Excitation and breakup of [pi]+[pi]--atoms at high energies

Abstract

The DIRAC collaboration at CERN currently aims at measuring the lifetime of pionium (π + π − -atoms) with an accuracy of 10%. This is a crucial test of Chiral Perturbation Theory which predicts the lifetime to be (2.9 ± 0.1) fs. To be able to determine if the theory is correct is very important for our understanding of the low energy limit of the Standard Model of particle physics. To be able to extract the lifetime from the measurement of the breakup probability, theoretical input is needed to describe the Coulomb interaction with the target atoms. The first order Born approximation of pionium–target atom interaction has, by now, been calculated completely. The main task of this work is to evaluate higher order corrections to the excitation cross-sections in Glauber theory. It has been found that they are non-negligible for heavy target materials (up to 15% difference compared to the Born approximation). Due to the apparent importance of higher order terms, a coupled channel calculation was performed to investigate the accuracy of the Glauber approximation. Finally, a Monte Carlo simulation of pionium moving through the target was done to investigate the influence of the different cross-sections of pionium–target interaction. It was found that for small and medium Z, the lifetime of pionium depends very little on the choice of the cross section, but for large Z targets, the differences become quite important.