Abstract
The standard Big Bang nucleosynthesis arguments are reviewed. The primordial He abundance is inferred from HeC and HeN and HeO correlations. The strengthened Li constraint as well as 2D plus 3He are used to limit the baryon density. The allowed number of neutrino families, Nν is delineated using the new value of τn = 890 ± 4 s (τ12 = 10.3 min) reported at this meeting. The formal statistical result is Nν = 2.6 ± 0.3 (1σ), providing a reasonable fit (1.3σ) to three families but making a fourth light (mν ⪅ 10 MeV) neutrino family exceedingly unlikely (≳ 4.7σ) (barring significant systematic errors either in D + 3He, and Li and/or 4He and/or τn). It is also shown that uncertainties induced by postulating a first-order quark-hadron phase transition do not seriously affect the conclusions.
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