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تسجيل مستخدم جديدFirst measurement of a long-lived $pi^+ pi^-$ atom lifetime
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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, $n_A^L= left.436^{+157}_{-61}right|_mathrm{tot}$, the lifetime of the 2$p$ state is found to be ${tau_{2p}=(left.0.45^{+1.08}_{-0.30}right|_mathrm{tot}) cdot10^{-11}}$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 $tau=(left.3.15^{+0.28}_{-0.26}right|_mathrm{tot})cdot 10^{-15}$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 $pipi$ scattering lengths with the aim to check QCD predictions.
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