of the meson or lepton, e.g., kaons or tau leptons. The couplings involving $\nu_\tau$ are the least constrained, so we regard them as the most robust. Accordingly, they are shaded in Fig. \[fig:constraint\] to indicate exclusion for all flavors.

An flavor-independent constraint can be obtained from Z-boson decay. If a heavy mediator is assumed, the limit is quite strong, $G \sim G_{F}$, as shown in Refs. [@Bilenky:1992xn; @Bilenky:1994ma; @Bilenky:1999dn], though the domain of applicability of that effective theory calculation ends below the Z-boson mass. We emphasize that though this is nominally a very strong limit, it does [*not*]{} rule out all of the parameter space above the diagonal line $G \sim G_{F} \sim 10^{-5} \;{\rm GeV}^{-2}$. If the calculation is extended to allow a light mediator, following Ref. [@Laha:2013xua], the result for the scalar mediator case (not shown) is comparable to the $g_{\alpha\tau}$ constraint in Fig. \[fig:constraint\] for mediator masses below the mass of Z-boson.

This combination of constraints shows a window of parameter space in the MeV range where model-independent astrophysical or cosmological constraints could be improved. The IceCube sensitivity, shown approximately by the three orange lines in Fig. \[fig:constraint\] and calculated in the next section, lies in this 