Licentiate Thesis - Studies of positron identification with the PAMELA calorimeter

by Ms Laura Rossetto (KTH)

Wednesday 10 Nov 2010, 13:00 → 15:00 Europe/Stockholm

FB42

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 approximately four 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. Simulations of electromagnetic and hadronic interactions in the calorimeter were performed with the purpose of studying π^0 production in hadronic cascades within the context of positron identification. The number of π^0s produced in hadronic showers was artificially boosted in order to study the consequences for positron identification. The positron selection criteria used to produce the widely discussed positron fraction published by the PAMELA Collaboration in 2009 were applied to simulations in the energy range 20 − 100 GeV. Results of this analysis show that the reported rise in the positron fraction cannot be accounted for by π^0 contamination. A new approach for positron identification based on shower profile variables in the calorimeter has been studied with the aim of extending positron identification beyond 100 GeV. The proton-to-positron flux ratio is ~10^4 at 100 GV. In the energy range 20 − 100 GeV a positron selection efficiency of ~0.50 was found together with a proton contamination of order of 10^−5.