EVIDENCE FOR A NARROW σ(770) RESONANCE AND ITS SUPPRESSION IN ππ SCATTERING FROM MEASUREMENTS OF π−p↑ → π−π+n ON POLARIZED TARGET AT 17.2 GeV/c

We present a new model independent amplitude analysis of reaction πp → π−π+n measured at CERN at 17.2 GeV/c on polarized target using a Monte Carlo method in an extended range of dipion mass 580–1080 MeV. The two solutions for the measured moduli |S|2Σ and |S|2Σ of the two S-wave transversity amplitudes S and S show resonant behaviour below 800 MeV corresponding to a scalar resonance σ(770). Simultaneous fits to |S|2Σ and |S|2Σ including f0(980) resonance give solution average mσ = 778 ± 16 MeV and Γσ = 142±33 MeV. The CERN data on polarized target supplemented by the assumption of analyticity of production amplitudes in dipion mass allow to determine the helicity amplitudes S0 (a1 exchange) and S1 (π exchange) from the fitted transversity amplitudes. The sign ambiguity in σ(770) contribution leads to two solutions. In the ”down” solution σ(770) is suppressed in the helicity flip amplitude S1 and thus also in π −π+ → π−π+ scattering. Most contribution of σ(770) to pion production is in the nonflip amplitude S0, or in πa1 → π−π+ scattering. In the ”up” solution the situation is reversed. The ”up” solution is excluded by unitarity in ππ scattering. The ”down” solution, and thus the evidence for σ(770), is in agreement with unitarity in both π−π+ → π−π+ and πa1 → π−π+ scattering. There are four ”down” solutions (1, 1), (2, 1), (1, 2) and (2, 2). The σ(770) resonance manifests itself as a broad resonant structure at ∼ 720 MeV in the flip amplitude |S1| in solutions (1, 1) and (2, 1). The contribution of σ(770) to |S1| is small in solutions (1, 2) and (2, 2). The metamorphosis of a narrow σ(770) in the nonflip amplitude S0 into a broad resonant structure in the flip amplitude S1 is a new phenomenon related to breaking of scale and chiral symmetry in QCD. Typeset using REVTEX electronic address: [email protected]