First Measurement of a Long-Lived π+π- Atom Lifetime

Abstract

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike π+π- 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 μm thick platinum foil, in which most of them dissociated. Analyzing the observed number of atomic pairs, nAL=436-61+157|tot, the lifetime of the 2p state is found to be τ2p=(0.45-0.30+1.08|tot)×10-11 s, not contradicting the corresponding QED 2p state lifetime τ2pQED=1.17×10-11 s. This lifetime value is three orders of magnitude larger than our previously measured value of the π+π- atom ground state lifetime τ=(3.15-0.26+0.28|tot)×10-15 s. Further studies of long-lived π+π- atoms will allow us to measure energy differences between p and s atomic states and so to discriminate between the isoscalar and isotensor ππ scattering lengths with the aim to check QCD predictions.