several assumptions, we can place 10-1 very strong constraints on the distance to the antimatter He/He limit (95% C.L.) domains by the γ-ray observation. But direct search for antimatters in the cosmic radiation is still meaningful. Smoot et al. (1975) In the BESS flights no antihelium candidate was found -2 (Ormes et al. 1997, Saeki et al. 1998, Sasaki et al. 10 Aizu et al. (1961) Evenson (1972) 2001). Sasaki et al. (2001) placed an upper limit on a ratio of antihelium to the helium nuclei of 6.8 × 10-7 (95% C.L.) in a rigidity range between 1 and 14 GV, Evenson (1972) o i -3 which is the most stringent upper limit ever achieved t 10 a Smoot et al. (1975) (Figure 2). Since too large uncertainties exist in the r x u models of the cosmic-ray propagation between galaxies, l f we cannot rule out existence of the antimatter domains Badhwar et al. (1978) m u i Golden et al. (1997) in the Universe from this null result. But this result is l 10-4 e the most direct evidence that our galaxy and its nearby h / consist of matters. m u i Buffington et al. (1981) l Based on these results from the BESS experiments, e h i we prepare a new balloon-borne experiment to carry t -5 Ormes et al. (1997) BESS-95 n 10 out further study of the low energy antiprotons A (Yamamoto et al. 2001, Yamamoto et al. 2002a). Saeki et al. (1998) BESS-93~95 Primary purpose of this “BESS-Polar” experiment is an intensive search for antiprotons of cosmic origin, which Alcaraz et al. (1999) AMS01 -6 should be a probe of the early Universe. Since the flux 10 of the primary antiprotons will be strongly suppressed Sasaki et al. (2001) BESS-1993~2000 by the solar wind, we need to have a high statistical observation at next solar minimum period. The high precision measurement enables to search for cosmic 10-7 10-1 110102 antiprotons with very high sensitivity, and the precise Rigidity (GV) antiproton spectrum will provide a basic data to check propagation models and to study solar modulation Fig. 2. Upper limit on the ratio of antiheliums to helium processes. nuclei. BESS-POLAR SPECTROMETER As shown in Figure 3, a concept of the BESS-Polar spectrometer is basically same as the one of the current BESS spectrometer (Ajima et al. 2000, Shikaze et al. 2000, Asaoka et al. 1998). A thin superconducting solenoid provides a strong magnetic field of 0.8 Tesla, and a tracking system consisting of a jet-type drift chamber (JET) and two cell-type drift chambers (Inner DC) measures a curvature of the trajectory of the incident particle. At the top and the bottom of the spectrometer, time-of-flight (TOF) plastic scintillator paddles will be mounted to measure PSB1-0046-02 3 TOF Counters Solenoid JET chamber Inner DC Middle TOF Silica Aerogel Cherenkov 00.51m TOF Counters Fig. 3. Cross-sectional view of the BESS-Polar spectrometer. the velocity and the energy deposit of the incident particle. A silica-aerogel Čerenkov counter will also be installed as a redundant particle identifier. We have to reduce the weight of the payload to meet requirements from the long duration flights at Antarctica and also reduce material thickness in the payload to measure the antiprotons at the lowest possible energy. A pressure vessel outside of the detector is eliminated to reduce the weight and the materials, so photomultipliers and high voltage supplies should work in vacuum. The silica-aerogel Čerenkov counter will be placed beneath the magnet to reduce the materials of the upper half of the spectrometer, since the counter is utilized for the antiproton identification in a high energy region. And an additional trigger scintillation counter system (Middle-TOF) is installed inside the magnet bore to keep high trigger efficiencies for the very low energy particles. Comparisons between the BESS and the BESS-Polar spectrometer are given in Table 1. Furthermore, in order to study very low energy antiprotons by long duration flights, we have to overcome several technical challenges. To measure antiprotons down to 100 MeV, we need to minimize material thickness along the trajectory of the incident particles. We have developed an ultra-thin superconducting solenoid. It is also inevitably required to develop a new power supply system for the long duration flights. Ultra-Thin Superconducting Solenoid The key technology to realize an ultra-thin superconducting solenoid is development of the high strength superconductor (Yamamoto et al. 1999). Recently new aluminum stabilizer was developed with micro alloying of 0.5 % nickel followed by a cold-work hardening. Figure 4 shows a scanning electron microscope (SEM) image of the cross section of the aluminum stabilizer (Wada et al. 2000). In an aluminum stabilizer of the superconductor, pure aluminum domain acts as a conductor and the aluminum-nickel composite part acts as reinforcing. When the current BESS solenoid was fabricated, yield strength of the superconductor is around 100 MPa, but now, as shown Table 1. Comparison between the BESS and the BESS-Polar spectrometer BESS BESS-Polar Acceptance (m2str) 0.3 0.3 Magnetic field (T) 1.0 0.8 Superconducting coil diameter (m) 1.0 0.9 Cryogen life (days) 5.5 20 JET/IDC diameter (m) 0.83 0.76 Weight (kg) 2,400 1,500 Power source Primary batteries Solar cells Minimum thickness for trigger generation 18 4.5 (g/cm2) Detectable antiproton energy (GeV) 0.18~4.2 0.1~4.2 MDR (GV) 200 150 4 250 SC Overall 240MPa 200 BESS-Polar ) a Al-Ni P M ( 150 Astromag h t Ai-Si g n LHC/ATLAS Al+0.5%Ni e Al-Ni r t Ordinal Copper 130MPa S 100 d l e i Y SSC/SDC Al-Zn/Si 50 Pure Al 0 1975 1980 1985 1990 1995 2000 2005 Year Fig 4.
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