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After observing and investigating the double-exotic $\pi^+\pi^-$ atom withthe ground state lifetime $\tau$ of about $3 \times 10^{-15}$~s, the upgradedDIRAC experiment at the CERN PS accelerator observes for the first timelong-lived states of the same atom with lifetimes of about $10^{-11}$~s andmore. The number of characteristic pion pairs resulting from the breakup(ionisation) of long-lived $\pi^+\pi^-$ atoms amounts to $436\pm61$,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 $\pi \pi$ scattering lengths.

First observation of long-lived $\pi^+ \pi^-$ atoms