Probing the Light Sterile Neutrino Through the Heavy Charged Higgs Decay on the LHC

We show the 13 TeV proton-proton collider simulation in a $\nu$-two-Higgs-doublet-model ($\nu$-THDM). The heavy charged Higgs bosons are produced in pairs through the electroweak processes and decay to the light sterile neutrinos. The light sterile neutrino further decays into a jet-like object with a muon in it. This helps us discriminate the signal from the backgrounds with the standard model jets.

I will show in this paper that a sterile neutrino emerged from the decay of a heavy charged Higgs boson can be probed by utilizing the muon-jet tagging technique at the LHC.
In this work, we only concern the m N < 100 GeV. When m H ± m N , the highly boosted sterile neutrino decays via the µ ± +jet+jet channel. Fig. 1 shows the complete diagram

Model Setup
We briefly show the Lagrangian of the ν-THDM, which is a variant of the type-I Two-Higgs doublet model [10]. There are two Higgs doublet fields, Φ 1,2 , with the hypercharge Y = 1 2 . Φ 2 couples with the Standard Model (SM) particles Q L , u R , d R , L L , e R through The Φ 1 is in charge of the neutrino, where the subscript i = 1, 2, 3 corresponds to the e, µ, τ lepton doublets, respectively. In this model, Y i can be significantly amplified by a sizeable tan β ≡ v 2 v 1 , keeping the effective coupling with the standard-model Higgs boson h SM small.

Background Analysis and the Cut Flow
We identify the sterile neutrino jet finding out the high-energy-fraction muons in a jet. For the SM backgrounds, b-jet might fake the signal through the semi-leptonic decay of a B-meson. The main irreducible background is therefore pp → bbl + l − , b → B + X → µ + ν + X. We also considered the pp → jb + l + l − and pp → j j + l + l − processes, in which a non-b-jet can also produce a muon inside.
We also calculated the important reducible pp → tt → bbl + l − + background. Considering the MET reconstruction efficiency and the large pile-up effect in the future, we will show both the results with and without this background, which two extreme cases are covered.
We select the signal events by some anti-mass window around the Z-boson mass, and the mass window around the H ± mass. Then a µ-jet will be identified if it carries more than 30% of the total jet energy. The events containing at least one tagged N-jet are suffixed by "-1N-jet" and the ones with two tagged N-jet by "-2N-jet".    In Fig. 2 and Fig. 3, we show the minimum efficiency, ε in the 3 ab −1 integrated luminosity at the LHC for the "no-tt" and tt cases respectively, required to obtain a 5σ significance. The ε is defined by multiplying all the branching ratios corresponding to each decay vertex in the process shown in Fig. 1. The left panels in the figures shows the "-2N-jet" and the right panels shows the "-1N-jet" results.

Summary
We have simulated the signal and backgrounds at a 13 TeV LHC for the production of a sterile neutrino with the mass m N < 100 GeV within the framework of a ν-THDM. With the muon-jet tagging technique, the QCD jet backgrounds have been eliminated and in some regions of the parameter space, the proposed 3000 ab −1 expected at the HL-LHC can be sensitive to the ε 0.01 cases. The reducible pp → tt may be crucial if the pile-up effects will not be improved. [11].