Novel Signatures of Heavy Neutral Lepton at Muon Collider

Abstract

The Higgs-strahlung process $\ell^+\ell^-\to Z h$ is one of the most important production channels of the standard model Higgs boson $h$ at the lepton colliders. The cross section reaches the maximum value slightly above the threshold $\sqrt{s}\sim m_Z+m_h$, and decreases as $\sim 1/s$ at high energies. In the gauged extension models, the new gauge boson $Z'$ and heavy Higgs boson $H$ exist after the symmetry breaking. The heavy Higgs-strahlung process $\ell^+\ell^-\to Z' H$ would also reach the maximum cross section around the threshold $\sqrt{s}\sim m_{Z'}+m_H$. Therefore, the future high energy lepton colliders, such as the TeV scale muon collider, are promising to probe this new process. If heavy neutral lepton $N$ is introduced to generate the tiny neutrino masses via seesaw mechanism, novel signatures could arise from $μ^+μ^-\to Z' H \to NN +NN \to 4 μ^\pm +4J$ and $μ^+μ^-\to Z' H \to μ^+μ^- +NN \to 3 μ^\pm μ^\mp +2J$, where the fat-jets $J$ come from the hadronic decay of $W$ bosons. In this paper, we investigate the same-sign tetralepton signature $4μ^\pm+4J$ and the same-sign trilepton signature $3μ^\pm μ^\mp + 2J$ at the 3 TeV and 10 TeV muon collider.

Figures (8)

• Figure 1: Branching ratio of new gauge boson $Z'$ in panel (a) and heavy Higgs $H$ in panel (b). We have set $m_N=200~\rm{GeV}$ in panel (a). Meanwhile, $m_{Z'}$ is fixed to 1000 GeV in panel (b).
• Figure 2: Cross section of the heavy Higgs-strahlung at muon collider. The red, blue, and green lines are the results for $g'=0.6,0.3,0.1$, respectively. In panel (a), the solid (dashed) lines are the results for $m_{Z'}=m_H=1000 ~(3000)$ GeV. In panel (b), we assume $m_{Z'}=m_H$. In panel (c), we fix $m_H=1000$ GeV. In panel (d), we fix $m_{Z'}=1000$ GeV. Solid (dashed) lines in panel (b)-(d) are the results at the 3 (10) TeV muon collider.
• Figure 3: Normalized distributions of muon transverse momentum $P_T(\mu)$, fat-jet transverse momentum $P_T(J)$, and invariant mass of fat-jet $m_J$. Left panels are the results of 3 TeV, and right panels are the results of 10 TeV. During the simulation of these plots, we have assumed the relation $m_{Z'}=m_H=3m_N$ for illustration.
• Figure 4: Normalized distributions of invariant mass of muon and fat-jet $m_{\mu J}$ and invariant mass of two reconstructed heavy neutral lepton $m_{NN}$. Left panels are the results of 3 TeV, and right panels are the results of 10 TeV. The relation $m_{Z'}=m_H=3m_N$ is also assumed.
• Figure 5: Left: The $5 \sigma$ discovery region of the same-sign tetralepton signature in the $g'-m_N$ plane, where we fix $m_{Z'}=m_H=3m_N$. Right: The $5 \sigma$ discovery region of the same-sign tetralepton signature in the $m_N-m_H$ plane, where we fix $m_{Z'}=m_H$ and $g'=0.6$.