Charge Symmetry Breaking in Close to Threshold

A.K. OPPER, K. Hicks
Ohio University

E. KORKMAZ, G.V. O'Rielly
University of Northern British Columbia

D.A. Hutcheon, R. Abegg, P.W. Green, L.G. Greeniaus
University of Alberta and TRIUMF

C.A. Davis, P.L. Walden, S. Yen

J.A. Niskanen
University of Helsinki


Charge symmetry breaking (CSB) in the strong interaction has been observed in a number of experiments, notably in precise measurements of the difference in analyzing power for elastic np scattering at TRIUMF [1,2] and IUCF [3]. These observations of CSB are not unexpected given the mass dependent terms of the QCD Lagrangian and the mass difference between the up and down quarks. While these observations of CSB can be understood as originating in the quark degrees of freedom the dynamics of the strong interaction are not understood. In fact, these small but observable manifestations of charge independence and charge symmetry breaking are needed to constrain our NN potential models which are based on meson exchange theory and QCD. These observables may also lead to further insight of hadronic structure. However to be most useful, future measurements of CSB must be more than additional observations; they must investigate aspects of the interaction which have not yet been tested.

The CSB effect that we will measure is the forward-backward asymmetry () in , which must be zero in the center of mass if charge symmetry is conserved. With the absence of a coulomb interaction between the two interacting nucleons, the electromagnetic interaction is reduced relative to the strong interaction making this one of the clearest indications of CSB. Calculations by Niskanen [4] give an angle integrated value of approximately for near 280 MeV with the dominant contributions being an order of magnitude larger than those of the elastic scattering CSB measurements. Thus, this measurement will complement other observations of CSB.

The experiment will be done with a 281 MeV neutron beam, a liquid hydrogen target, and the SASP spectrometer at TRIUMF. With these kinematics and the large acceptance of SASP the full deuteron distribution will be detected in one setting of the spectrometer thereby eliminating many systematic uncertainties. The false instrumental asymmetry will be measured and many remaining systematic uncertainties will be reduced by measuring the deuteron distribution from which, due to the indistinguishability of the two protons, must be symmetric in the center of mass. The and reactions have been studied near threshold with the MRS in TRIUMF experiments 466 [5] and 552 [6], respectively. By exploiting the techniques and hardware that were developed for these two experiments, and with the factor of four increase in acceptance of SASP over the MRS, we intend to measure with an overall precision of approximately .

  1. R. Abegg, et al. , Phys. Rev. Let. 56, 2571 (1986); Phys. Rev. D 39, 2464 (1989).

  2. R. Abegg, et al. , Phys. Rev. Let. 75, 1711 (1995).

  3. S.E. Vigdor, et al. , Phys. Rev. C 46, 410 (1992).

  4. J.A. Niskanen, private communication.

  5. D.A. Hutcheon, et al. , Nucl. Phys. A535, 618 (1991).

  6. E. Korkmaz, et al. , Nucl. Phys. A535, 637 (1991).

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Richard T. Waters
14 August 1996