Measurement of the breakup probability of pi+ pi- atoms in a nickel target with the DIRAC spectrometer

The DIRAC Collaboration aims to determine the lifetime of the pionium atom, a pi+pi- bound state, by measuring the breakup probability of such a bound state. This breakup probability measurement with the DIRAC spectrometer at CERN searches for an excess of pi+pi- pairs from breakup at very low relative momenta (Q < 4 MeV/c) on top of a dominant background coming from correlated and uncorrelated pion pairs from the proton beam - target interaction. The number of found pi+pi- pairs from breakup can be put in relation to the number of produced atoms to yield the breakup probability. The number of produced atoms is related to the number of produced pi+pi- pairs with Coulomb final state interaction through the production process. The Nickel target data amounts to the biggest measured homogeneous sample recorded by the DIRAC collaboration, which translates into a small statistical error. Systematical influences are studied and shown to be not dominant. The measurement of the breakup probability and hence the determination of the life time of pionium allows to calculate the difference between the S-wave isoscalar and isotensor pi pi scattering lengths using only quantum mechanics. This calculation can be used to cross check the prediction coming from Chiral Perturbation Theory.