Experimental Investigation of $\pi^+\pi^-$ and $K^+K^-$ atoms

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 $\mu$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 $\pi\pi$ scattering lengths with the aim to check QCD predictions. At the same setup, there were identified more than 7000 $K^+K^-$ pairs with effective mass less than $2M_K + 5$ MeV. In the distributions of $K^+K^-$ pairs there is a strong signature of the Coulomb enhancement: the number of pairs increases with decreasing of the relative momentum in the pair c.m.s. The observed number of $K^+K^-$ pairs with small relative momentum will allow us to evaluate for the first time the number of the produced $K^+K^-$ atoms.