2008 / 09 23 5 Testing lepton universality , the π → eν / π → μ ν branching ratio
نویسندگان
چکیده
Determinations of Re/μ are based on observing final states with positrons emerging from a stopped π beam. Decays π → eν(γ) result in positrons appearing with the pion lifetime (26 ns) and with an energy distribution peaking at Ee+ = mπc/2. Decays π → μν(γ) result in positrons too, through the subsequent decay μ → eνν. The latter process leads to a time distribution first rising with the pion lifetime and then falling with the muon lifetime of 2.2 μs, and to a continuous energy distribution with Ee+ ≤ mμc/2. Whereas ≈ 98% of the π → eν(γ) decays are unambiguously identified by their energy, in the remaining ≈ 2% the observed energy leaks into the region below mμc /2 dominated by π → μν(γ). For this reason it is crucial to know (measure and/or simulate) the full π → eν(γ) energy distribution.
منابع مشابه
Improved Measurement of the π→eν Branching Ratio.
A new measurement of the branching ratio R_{e/μ}=Γ(π^{+}→e^{+}ν+π^{+}→e^{+}νγ)/Γ(π^{+}→μ^{+}ν+π^{+}→μ^{+}νγ) resulted in R_{e/μ}^{exp}=[1.2344±0.0023(stat)±0.0019(syst)]×10^{-4}. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1%.
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