First measurement of a long-lived ${\pi }^{+}{\pi }^{-}$ atom lifetime

Abstract

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

arXiv comment7 pages, 8 figures

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• Possible manifestation of the 2p pionium in particle physics processes
• Pionium as a source of false events in the $K\to \pi \nu \bar{\nu }$ decays