1. Broadhurst D J 1972Phys. Rev. D5 2615
2. Cooper F and Pagels H 1970Phys. Rev. D2 228
3. Drell S D and Yan T M 1970Phys. Rev. Lett. 24 181
4. Drell S D, Levy D J, and Yan T M 1970Phys. Rev. D1 1617
5. Goldberger M L, Soper D E and Guth A H 1976Phys. Rev. D14 1117