Evidence for a narrow sigma(770) resonance and its suppression in pipi scattering from measurements of pi(-)p->pi(-)pi(+)n on polarized target at 17.2 GeV/c

Abstract

We present a new model independent amplitude analysis of reaction pi(-)p->pi(-) pi(+)n measured at CERN at 17.2 GeV/c on polarized target using a Monte Carlo method for dipion mass 580-1080 MeV. The two solutions for measured moduli |Sbar|^2 and |S|^2 of the two S-wave transversity amplitudes Sbar and S show res onant behaviour below 800 MeV corresponding to a scalar resonance sigma(770). Simultaneous fits to |Sbar|^2 and |S|^2 include f_0(980). The CERN data on polar ized target supplemented by the assumption of analyticity of production amplitud es in dipion mass allow to determine the helicity amplitudes S_0 (a_1 exchange) and S_1 (pi exchange) from fitted transversity amplitudes. The sign ambiguity in sigma(770) contribution leads to 2 solutions. In the "down" solution sigma(770) is suppressed in the helicity flip amplitude S_1 and thus also in pi(-)pi(+)-> pi(-)pi(+) scattering. Most contribution of sigma(770) to pion production is in the nonflip amplitude S_0, or in pi(-)a_1(+)->pi(-)pi(+) scattering. In the "up" solution the situation is reversed. The "up" solution is excluded by unitarity in pipi scattering. The "down" solution, and thus the evidence for sigma(770), is in agreement with unitarity in both pi(-)pi(+)->pi(-)pi(+) and pi(-)a_1(+)-> pi(-)pi(+) scattering. There are 4 "down" solutions (1, 1bar), (2, 1bar), (1, 2bar) and (2, 2bar). The sigma(770) resonance manifests itself as a broad resonant structure at ~ 720 MeV in the flip amplitude |S_1|^2 in solutions (1, 1bar) and (2, 1bar). The contribution of sigma(770) to |S_1|^2 is small in solutions (1, 2bar) and (2, 2bar). The metamorphosis of a narrow sigma(770) in the nonflip amplitude S_0 into a broad resonant structure in the flip amplitude S_1 is a new phenomenon related to breaking of scale and chiral symmetry in QCD.