Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks

TL;DR

This work evaluates lepton-number violation at the LHC in a leptoquark realization of the Zee model, where Majorana neutrino masses arise radiatively at one loop. After mapping the viable parameter space under neutrino data, cLFV, EW precision, and LHC leptoquark constraints, the authors identify three LNV production channels—QCD and electroweak leptoquark pair production, plus single LQ production—as the main pathways to the final state $pp \to \ell^{\pm}\ell'^{\pm} + \text{jets}$, with QCD pair production dominating. A detector-level collider analysis using CMS background estimates projects HL-LHC sensitivity up to $m_{ m LQ} \sim 1.5$ TeV for this LNV signature, corresponding to ~2σ sensitivity around $L \sim 790~\mathrm{fb}^{-1}$. Observing such a signal would provide direct evidence of LNV and the Majorana nature of neutrinos, and would complement neutrinoless double beta decay searches by probing multiple lepton-flavor combinations.

Abstract

We investigate the prospects for observing lepton number violation (LNV) by two units, $|ΔL| = 2$, at the LHC within the leptoquark variant of the Zee Model, where Majorana neutrino masses arise radiatively at one-loop. The model features an $SU(2)_L$ doublet and singlet leptoquarks, whose interactions produce a distinctive same-sign dilepton plus jets signature, $pp \to \ell^{\pm}\ell'^{\pm} + \text{jets}$. Taking into account current experimental constraints, we identify the dominant production channels for this LNV signal and perform a detailed collider analysis. We find that the HL-LHC can probe leptoquark masses up to $m_{\rm LQ} \sim 1.5~\mathrm{TeV}$ with this process. Observation of this signal would provide a direct test of LNV and would unambiguously establish the Majorana nature of neutrinos.

Figures (7)

• Figure 1: Feynman diagram representing neutrino mass generation at one-loop in the Leptoquark variant of the Zee Model.
• Figure 2: Upper limits on the leptoquark pair production cross-section times branching fraction, $pp \to \mathrm{LQ}\,\overline{{\rm LQ}}\to X$, as a function of leptoquark mass $m_{\mathrm{LQ}}$, from searches in the $X = \{qe qe,\, q\mu q\mu,\, cece,\, c\mu c\mu,\, be be,\, b\mu b\mu\}$ channels by ATLAS ATLAS:2020dsk and CMS CMS:2024bnj at the $\sqrt{s}=13~\text{TeV}$ LHC.
• Figure 3: Parameter space is shown in the $m_{\omega^{2/3}}$-$m_{X_{1}}$ plane. The blue shaded region is excluded by constraints from $\Delta T$ at 95% CL, while the yellow shaded region is ruled out by leptoquark pair production searches at the $\sqrt{s}=13~$TeV LHC ATLAS:2020dskCMS:2024bnj. The green shaded region denotes the parameter space consistent with current constraints. Results are shown for the benchmark scenario $\{\theta_\mathrm{LQ} = 0.2,\ \lambda_7 = 0.5,\, c \equiv \lambda_{ij}/\lambda_{ij}^\prime = 0.1, \ m_1 = 0.04~\mathrm{eV}\}$.
• Figure 4: Representative Feynman diagrams illustrating the QCD pair production of leptoquarks (left), electroweak pair production (center), and single leptoquark production (right), leading to LNV signatures $pp \to \ell^{\pm}\ell'^{\pm} +$ jets at the LHC.
• Figure 5: Diagrams illustrating the connection between Majorana neutrino mass generation and LNV signatures at the LHC Schechter:1981bdBabu:2022ycv. The left, center, and right diagrams for neutrino mass generation correspond, respectively, to the LHC processes shown in the left, center, and right panels of Fig. \ref{['fig:lnv_feyn']}.