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

Abstract

After observing and investigating the double-exotic ${\pi }^{+}{\pi }^{-}$ atom with the ground state lifetime $\tau$ of about $3\times 10^{-15}$~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 $10^{-11}$~s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived ${\pi }^{+}{\pi }^{-}$ atoms amounts to $436\pm 61$, 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 $\pi \pi$ scattering lengths.

13 pages, 7 figures