Lepton number violation from Higgs/$Z$ decays into a pair of right-handed neutrinos

Abstract

We explore signatures of Lepton Number Violation (LNV) from decays of the discovered Higgs ($h$) and Z bosons into a pair of Right-Handed Neutrinos (RHNs). Due to the Majorana nature of RHNs, the final state can be the same-sign dilepton plus jets leading to 2 units of LNV. As a simple but plausible scenario, we investigate such a signal in models with a new $U(1)_X$ gauge symmetry which naturally introduces three RHNs for gauge anomaly cancellation, and is spontaneously broken down by a vacuum expectation value of an isospin singlet scalar field ($φ$). In this scenario, $h$ and the Z boson can decay into a pair of RHNs via the $h$-$φ$ mixing and the $Z$-$Z'$ mixing with $Z'$ being a new massive gauge boson, respectively. Estimating the LNV signal and corresponding backgrounds for the $\ell^{\pm} \ell^{\pm} 4j$ final states, we find bounds on the $h$-$φ$ mixing and the $Z$-$Z^\prime$ mixing as a function of a mass of RHNs at Higgs factories, e.g., at High-Luminosity LHC with $\sqrt{s}=$ 14 TeV, future $e^-e^+$ colliders at $\sqrt{s}=$ 250 GeV, muon colliders at $\sqrt{s}=$ 125 GeV as well as at Z factories, e.g., CEPC and FCC-ee at $\sqrt{s}=$ 91.2 GeV. We also discuss limits on the scalar mixing at $e^-μ^+$ and $μ^+ μ^+$ collisions ($μ$TRISTAN) with $\sqrt{s}=$346 GeV/775 GeV and 2 TeV, respectively.

Figures (2)

• Figure 1: Left: Branching ratios of $N$ as a function of $m_N$, where $q,q'\neq t$. Right: Branching ratios of $h\to NN$ as a function of $m_N$ for $\sin^2\alpha = 0.1$ (solid), 0.01 (dashed) and 0.001 (dotted).
• Figure 2: Left: LNV signature can be explored from the Higgs decay with the significance $S_{\rm LNV} = 2$ on each curve in the $m_N^{}$-$\sin^2\alpha$ plane. The horizontal lines show the upper limit on $\sin^2\alpha$ from the Higgs coupling measurements and direc searches for $\phi$. Right: LNV signature can be explored from the $Z$ decay with the significance $S_{\rm LNV} = 2$ and 5 assuming $\epsilon_{\rm mis}=0.01$ (red) and $0.001$ (blue) on each curve in the $m_N^{}$-$|\zeta|$ plane.