Dynamics of the Pionium with the Density Matrix Formalism
  The evolution of pionium, the $\pi^+ \pi^-$ hydrogen-like atom, while passing through matter is solved within the density matrix formalism in the first Born approximation. We compare the influence on the pionium break-up probability between the standard probabilistic calculations and the more precise picture of the density matrix formalism accounting for interference effects. We focus our general result in the particular conditions of the DIRAC experiment at CERN. 
L Afanasyev (1), C Santamarina (2), A Tarasov (1) and O Voskresenskaya   (1) ((1) Joint Institute for Nuclear Research, (2) ...