Orito Group

BESS cosmic ray detector being tuned-up prior to the first balloon launching at Lynn Lake, Canada. The detector functioned perfectly during 17-hours' flight at the altitude of 37 km, and was safely recovered with 4 million cosmic ray events recorded during the flight. With the detector, we expect to perform a variety of astroparticle physics: 1) Very sensitive search for anti-helium and low-energy spectra of e⁺,e^-, p, He, and various light isotopes; 3) Detection of GeV gamma-rays from quasers and other sources; and 4) Precision measurements of cosmic mu^+- flux and the determination of atomospheric neutrino spectra. The state-of-the-art techniques, developed for particle physics experiments at colliders, were applied to this detector such as thin superconducting solenoidal magnet, precision drift chambers and high-speed data aquisition system.

Subjects:

(1)OPAL experiment at LEP e⁺e^- collider and preparation for Japan Linear e⁺e^- Collider; (2)research on astroparticle physics with balloon-borne high resolution spectrometer(BESS experiment); (3) Studies on possible future particle physics experiments.

Member: Shuji Orito, Koji Yoshimura

1)OPAL experiment:Precision tests of the Standard Model have been further performed based on the total 5 millions Z⁰ events collected since 1989. In '97, data were taken at a new energy of 183 GeV, resulting to a new lower limit of 84.2GeV for the mass of Higgs particle. The super-symmetric particles have been searched for, providing various constraints on their masses and couplings. W mass is determined to be of 80.35+-0.12+-0.07 GeV, using 873 W⁺W^- events detected at 183 GeV.

(2) Energy spectrum of cosmic antiproton was obtained[1] in the range 0.2 to 1.4GeV based on 43 events unambiguously detected in BESS'95 data. A new limit on cosmic antihelium, a factor 30 improvement over previous experiments, was published[2]. The BESS detector had its fourth successful flight at Canada in July '97, and was safely recovered with recorded 16 millions cosmic-ray events at the solar minimum. A background-free and positive(mass-identified) detection of 400 antiprotons has been achieved in the energy range 0.2 to 3 GeV by utilizing the newly installed aerogel Cherenkov counter and the scintillation hodoscopes with 50ps timing resolution. Analysis is underway on '97 data to obtain the absolute flux and the energy spectrum of cosmic ray antiprotons, by which one can conduct a sensitive search for primary antiprotons from novel sources such as the evaporating primordial black holes or the annihilation of neutralino dark matter. Analysis is being completed to determine the absolute fluxes and the energy spectrum of cosmic ray protons and heliums, as well as various light isotopes. Vertical flux of ground muon was measured at Lynn Lake Canada (rigidity cut-off 0.3GV/c).

(3) Possible experimental set-ups are examined for the search of mu⁺ --> e⁺ gamma decay at levels down to 10^-14. R/D works started on the liquid Xe scintillation detector for the gamma-rays. Detailed simulations were performed to design the e⁺ spectrometer.

(4)Studies and proposal[3] on the search for neutralino decay at hadron colliders such as LHC have bean performed.

References:

[1] H.Matsunaga et.al: Measurement of Low-Energy Cosmic-Ray Antiprotons at Solar Minimum, UT-ICEPP 98-02 (May,1998)
[2] T.Saeki et.al: A new limit on the flux of cosmic antihelium,Phys.lett. B 422 (1998)
[3] K.Maki, and S.Orito: Hadron colliders as the "neutralino factory": Search for a slow decay of the ligh test neutralino at the CERN LHC, Phys. Rev. D 57 (1997) 554