$N\bar N$ annihilation at rest into five pions

Abstract

$N\bar N$ annihilation at rest into five pions was studied in the ASTERIX spectrometer by stopping antiprotrons from LEAR in a H₂ or D₂ gas target. In$\bar pp$ annihilations in H₂ leading to the π⁺π⁻π⁺π⁻π⁰ final state, the invariant mass spectra of two, three, and four pions show no evidence for any new narrow states. In$\bar pn \to \pi ^ + \pi ^ - \pi ^ + \pi ^ - \pi ^ -$ the difference of the four pion mass spectra recoiling against a π⁻ and those recoiling against a π⁺ shows a resonance-like structure (“ζ (1480)”), confirming a previous analysis with a smaller event sample. The ζ (1480) mass is shown to depend strongly on the momentum of the spectator proton. In H₂, we searched for the ζ (1480) in the reaction$\bar pp \to \pi ^0 \zeta$ (1480), ζ (1480) →π⁺π⁻π⁺π⁻ to clarify whether the ζ (1480) is identical to the 4π-mode of thef ₂ (1520). No evidence for this decay mode was found. A prominent resonance in$\bar pp$ innihilation into five pions is the ω meson. We find two quasi-two-body intermediate states:$\bar pp \to \rho \omega$ and$\bar pp \to \pi ^ \pm b_1^ \mp$ (1235). The mass and width of theb ₁ (1235) are determined to be:

$$M_{bI} = 1225 \pm 5\user2{MeV, }\Gamma _{b1} = 113 \pm 12MeV.$$

((1))