Neutrino mass and invisible Higgs decays at the LHC

The discovery of the Higgs boson suggests that neutrinos also get their mass from spontaneous symmetry breaking. In the simplest ungauged lepton-number scheme, the Standard Model Higgs now has two other partners: a massive $CP$-even scalar, and the massless Nambu-Goldstone boson, called the Majoron. For weak-scale breaking of lepton number the invisible decays of the $CP$-even Higgs bosons to the Majoron lead to potentially copious sources of events with large missing energy. Using LHC results, we study how the constraints on invisible decays of the Higgs boson restrict the relevant parameters, substantially extending those previously derived from LEP and potentially shedding light on the scale of spontaneous lepton-number violation.

Figure 1

$m_{H_1}$ versus $\sin \alpha$ in the model for $v_1=1000\mathrm{GeV}$. The blue region is the region excluded by LEP results. The red, cyan, and magenta regions correspond to an invisible BR excluded at 75%, 50%, and 25%, respectively.

Figure 2

$m_{H_1}$ versus $\sin \alpha$ in the model for $v_1=1000\mathrm{GeV}$ (left panel) and $v_1\in [500,1000]\mathrm{GeV}$ (right panel). The blue region is the region excluded by LEP results. The red region corresponds to the points excluded by the LHC as discussed in the tex,t and the points in the green region pass all constraints.

Figure 3

Left panel: ${\mu }_{VV}$ versus $\mathrm{BR}(H_2\to \mathrm{Inv})$. Right panel: The same for $\mathrm{BR}(H_1\to \mathrm{Inv})$. The color code is as in Fig. 2.

Figure 4

Left panel: $\mathrm{BR}(H_2\to \mathrm{Inv})$ as a function of $\mathrm{BR}(H_1\to \mathrm{Inv})$. Right panel: $m_{H_1}$ as a function of $\mathrm{BR}(H_1\to \mathrm{Inv})$. The color code is as in Fig. 2.

Figure 5

Correlation between ${\mu }_{ZZ}$ and ${\mu }_{\gamma \gamma }$. The color code is as in Fig. 2.