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4 Universality improved B (τ → e ν ν) and Rhad
Following Ref. [63], we assume lepton universality to
obtain a more precise experimental determination of B e = B (τ →
eνeντ) using the tau branching fraction to muon and the tau
lifetime, by averaging the B e direct measurement,
the B e determination from assuming that gµ/ge = 1 hence (see
also Section 3) B e = B µ·
f(me2/mτ2)/f(mµ2/mτ2), and B e from assuming that gτ/gµ=1
hence B e = B (µ → e νe νµ)· (ττ/
τµ) · (mτ/mµ)5 · f(me2/mτ2)/f(me2/mµ2)
· (δγτδWτ)/(δγµδWµ)
where B (µ → e νe νµ) = 1. Accounting for statistical
correlations, we obtain
| B euni = (17.839 ± 0.028)%.
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We use B euni to obtain the ratio
| Rhad = | | = 3.6279 ± 0.0094.
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Here Γ(τ → hadrons) is obtained by summing all tau
hadronic decay modes.
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