A NEW LOOK AT NEUTRINO LIMITS FROM BIG-BANG NUCLEOSYNTHESIS

We take a fresh look at the limits on the number of neutrino flavors derived from big bang nucleosynthesis, In particular, recent measurements of the He-4 abundance enable one to estimate the primordial 4He mass fraction at Y-p = 0.232 +/- 0.003 (stat) +/- 0.005 (syst). For a baryon to photon ratio, eta, consistent with the other light elements, this leads to a best fit for the number of neutrino flavors N-nu < 3 (the precise number depends on eta) indicating a very strong upper limit to N-nu. Here, we derive new upper limits on N-nu, paying special attention to the fact that the best estimate may lie in an unphysical region (N-nu < 3 if all three neutrino flavors are light or massless; the lower bound to N-nu may even be as low as 2, if the small window for a nu(tau) mass is exploited). Our resulting upper limits therefore depend on whether N-nu greater than or equal to 2 or 3 is assumed. We also explore the sensitivity of our results to the adopted value of eta and the assumed systematic errors in Y-p.