content_image/337440.jpg content_image/337444.jpg
<img src='content_image/337440.jpg'>

where underlined parts in the final states are observed as missing energies. 

## The Zee-Babu model 

In the Zee-Babu model, the decay branching pattern of ω ± is determined by the relative magnitudes of the coupling constants f$_{ij}$ . As a reference scenario, we take a parameter set 

<img src='content_image/337444.jpg'>

which satisfies the neutrino data for the normal mass hierarchy. In this scenario, the rate in final states ℓ $^{+}$ℓ − = e $^{+}$e $^{−}$, e $^{±}$µ$^{∓}$, e $^{±}$τ $^{∓}$, µ $^{+}$µ $^{−}$, µ $^{±}$τ $^{∓}$, τ $^{+}$τ − is given by 2, 13, 13, 19, 36, 19, respectively. In Fig. 2, the differential cross section dσ/d cos θ$_{µ}$ for the signal e $^{+}$e − → ω $^{+}$ω − → µ $^{−}$τ + (+missing energy) is shown for √$_{s}$ = 1 TeV as a function of cos θ$_{µ}$ , where θ$_{µ}$ is the angle between the outgoing muon and the beam axis. The main background comes from the W pair production, which is also plotted. The angle cut ( e.g. cos θ$_{µ}$ < − 0 . 5) improves the ratio of the signal and the background. 6 

