Experimental investigation of π + π − and K + K − atoms

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time longlived hydrogen-like π+π− 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 μm thick platinum foil. Analysing the observed number of atomic pairs, nA = 436 +157 −61 ∣∣ tot, the lifetime of the 2p state is found to be τ2p = (0.45+1.08 −0.30 ∣∣ tot) ·10 −11s, not contradicting the corresponding QED 2p state lifetime τ 2p = 1.17 ·10−11s. This lifetime value is three orders of magnitude larger than our previously measured value of the π+π− atom ground state lifetime τ = (3.15+0.28 −0.26 ∣∣ tot) · 10 −15s. Further studies of long-lived π+π− atoms will allow to measure energy differences between p and s atomic states and so to determine ππ 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 2MK +5 MeV. In the distributions of K+K− pairs there is a strong signature of the Coulomb