Mass and width of sigma(750) scalar meson from measurements of piN->pi(-)pi(+)N on polarized targets

Abstract

The measurements of reactions $π^- p_\uparrow \to π^- π^+ n$ at 17.2 GeV/c and $π^+ n_\uparrow \to π^+ π^- p$ at 5.98 and 11.85 GeV/c made at CERN with polarized targets provide a model-independent and solution-independent evidence for a narrow scalar state sigma(750). The original chi^2 minimization method and the recent Monte Carlo method for amplitude analysis of data at 17.2 GeV/c are in excellent agreement. Both methods find that the mass distribution of the measured amplitude $|\overline S |^2Σ$ with recoil transversity ``up'' resonates near 750 MeV while the amplitude $|S|^2Σ$ with recoil transversity ``down'' is large and nonresonating. The amplitude $|S|^2Σ$ contributes as a strong background to S-wave intensity I_S = (|S|^2 + |\overline S |^2)Σ$ and distorts the determinations of $σ$ resonance parameters from $I_S$. To avoid this problem we perform a series of Breit-Wigner fits directly to the measured distribution $|\overline S |^2Σ$. The inclusion of various backgrounds causes the width of sigma(750) to become very narrow. Our best fit with $t$-averaged coherent background yields $m_σ= 753 \pm 19$ MeV and $Γ_σ= 108 \pm 53$ MeV. These values are in excellent agreement with Ellis-Lanik theorem for the width of scalar gluonium. The gluonium interpretation of $σ(750)$ is also supported by the absence of $σ(750)$ in reactions $γγ\to ππ$. We also show how data on polarized target invalidate essential assumptions of past determinations of $ππ$ phase shifts .