Signatures of Long-Lived Heavy Neutral Leptons from Neutrinophilic Charged Higgs Pair Production at the LHC

Abstract

In the neutrinophilic Higgs doublet framework, the neutrino Dirac Yukawa couplings can be sizable because of the small vacuum expection value of the extra Higgs doublet, even for a low seesaw scale. Due to this structure, the neutrinophilic charged Higgs bosons, once created, decay dominantly into heavy neutral leptons (HNLs) and charged leptons. This is a new mechanism to produce a gauge singlet HNL without suppressed cross sections. In the standard seesaw, one HNL can be long-lived, when the lightest neutrino is sufficiently light. We investigate displaced vertex signatures of the long-lived HNLs produced from the decays of the charged Higgs pair at the high luminosity LHC. We consider one displaced vertex as well as two displaced vertices signatures and perform a dedicated simulation to identify the displaced leptons. We find that high statistical significance can be achieved for the observation of one displaced vertex for charged Higgs pair production cross section $>\mathcal{O}(1)$ fb. On the other hand, the observation of two displaced vertices is challenging even for charged Higgs pair production cross section of $\mathcal{O}(10)$ fb.

Figures (7)

• Figure 1: Limits on the VEV of the neutrinophilic second Higgs doublet $v_2$ from the $\mu \to e \gamma$ constraint. Left: Normal hierarchy (NH) scenario. Right: Inverted hierarchy (NH) scenario.
• Figure 2: Charged Higgs pair production cross section at the 14 TeV LHC.
• Figure 3: Top: $H^\pm$ branching ratios to the long-lived HNLs in normal hierarchy (NH) (left) and inverted hierarchy (IH) (right). Bottom: Long-lived HNL branching ratios in NH (left) and IH (right).
• Figure 4: Top: Proper decay lengths of the HNL states in normal hierarchy (NH). Bottom: Proper decay lengths of the HNL states in inverted hierarchy (IH). The vertical shaded region is excluded by the Planck upper limit on the sum of neutrino masses.
• Figure 5: Feynman diagram for the production of long-lived HNLs ($N_1$ in the normal hierarchy (NH) and $N_3$ in the inverted hierarchy (IH)) from $H^\pm$ pair production. The displaced vertices (DVs) associated with the long-lived HNL decays are shown in red.