We describe the observation of the isospin violating and charge symmetry breaking reaction dd -> alpha pi(0) just above threshold. Measurements using a magnetic channel (gated by two photons) of the He-4 scattering angle and momentum (from time of flight) permitted reconstruction of the pi(0) "missing mass," the quantity used to separate distributions of He-4 + pi(0) events from a continuum containing He-4 + gamma + gamma and He-4 + pi(0 )+ gamma events. A review has been completed of the apparatus, relevant GEANT simulations, various corrections, and the d + p scattering cross section used as a luminosity calibration. The new values of the total cross section for neutral pion production are 14.3 +/- 2.2 (stat) +/- 1.6 (sys) pb at 228.5 MeV and 17.3 +/- 3.4 (stat) +/- 2.4 (sys) pb at 231.8 MeV. The uncertainty remains dominated by statistical errors. These cross sections arise from the down-up quark mass difference and quark electromagnetic effects.

We report angular distribution measurements of the differential cross section sigma(theta) and the analyzing powers iT(11)(theta),T-20(theta), and T-22(theta) for deuteron-deuteron elastic scattering at 231.8 MeV. These data are compared to calculations based on the lowest order terms in the Born series expansion of the Alt-Grassberger-Sandhas equation for four nucleons interacting through the CD Bonn potential.

The authors conclude, using the current world data around 50 MeV there is no question that experimental values of e1 and 1P1 are significantly different from theoretical prediction. (AIP)Received 20 December 1993DOI:https://doi.org/10.1103/PhysRevLett.72.2665©1994 American Physical Society

The polarized target asymmetry ${\mathit{A}}_{\mathit{N}}$ for ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\rightarrow}\ensuremath{\gamma}n has been measured by coincident detection of the \ensuremath{\gamma} and the neutron at several angles across the \ensuremath{\Delta} resonace energy region. A high-resolution NaI \ensuremath{\gamma} spectrometer together with good time-of-flight information for the neutron resulted in excellent separation of this radiative capture reaction from the much higher cross-section charge exchange reaction ${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{0}$n. Statistical uncertainties are \ensuremath{\Delta}${\mathit{A}}_{\mathit{N}}$=0.02-0.04, representing a significant improvement over previous results. The new data are discussed in the context of recent theoretical models.

We report a first measurement of the spin-correlation parameter ${\mathit{A}}_{\mathit{z}\mathit{z}}$ in neutron-proton scattering at 67.5 MeV. The results, obtained in the angular range 105\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}170\ifmmode^\circ\else\textdegree\fi{} with typical accuracies of 0.008, are highly sensitive to the $^{3}$${\mathit{S}}_{1}$${\mathrm{\ensuremath{-}}}^{3}$${\mathit{D}}_{1}$ mixing parameter ${\mathrm{\ensuremath{\epsilon}}}_{1}$. A phase-shift analysis based on the current world data yields a value of ${\mathrm{\ensuremath{\epsilon}}}_{1}$ significantly higher than predicted by modern potential models.