Ph.D. Thesis: PAMELA measurements of high energy cosmic ray positrons
Monday 11 June 2012
to 16:15 at
Laura Rossetto (KTH, Dept. of Physics)
PAMELA is a satellite-borne experiment mounted on board of the Russian Resurs DK1 satellite which was launched from the Baikonur cosmodrome in Kazakhstan on June 15th 2006. The satellite orbits around the Earth on a semi-polar and elliptical trajectory and PAMELA has been acquiring data for almost six years. The detector was designed and optimised for the study of the antimatter component in the cosmic radiation. The PAMELA apparatus consists of a time-of-flight system, a permanent magnetic spectrometer, an electromagnetic calorimeter, a neutron detector and an anticoincidence system. Combining information from different detectors and in particular from the calorimeter, positrons can be identified from the significant background due to cosmic ray protons.
The interest in cosmic ray positron measurements considerably increased in the last years because of new experimental results. The positron fraction measured by the PAMELA detector clearly increases with energy above 10 GeV. This is not in agreement with a pure secondary positron production, thus indicating a probable primary origin of positrons. In this context, a measurement of the positron fraction and of the positron flux up to the maximum energy permitted by the PAMELA design becomes extremely important.
In view of extending positron measurements, a method for positron identifica- tion has been studied. The method uses longitudinal and transverse shower profile variables in the calorimeter for separating electromagnetic and hadronic showers. This method has been first tested on simulated positron and background proton events produced in two different energy ranges (20 − 100 GeV and 100 − 300 GeV). Proton contamination can arise in identified positron events due to the production of neutral pions which decay electromagnetically. Positron and electron events have been identified from positively and negatively charged particles in flight data, al- lowing the positron fraction and the positron flux to be reconstructed up to an energy of ∼ 300 GeV. As a cross-check, a multivariate approach has also been applied to flight data in order to estimate the number of positron and electron events at energies greater than 100 GeV. The positron fraction obtained with these two different methods are in good agreement within statistical uncertainties. The resulting positron flux measurement shows a rise at energies greater than 100 GeV.