W Franklin, S Wissink, A Bacher, A Betker, T Black, S Choi, K Jiang, W Schmitt, J Sowinski, E Stephenson, C Yu
Current potential models1 of the nucleon-nucleon (NN) interaction can describe much of the low-and intermediate-energy NN scattering database with a X 2 per datum fairly close to 1. In spite of these successes, however, there remains significant controversy over the value of one of the most fundamental parameters in such models, the pion-nucleon coupling constant g:. Difficulties encountered initially in reproducing both the pp scattering data and the ground-state properties of the deuteron2
more » ... to speculation as to whether this coupling could be strongly charge-dependent.3 Though later analyses of the NN (both pp and np) (Ref. 1) and ?rN (Ref. 4) data showed no clear signature for charge-dependence, the large value for gZk extracted in a recent study of the np cross section at back angles5 has only served to muddy the waters further. An additional complication, and one whose severity is difficult to gauge, arises from the fact that those who model the interaction have generally employed different criteria for data selection, different conventions for renormalizing data sets, and have emphasized different energy ranges in their analysis. Thus, discrepancies at the few percent level for remain, even when only pp scattering is ~onsidered.~ 7 ' On the experimental side, while it is clear that some of the existing data must be in error (e.g., when two measurements of nominally the same quantity differ by 4 or 5 standard deviations), it is also true that some of the most critical kinematic regimes remain relatively unexplored. Detailed analysis and simulations have shown7v8 that precise knowledge of a specific set of spin observables, over the kinematic region where single pion exchange should dominate the hadronic interaction (q 0.3-0.8 fm-') , can provide significant constraints on the value of g:. To identify these observables, we note that in a momentum-space representation, the first-order potential for the exchange of a pseudo-scalar particle takes the form:' where MN is the (average) nucleon mass and ak is the Pauli spin matrix for the kth nucleon. By measuring an appropriate combination of spin-transfer observables, one can isolate that component of the NN interaction which exhibits a spin-longitudinal structure identical to that found in Eq. (1). Explicitly, this component-the "6" amplitude in the KMT form~lation,'~ for example-can be expressed in the following model-independent manner: In conjunction with the DNN1 data measured recently at IUCF," the measurements described here constitute a robust set of high precision data that will allow us to map out the momentum-transfer dependence of the S amplitude in a regime where it is expected to be changing rapidly, due largely to single pion exchange. Data for experiment E383 were acquired in three runs during 1995-96. Measurements of the lab-frame observables DLLl, DLs/, DSLl, and Dss/ were completed at 5 angles (Blab x so, 7.5", l o 0 , 12", and 15"), at an average beam energy of 197.9 MeV. The basic measurement involved the scattering of protons, whose polarization vector had been precessed to lie in the (horizontal) reaction plane, from a thin CH2 foil target. The higher-energy, forward-going protons were momentum analyzed with the K600 spectrometer and focal plane detectors, and their sideways polarization components determined using elastic scattering from natural carbon in the focal plane polarimeter (FPP). The low-energy recoil protons were detected with a thin (500 pm) stopping silicon microstrip detector located inside the scattering chamber. No coincidence requirement between the protons was imposed in hardware, so that a 'singles' analysis of the focal plane data, after suitable background subtraction, was possible. This detection scheme, developed during the DNN/