High-precision measurement of polarized-Σ−beta decay

Abstract

We report the results of a polarized-${\Sigma }^{\mathrm{-}}$ beta-decay experiment carried out in the Fermilab Proton Center charged-hyperon beam. These results are based on 49 671 observed ${\Sigma }^{\mathrm{-}}$→${\mathrm{ne}}^{\mathrm{-}}$ν¯ decays. The ${\Sigma }^{\mathrm{-}}$ beam had a nominal momentum of 250 GeV/c and was produced by 400-GeV/c protons im- pinging on a Cu target. At a production angle of 2.5 mrad, the polarization was (23.6±4.3)%. The decay asymmetries of the electron (${\alpha }_e$=-0.519±0.104), neutron (${\alpha }_n$=+0.509±0.102), and antineutrino (${\alpha }_{\nu }$=-0.230±0.061) were measured and used to establish sign and approximate magnitude of the axial-vector-to-vector form-factor ratio $g_1$/$f_1$. The form-factor ratios ‖$g_1$/$f_1$‖ and $f_2$/$f_1$ were determined most sensitively from the neutron and electron center-of-mass spectra, respectively. We obtain ‖${\mathit{g}}_1$/${\mathit{f}}_1$-0.237${\mathit{g}}_2$/${\mathit{f}}_1$‖=0.327±0.007±0.019 and ${\mathit{f}}_2$(0)/${\mathit{f}}_1$(0)=-0.96±0.97±0.13, where the stated errors are statistical and systematic, respectively. A general fit that includes the asymmetries and makes the conventional assumption $g_2$=0 gives the final value $g_1$(0)/$f_1$(0)=-0.328±0.019. The data are also compatible with positive values for $g_2$/$f_1$ combined with corresponding reduced values for ‖$g_1$/$f_1$‖. .AE