Detection of pi+pi- atoms with the DIRAC spectrometer at CERN

Abstract

The goal of the DIRAC experiment at CERN is to measure with high precision the lifetime of the π+π− atom (A2π), which is of order 3 × 10−15 s, and thus to determine the s-wave ππ-scattering lengths difference |a0 − a2|. A2π atoms are detected through the characteristic features of π+π− pairs from the atom break-up (ionization) in the target. We report on a first high statistics atomic data sample obtained from p–Ni interactions at 24 GeV/c proton momentum and present the methods to separate the signal from the background.