Differential cross sections and analyzing powers for the pp ! dp 1 reaction have been measured to within T cm p 85 keV of threshold. The S-wave strength is extracted and found to be ϳ13% larger than that determined from recent measurements of np ! dp 0 . Both the analyzing power and the anisotropy in the angular distribution show the expected energy dependence and these data are used to obtain values for the two contributing P-wave amplitudes. [S0031-9007(96)00564-9] PACS numbers: 13.75. Cs, 24.70. + s, 25.40.Ve Threshold pion production has received renewed attention in recent years both because technological developments have made high resolution experiments possible and because of the realization of the pivotal role played by this fundamental process. These threshold reactions provide direct information on the nuclear axial charge and indirect information regarding the pNN form factor and certain weak nuclear transitions [1-3]. Additionally, precision measurements of the pp ! dp 1 cross section and spin observables could potentially distinguish between competing production mechanisms such as mesonexchange currents (MEC) and off-shell rescattering.Considering that the NN ! ͑NN͒p reaction has been extensively studied for 50 years, it may seem surprising that there could be anything new to learn about it. However, the vast majority of the work is concentrated at energies where contributions from resonant production mechanisms dominate. Only a handful of experiments examine the threshold region where it is expected that nonresonant mechanisms are important [4][5][6][7][8].We present in this Letter a measurement of the cross section and analyzing power for the pp ! dp 1 reaction at nine energies between 287.7 and 294.1 MeV (E thresh 287.511). The lowest energy corresponds to T cm p ϳ 85 keV; the lowest energy at which analyzing powers were previously measured is more than 1 MeV above threshold [6].The measurements were carried out at the electroncooled proton storage ring (the Cooler Ring) at the Indiana University Cyclotron Facility (IUCF) using an internal H 2 gas jet target. The proton beam was 70% polarized and had an energy spread of approximately 60 keV FWHM.Sufficiently close to threshold, the pions, which are directly detected, are confined to a narrow cone about the beam direction with an opening angle given by sinu max h͞b cm g cm , where b cm and g cm are the boost parameters of the center-of-mass system and h p cm p ͞m p c. This energy dependence of the pion is used to determine the absolute beam energy to within 620 keV. To our knowledge, no other investigation of threshold pion production has obtained this level of energy precision which becomes essential at the lower end of the covered energy range. The detector (Fig. 1) contained left-right symmetric plastic scintillators which measured DE and E of the forward-going particles. Particles not stopping in the DE and E counters (E1 and E2, respectively) were vetoed by a double layer of plastic scintillators. Pions with energies less than 24 MeV and p...

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Cross sections for pp→pn ϩ have been measured at incident proton energies of 294.3, 299.3, 306.3, 314.1, and 319.2 MeV (0.11ϽϽ0.42) to investigate hadronic ϩ production near threshold. Pion angular distributions are presented for 294.3, 299.3, and 319.2 MeV. At 294.3 and 299.3 MeV they are consistent with isotropy, indicating s-wave dominance for the angular momentum of the ϩ pn system. The shapes of the pn, n ϩ , and p ϩ invariant mass distributions provide further evidence for dominance of s-wave mechanisms close to threshold. At 319.2 MeV significant anisotropy indicates contributions from higher partial waves. The total cross sections show an energy dependence consistent with an early partial conservation of axial current based calculation and with a recent calculation that considers only s-wave contributions. Tentative values for the strongest contributing partial wave amplitudes are presented. ͓S0556-2813͑97͒01307-1͔

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