A New Measurement of Low Energy Antiprotons In the Cosmic Radiation

New measurements of the antiproton flux and the antiproton-to-proton flux ratio at the top of the atmosphere between 80 MeV and 2.0 GeV are presented. The measurement was conducted from July 2006 to March 2008 with the PAMELA satellite experiment. This is a period of minimum solar activity and negative solar polarity and the PAMELA measurement is the first observation of antiprotons during this particular solar state. The PAMELA instrument comprises a permanent magnet spectrometer, a scintillator based time-of-flight system, an electromagnetic calorimeter and an anticoincidence shield. These detectors can identify antiprotons from a background of cosmic-ray electrons and locally produced pions. The PAMELA instrument is mounted on the Resurs DK1 satellite that was launched from the Baikonur Cosmodrome on June the 15th into a semi-polar orbit with an inclination of 70o. During approximately 500 days of data collection 170 antiprotons were identified. The derived antiproton spectrum shows a steep increase up to 2 GeV as expected for pure secondary production of galactic antiprotons. The antiproton flux is over-estimated by most current models of secondary production compared to PAMELA results. There are no indications of the excess of antiprotons at low energy predicted by theories of primordial black hole evaporation. The antiproton-to-proton flux ratio is in agreement with drift models of solar modulation, which are also favoured by recent PAMELA measurements of the positron fraction.