The Balloon-borne Experiment with Superconducting Spectrometer (BESS) launched a cosmic-ray spectrometer from Antarctica on 13 December. BESS, a collaboration between the US and Japan, has been studying cosmic rays since 1993 with balloon flights over northern Canada, but this was its first flight in Antarctica with a completely new instrument.
The 2 t detector was carried by a 1,000,000 m3 balloon from Williams Field near the US McMurdo Station. It flew to altitudes of 37-39 km for a period of 8 days and 17 hours. Flight operations were carried out by the National Scientific Balloon Facility (NSBF) as part of the United States Antarctic Program, supported by NASA and by the National Science Foundation (NSF).
With this new detector the BESS group is continuing the systematic study of low-energy antiprotons in cosmic radiation. These rare particles are a unique probe for understanding elementary particle phenomena in the early universe.
Most cosmic-ray antiprotons are produced in collisions of primary cosmic-ray nuclei with the interstellar gas. However, if an excess of low-energy antiprotons beyond those expected from standard processes is observed, measurements from BESS may provide evidence for the primary origin of some cosmic-ray antiprotons through processes such as the evaporation of primordial black holes or the decay of possible forms of dark matter.
BESS has detected more than 2000 low-energy antiprotons in eight flights from northern Canada over the past 11 years. Most of the antiprotons measured by BESS are clearly secondary products of primary cosmic rays. However, the data obtained during the last solar minimum in the sunspot cycle (which occurred in 1996) suggest a spectrum flatter than expected in the low-energy region, and hence the exciting possibility of novel origins for cosmic antiprotons.
BESS also searches for antihelium in the cosmic radiation, the detection of which would have profound significance for both cosmology and particle physics. Unlike antiprotons, antihelium has a vanishingly small probability of creation by cosmic rays. Furthermore, our current understanding is that the universe is baryon-asymmetric, with an overwhelming dominance of matter over antimatter, and that antimatter stars or galaxies do not exist. The discovery of a single antihelium event would change this view.
The analysis of the BESS data has found no evidence for antihelium while recording more than 7 million helium nuclei, establishing the most stringent upper limit to the existence of antihelium and supporting baryon asymmetry.
In 2001 the BESS group started a project to improve the statistics and to lower the energy threshold of the detector. They developed a new instrument with a much thinner superconducting solenoid magnet and detector system and without an outer pressure vessel. The cryogen lifetime of the new magnet has also been greatly improved, and at polar latitudes a solar-power system increases flight times by more than an order of magnitude compared with typical one-day flights in Canada.
During the 2004 BESS-Polar flight, the data from some 900 million cosmic rays, totalling about 2 Tb, was recorded on an array of on-board hard disks. Following the flight around Antarctica, BESS-Polar descended by parachute to a landing site on the Ross Ice Shelf approximately 900 km from its launch point. A recovery crew was flown to the area, and in a series of flights from the remote Siple Dome Camp to the landing location the data disks and the remainder of the instrument and payload were recovered successfully.
• BESS is a collaboration between KEK, the NASA Goddard Space Flight Center, the University of Tokyo, Kobe University, the Institute of Space and Astronautical Science of JAXA, and the University of Maryland.
