Search

Angular distributions for the differential cross section and three deuteron analyzing powers {ital iT}{sub 11}, {ital T}{sub 20}, and {ital T}{sub 22} of the reaction {ital d}{searrow}{ital p}{r_arrow}{sup 3}He{pi}{sup 0} have been measured over the whole angular domain at 20 energies close to threshold (0.03{lt}{ital T}{sub {pi}}{sup c.m.}{lt}10.2 MeV). The differential cross section and tensor analyzing power {ital T}{sub 20} both show strong variation in energy and angle due to interference between {ital S}- and {ital P}-wave pion production, whereas {ital iT}{sub 11} and {ital T}{sub 22} remain consistent with zero over the whole experimental range. All the data at different energies and angles fall on universal curves when plotted as functions of the single variable {ital p}{sub {pi}}cos{Theta}, evaluated in the c.m. The broad features of the results are in line with theoretical expectations. {copyright} {ital 1996 The American Physical Society.}

A search for the lepton-family-violating decay {eta}{r_arrow}{mu}{ital e} and the rare decay {eta}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}} yields the following branching ratio ({ital B}) upper limits at a 90{percent} confidence level: {ital B}({eta}{r_arrow}{mu}{ital e}){lt}6{times}10{sup {minus}6} and {ital B}({eta}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}}){lt}2{times}10{sup {minus}4}. This is the first direct search for {eta}{r_arrow}{mu}{ital e}. The measurements were carried out at the SPES2 tagged {eta} facility at Laboratoire National Saturne in the course of a measurement of {ital B}({eta}{r_arrow}{mu}{sup +}{mu}{sup {minus}}). {copyright} {ital 1996 The American Physical Society.}

The reaction {ital pd}{r_arrow}{sup 3}He{eta} has been studied with the SPES2 spectrometer, for proton energies between 0.2 MeV and 11 MeV above threshold. The total cross section rises from 0.25 to 0.40 {mu}b in this region. The observed energy dependence can be attributed to the strong final state interaction. The angular distribution is nearly isotropic, consistent with {ital S} wave production. A measurement of the {ital pd}{r_arrow}{sup 3}He{pi}{sup +}{pi}{sup {minus}} cross section in the threshold region is also presented. The setup, which is capable of producing 10{sup 5} tagged {eta}/s with only a few percent background, is used to investigate rare {eta} decays. {copyright} {ital 1996 The American Physical Society.}

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.

First results on π−d → are reported. The measurements were made using 8 specially designed neutron counters, which were carefully calibratedin situ. The differential cross sections atTπ=142, 180, 217, and 254 MeV were obtained at four angles between 0° and 90°, they are compared to π+d →pp data measured at the same energies and angles with the same setup. At every beam energy, the shape of the angular distributions of π−d →nn and π+d →pp is the same to ±2%. The absolute cross sections differ by 1 to 10%. The error in this comparison is ±4% implying a small violation of charge symmetry.

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.

The verification of the existing sets of πN scattering phases is sought via the determination of the A and R spin rotation parameters in π+p and π−p elastic scattering. The preliminary results of this first measurement of these parameters for incident pion momenta from 427 to 657 MeV/c, show agreement with the gross features of the partial‐wave analyses by the groups from VPI, Karlsruhe‐Helsinki, and Carnegie‐Mellon‐LBL. There is room for improvement.