First Measurement of a Long-Lived π^{+}π^{-} Atom Lifetime.

B Adeva ,L Afanasyev,J Schacher,T Vrba,V Brekhovskikh,D Drijard,K Gritsay,D Dumitriu,V Yazkov,J Saborido,A Romero Vidal,C Guaraldo,S Horikawa,M Nikitin,S Aogaki,A Lamberto,M Pentia,Y Yoshimura,J Martincik,A Anania,V Olchevskii,P Zrelov,L Kruglova,A Benelli,M Gugiu,Z Hons,A Gorin,E Kulish,M Hansroul,P Doskarova,V D Ovsiannikov ,V.i Rykalin ,Yoshihisa Iwashita ,A Ryazantsev ,J Smolík ,D Flueraşu ,V.v Karpukhin ,T Urban ,L.l Nemenov ,R Lednický ,A Penzo ,F Takéutchi ,P Chliapnikov ,T Trojek ,P Průša ,V.v Kruglov ,A Lanaro ,S Trusov ,Kenji Okada ,M Pló ,A Kulikov ,M Chiba ,A Sidorov ,T Čechák ,C Mariñas ,A Dudarev ,M Kobayashi ,J Klusoň ,G F Rappazzo ,O.e Gorchakov

doi:10.1103/physrevlett.122.082003

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 96mm between the target and a 2.1 μm thick platinum foil, in which most of them dissociated. Analyzing the observed number of atomic pairs, n_{A}^{L}=436_{-61}^{+157}|_{tot}, the lifetime of the 2p state is found to be τ_{2p}=(0.45_{-0.30}^{+1.08}|_{tot})×10^{-11} s, not contradicting the corresponding QED 2p state lifetime τ_{2p}^{QED}=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.

Highlights

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike πþπ− atoms, produced by protons hitting a beryllium target
These atoms are produced in s states and are distributed according to n−3, n being the principal quantum number [8]
The precision of the ratio R is 1.2% [13], and the scattering lengths and their combination have been calculated in low-energy QCD

Summary

Introduction

The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike πþπ− atoms, produced by protons hitting a beryllium target. We use our measured value of the A2π ground state lifetime τ 1⁄4 3.15 × 10−15 s [3] and the spectrum of the atom lab momentum in the range 3–8 GeV=c with the average atom momentum hpAi 1⁄4 4.44 GeV=c, extracted from the experimental distribution of the Coulomb pairs.

Results

Conclusion