Search

A new measurement of the branching ratio for the decay \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$, made at the SATURNE II proton synchrotron, resulted in \ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$)/\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}all)= [5.7\ifmmode\pm\else\textpm\fi{}0.7(stat.)\ifmmode\pm\else\textpm\fi{}0.5(syst.)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$. The reaction pd${\ensuremath{\rightarrow}}^{3}$He\ensuremath{\eta} close to threshold yielded 800 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ tagged \ensuremath{\eta}'s in a narrow momentum band around 257 MeV/c. Muon pairs were detected in two range telescopes. The data obtained consist of 114 events \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$ on a background of 14 events. The new value for the branching ratio is 1.3\ifmmode\pm\else\textpm\fi{}0.2 times the unitarity lower limit, consistent with most quark and vector meson dominance models which describe the decay as an electromagnetic transition with a two-photon intermediate state. The result resolves the discrepancy between the two previous measurements of this branching ratio.

The mass of the η meson has been measured through the study of the d p→ 3 He η reaction near threshold, the beam having been calibrated through comparison with three other nuclear reactions. The value obtained is m η =547.30±0.15 MeV/ c 2 , where the error bar includes both systematic and statistical effects. This is much more precise and significantly lower than the Particle Data Group average, through it is consistent with a previous counter experiment.

The spin-rotation parameters A and R for ..pi../sup +/p and ..pi../sup -/p elastic scattering have been measured at pion momenta from 471 to 625 MeV/c using a frozen spin target polarized in the scattering plane and a recoil-proton polarimeter. Our results show that the major features of the recent ..pi..N partial-wave analyses (PWA's) are correct. This is the first time such confirmation has been possible. Some deviations outside the errors indicate the need for a new round of PWA's.

The eight neutron detectors described in the preceding paper have been calibrated with tagged neutrons from the reaction π−d → nn for Tn = 83–290 MeV. The neutron detection coefficient λ is (8.7 ± 0.2) × 10−3/(cm path length) at 83 MeV and decreases monotonically to (7.6 ± 0.5) × 10−3/(cm path length) at 290 MeV. Measurements were made repeatedly for different configurations to show the internal consistency of the data. Our results are in good agreement with the calibration data for neutron counters reported by Betti et al. but not with the work of Strong et al. Our data support the energy dependence of the efficiencies calculated by the Kent State University neutron efficiency code.