-1 -0.5 0 0.5 1 cos  0 1 10 100 1000 Diff. cross section [fb] Signal BG Zee-Babu model s = 1TeV  + missing energy WW FIG. 2: The differential cross section of e $^{+}$e − → ω $^{+}$ω − → µ $^{−}$τ $^{+}$(+missing energy) as a function of the angle between the outgoing muon and the beam axis in the Zee-Babu model for √ s = 1 TeV. The rate of µ $^{−}$τ $^{+}$(+missing energy) from main background e $^{+}$e − → W $^{+}$W − is also shown. where underlined parts in the final states are observed as missing energies. 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 m$_{ω}$ = 300 GeV , m$_{k}$ = 1200 GeV , µ = 800 GeV , f$_{eµ}$ = f$_{eτ}$ = 0 . 013 , f$_{µτ}$ = 0 . 027 , g$_{ee}$ = 0 . 17 , g$_{µµ}$ = 1 . 8 , g$_{τ τ}$ = 0 . 0061 , g$_{eµ}$ = 5 . 7 × 10 − $^{5}$, g$_{eτ}$ = 0 . 011 , g$_{µτ}$ = − 0 . 081 , (11) The Zee-Babu model 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 11 6 Although in this Letter we mainly discuss the case where m$_{k}$ is at the TeV scale, we just comment on the case of lighter k $^{±±}$. In such case, the pair production of k $^{++}$k −− can be a clear signature of this model, whose signal is the like-sign dilepton pairs with opposite direction [3, 9].