Antiproton-proton annihilation at rest in H₂ gas into π⁺ π⁻ π⁰

Abstract

Antiproton-proton annihilation at rest into π⁺ π⁻ π⁰ has been studied by stopping antiprotons from LEAR in a H₂ gas target at normal temperature and pressure. The reaction is observed in coincidence withL X-rays which are emitted in the cascade of$\bar pp$ atoms. The X-rays populate one of the angular momentumL=1 states of a$\bar pp$ atom which then annihilates. A spinparity analysis of the π⁺ π⁻ π⁰ Dalitz plot gives contributions from the threeP states from which annihilation into three pions is allowed. The most striking features of the Dalitz plot are production of charged and neutral ϱ(770) and off ₂(1270) mesons. Production of ϱ(770) mesons proceeds via the isoscalar part of the¹ P ₁ state and the isovector part of the³ P ₁ state.f ₂(1270) mesons are produced abundantly fromP states while their production fromS states is very weak. The π⁺ π⁻ invariant mass spectrum shows a third strong peak at a mass of 1565±20 MeV/c² and a width of 170±40 MeV/c², which we call AX(1565). No reasonable description of the Dalitz plot is obtained without introducing the AX(1565) as a new isoscalar particle. A phase shift analysis shows a phase movement which is fully compatible with a resonance withJ ^PC=2⁺⁺. The most likely interpretation of the new resonance is a multiquark$(qq\overline {qq} or N\bar N)$ state.