Probing the origin of neutrino masses and mixings via doubly charged scalars: Complementarity of the intensity and the energy frontiers

We discuss how the intensity and the energy frontiers provide complementary constraints within a minimal model of neutrino mass involving just one new field beyond the Standard Model at accessible energy, namely a doubly charged scalar $S^{++}$ and its antiparticle $S^{--}$. In particular, we focus on the complementarity between high-energy LHC searches and low-energy probes such as lepton flavor violation. Our setting is a prime example of how high- and low-energy physics can cross-fertilize each other.

Figure 1

Neutrino mass generation for the example of the $m_{ee}^{\nu }$ element of the full neutrino mass matrix. Similar diagrams are responsible for the full matrix $m_{ab}^{\nu }$, where $a,b=e$, $\mu$, $\tau$.

Figure 2

Example diagrams for $\mu \to e\gamma$ (left) and $\mu –e$ conversion long-range (middle) and short-range (right) contributions. The parts inside the large rectangular frames happen inside the nucleus, which necessitates the distinction of long and short range.

Figure 3

$S^{++}$ production channels at the LHC: Drell-Yan (top); vector boson fusion (bottom).

Figure 4

Particle physics parts ${\mathrm{\Xi }}_{\text{particle}}$ as functions of the scalar mass $M_S$ for the red/purple/blue scenarios. Different limits from ${\mu }^{-}–e^{-}$ conversion and from LHC searches are indicated; see text for details.