BEHAVIOR OF THE NEUTRINO MAGNETIC FORM-FACTOR IN A RENORMALIZABLE MODEL

We have recently shown that the neutrino magnetic form factor mu(v)(q( 2)) could be considerably enhanced at low momentum transfers (q(2) les s than or equal to m(N)(2)) by introducing an extra light neutral ferm ion N with mass m(N) and nonzero magnetic moment. It was assumed that the magnetic moment of N would originate in a renormalizable way at a heavy scale M. While the enhancement of the neutrino magnetic moment w as unambiguous, we stressed that, in this effective-Lagranian approach , there is an ambiguity in the behavior of mu(v)(q(2)) in the interval m(N)(2) much less than q(2) much less than M-2. This is not unexpecte d in the presence of an unrenormalizable effective theory (a particle with a bare magnetic moment). In a simple renormalizable model for the magnetic moment of particle N, we 2 show how a two-loop calculation s olves the ambiguity. In the domain m(N)(2) much less than q(2) much le ss than M-2, We confirm OUT previous result obtained by using a sharp cutoff. As might have been expected, correct results are deduced throu gh dimensional regularization in the full theory but not in the effect ive-Lagrangian approach.