Stars as Laboratories for Fundamental Physics - MPP Theory Group

Radiative Particle Decays 457
Fig. 12.3. Bounds on ν e -ν h mixing from the absence of ν h → ν e e + e − decays.
Meson decays: (a,b) Leener-Rosier et al. (1986) and (c) Bryman et al. (1983).
Reactor neutrinos: (d) Hagner et al. (1995). Absence of reactor neutrino
oscillations: (e) Zacek et al. (1986). Absence of solar positrons: Toussaint
and Wilczek (1981); see also Sect. 12.3.2.
These results are based on ν e sources which produce ν h by their
mixing which also leads to the subsequent ν h → ν e e + e − decay. Of the
known sequential neutrinos ν h can be identified only with ν 3 , the dominant
mass component of ν τ with an allowed mass of up to 24 MeV.
If one makes this identification, stronger limits are obtained from direct
ν τ ≈ ν 3 sources. One example is the beam stop at the Big European
Bubble Chamber (BEBC) where a strong flux of charmed strange
mesons 72 D s was produced which subsequently can decay as D s → τν τ
besides the dominant hadronic modes (WA66 Collaboration 1985). According
to Babu, Gould, and Rothstein (1994) who quote a private
communication from the WA66 collaboration, a model-independent
constraint from the BEBC experiment is τ γ /m 3 > 0.15 s/MeV or
µ eff < 1.1×10 3 µ B m −2
eV . (12.9)
Another constraint is τ e + e −/m 3 > 0.18 s/MeV. As a constraint on
the mixing amplitude, |U e3 | 2 < 1.6×10 5 m −6
MeV, it is weaker than those
shown in Fig. 12.3. However, as it is based on a direct ν τ flux it is valid
even if the decays are not induced by mixing but by exotic intermediate
states (Babu, Gould, and Rothstein 1993).
72 The D s used to be called F as in the quoted reference.