The spin dynamics of an anisotropic fermi superfluid: (3He?). A. J. Leggett. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14850 and School of Mathematical and Physical Sciences, University of Sussex, Falmer, Brighton BN1 9QH, Sussex, UK, and Department of Physics, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

• Publisher: Elsevier Science
• Year: 1974
• Tongue: English
• Weight: 170 KB
• Volume: 84
• Category: Article
• ISSN: 0003-4916
• DOI: 10.1016/0003-4916(74)90323-6

✦ Synopsis

We discuss how massless particle reactions may be incorporated into standard S-matrix theory. The crucial element for doing so is a low energy zero. Examples of reactions where such zeros occur are weak interaction processes involving neutrinos, chirally symmetric massless pion scattering and two photon exchange between neutral systems. These zeros make two body unitarity a good approximation for sufficiently low energy despite the coalescence of multiparticle thresholds. Through two body unitarity, these zeros produce lines of zeros in the absorptive parts and double spectral functions. These lines of zeros are the S-matrix analog of the requirement of an infrared finite field theory. Not only do they produce finite total cross sections at finite energies, but they also allow both upper and lower bounds to be derived for these cross sections at high energies. This upper bound is our main result. If a plausible smoothness assumption is made, we find oror < fl (where B is arbitrarily small). In particular, the experimentally observed linear rise of the neutrino proton cross section cannot continue indefinitely.