First observation of long-lived π + π − atoms

Abstract After observing and investigating the double-exotic (a double- exotic atomis a bound system, in which both oppositely charged components are unstable particles like μ , π , K , … ) π + π − atom with the ground state lifetime τ of about 3 × 10 − 15 s , the upgraded DIRAC experiment at the CERN PS accelerator observes for the first timelong- livedstates of the same atom with lifetimes of about 10 − 11 s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived π + π − atoms amounts to 436 ± 61 , corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between p and s atomic states and so to determine ππ scattering lengths.