The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike ${\pi }^{+}{\pi }^{-}$ atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96 mm between the target and a $2.1\mu \mathrm{m}$ thick platinum foil, in which most of them dissociated. Analyzing the observed number of atomic pairs, $n_A^L={436_{-61}^{+157}|}_{\mathrm{tot}}$, the lifetime of the $2p$ state is found to be ${\tau }_{2p}=({0.45_{-0.30}^{+1.08}|}_{\mathrm{tot}})\times 10^{-11}\mathrm{s}$, not contradicting the corresponding QED $2p$ state lifetime ${\tau }_{2p}^{\mathrm{QED}}=1.17\times 10^{-11}\mathrm{s}$. This lifetime value is three orders of magnitude larger than our previously measured value of the ${\pi }^{+}{\pi }^{-}$ atom ground state lifetime $\tau =({3.15_{-0.26}^{+0.28}|}_{\mathrm{tot}})\times 10^{-15}\mathrm{s}$. Further studies of long-lived ${\pi }^{+}{\pi }^{-}$ atoms will allow us to measure energy differences between $p$ and $s$ atomic states and so to discriminate between the isoscalar and isotensor $\pi \pi$ scattering lengths with the aim to check QCD predictions.

• Received 29 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.082003

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Published by the American Physical Society