First observation of long-lived π+π− atoms

After observing and investigating the double-exotic (a double-exotic atom is a bound system, in which both oppositely charged components are unstable particles like μ,π,K,…μ,π,K,…) π+π−π+π− atom with the ground state lifetime τ   of about 3×10−15 s3×10−15 s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about 10−11 s10−11 s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived π+π−π+π− atoms amounts to 436±61436±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.