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 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, n L A = 436 +157 −61 tot , the lifetime of the 2p state is found to be τ 2p = ( 0.45 +1.08 −0.30 tot ) · 10 −11 s, not contradicting the corresponding QED 2p state lifetime τ QED 2p = 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 +0.28 −0.26 tot ) · 10 −15 s. 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. (J.Schacher) 1 deceased cesses, these atoms are produced in s states distributed according to n −3 , n being the principal quantum number [8].The decay probability of short-lived π + π − atoms (A 2π , pionium) in s states is dominated (99.6%) by the annihilation process [9-13] π + + π − → π 0 + π 0 and is given by the ππ s-wave scattering lengths combination |a 0 − a 2 | (a I is the ππ scattering length for isospin I):

Physics Beyond Colliders: QCD Working Group Report

“…−61 atomic pairs from the breakup of long-lived A 2π atoms was observed by DIRAC [117]. From this sample, the collaboration extracted the lifetime of the 2p state, which is three orders of magnitude larger than the one of A 2π in the ground state (τ 2p = 0.45 +1.08 −0.30 × 10 −11 s vs τ 1s = 3.15 +0.28 −0.26 × 10 −15 s) [118]. At the SPS, the 450 GeV/c proton beam and experimental adaptations envisaged for DIRAC++ (see below) open up new possibilities for study of long-lived ππ and πK atoms [107].…”