We report on a new measurement of the cosmic ray antiproton spectrum. The
data were collected by the balloon-borne experiment CAPRICE98 which was flown
on 28-29 May 1998 from Fort Sumner, New Mexico, USA. The experiment used the
NMSU-WIZARD/CAPRICE98 balloon-borne magnet spectrometer equipped with a gas
Ring Imaging Cherenkov (RICH) detector, a time-of-flight system, a tracking
device consisting of drift chambers and a superconducting magnet and a
silicon-tungsten calorimeter. The RICH detector was the first ever flown
capable of mass-resolving charge-one particles at energies above 5 GeV.
A total of 31 antiprotons with rigidities between 4 and 50 GV at the
spectrometer were identified with small backgrounds from other particles. The
absolute antiproton energy spectrum was determined in the kinetic energy region
at the top of the atmosphere between 3.2 and 49.1 GeV. We found that the
observed antiproton spectrum and the antiproton-to-proton ratio are consistent
with a pure secondary origin. However, a primary component may not be excluded.Comment: 39 pages, 11 Postscript figures, uses AAS LATEX style; changes in
sections 3.1.1, 3.3, 3.4 and 6, Figure 8 modified, 2 figures added, typos
correcte