arXiv:nucl-th/0104027v1 7 Apr 2001 1 Nucleon-Nucleon Scattering in a Three-Dimensional Approach I. Fachruddina, Ch. Elsterb, W. Gl¨ocklea aInstitut f¨ur Theoretische Physik II, Ruhr-Universit¨at Bochum, D-44780 Bochum bInsitut f¨ur Kernphysik, Forschungszentrum J¨ulich, and Institute for Nuclear and Particle Physics, Ohio University, Athens, OH 45701 Two-nucleon scattering at intermediate energies of a few hundred MeV requires quite a few angular momentum states in order to achieve convergence of e.g.scattering ob- servables. This is even more true for the scattering of three or more nucleons upon each other. An alternative approach to the conventional one, which is based on angular mo- mentum decomposition, is to work directly with momentum vectors, specifically with the magnitudes of momenta and the angles between them.We formulate and numerically illustrate [1] this alternative approach for the case of NN scattering using two realistic interaction models, the Argonne AV18 [2] and the Bonn-B [3] potentials. The momentum vectors enter directly into the scattering equation, and the total spin of the two nucleons is treated in a helicity representation with respect to the relative momentaqof the two nucleons. The momentum-helicity states are given as |q; ˆqSΛ〉 ≡ |q〉 |ˆqSΛ〉=|q〉R(ˆq)∑ m1m2 C(1 2 1 2S;m1m2Λ) ∣ ∣ ∣ ∣ˆz1 2m1 〉 ∣ ∣ ∣ ∣ˆz1 2m2 〉 ,(1) whereR(ˆq) = exp(−iSzφ) exp(−iSyθ) is the rotation operator,Sz, Sythe components of S=1 2(σ1+σ2), and Λ the eigenvalue of the helicity operatorS·ˆq. Introducing parity and two-body isospin states|tmt〉, the antisymmetrized two-nucleon state is given by |q; ˆqSΛ;t〉πa=1 √2 (1−ηπ(−)S+t)|t〉 |q; ˆqSΛ〉π,(2) with the parity eigenvaluesηπ=±1 and|q; ˆqSΛ〉π≡1√2(|q〉+ηπ|−q〉)|ˆqSΛ〉. With these basis states we formulate the Lippmann-Schwinger (LS) equations for NN scattering. In the singlet case,S= 0, there is one single equation for each parity, TπSt 00(q′,q) =VπSt 00(q′,q) + 1 4 ∫ d3q′′VπSt 00(q′,q′′)G0(q′′)TπSt 00(q′′,q)(3) Using rotational and parity invariance one finds in the triplet case,S= 1, a set of two coupled LS equations for each parity and each initial helicity state TπSt Λ′Λ(q′,q)=VπSt Λ′Λ(q′,q) + 1 2 ∫ d3q′′VπSt Λ′1(q′,q′′)G0(q′′)TπSt 1Λ(q′′,q) + 1 4 ∫ d3q′′VπSt Λ′0(q′,q′′)G0(q′′)TπSt 0Λ(q′′,q).(4)
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