Status of the Zee–Babu model for neutrino mass and possible tests at a like-sign linear collider

We provide an updated scan of the allowed parameter space of the two-loop Zee–Babu model for neutrino mass. Taking into account most recent experimental data on <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>μ</mi><mo stretchy="false">→</mo><mi>e</mi><mi>γ</mi></math> as well as the mixing angle <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>θ</mi></mrow><mrow><mn>13</mn></mrow></msub></math> we obtain lower bounds on the masses of the singly and doubly charged scalars of between 1 and 2 TeV, with some dependence on perturbativity and fine-tuning requirements. This makes the scalars difficult to observe at LHC with 14 TeV even with optimistic assumptions on the luminosity, and would require a multi-TeV linear collider to see the scalar resonances. We point out, however, that a sub-TeV linear collider in the like-sign mode may be able to observe lepton flavor violating processes such as <math altimg="si3.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup></math> due to contact interactions induced by the doubly charged scalar with masses up to around 10 TeV. We investigate the possibility to distinguish the Zee–Babu model from the Higgs triplet model using such processes