Long-lived Left-Right signals at the FCC-ee

TL;DR

This work provides the first comprehensive analysis of displaced heavy Majorana neutrino signatures in the minimal Left-Right Symmetric Model at future $e^+e^-$ colliders. By combining analytic calculations of decay widths and production cross sections across gauge, scalar, and fusion channels with a dedicated vertexing-based detector simulation, it demonstrates that FCC-ee can probe left-right symmetry scales into the multi-TeV (and in some channels tens to hundreds of TeV) regime. The methodology enables full kinematic reconstruction of long-lived neutrinos and their parent states, allowing discrimination among production mechanisms and direct access to model parameters such as $m_N$, $M_{W_R}$, $\theta$, and the Yukawa structure. The results show strong sensitivity gains over the LHC, underscoring the FCC-ee’s potential to illuminate the origin of neutrino masses and the nature of lepton-number violation in LRSM.

Abstract

We give an extensive discussion of the displaced signals of heavy Majorana neutrino production at future electron-positron colliders operating at various proposed energies in the context of the Left-Right symmetric model. A comprehensive collection of channels is taken into account, ranging from those featuring $W$ and $W_R$ mediation to those induced by scalar mixing and gauge/scalar boson fusion, with connections to the mechanism of neutrino mass origin. The emerging signatures feature possibly multiple displaced heavy neutrinos that are in some cases accompanied by prompt activity and forward leptons. We derive the corresponding total production rates and differential distributions, which allow us to differentiate the channels and have analytical estimates of the signal yield. We then develop realistic estimates of the selection efficiencies using a dedicated vertexing algorithm which establishes the displaced decay positions and supplies a reliable proxy for reconstructing the full four-momenta of long-lived particles. This allows to determine the realistic reaches in the parameter space of the Left-Right symmetric model across the various channels, and we show that these can strongly surpass the LHC ones, demonstrating that future lepton colliders are sensitive to left-right symmetry breaking scales in the deep multi-TeV regime.

Figures (24)

• Figure 1: Cross sections for various heavy neutrino ($N$) and scalar ($\Delta$) production channels, possibly together with SM neutrinos or a $Z$ boson. The solid lines show the analytic results discussed in the text, while the circles correspond to MadGraph5_aMC@NLO simulations using the LRSM model file of Kriewald:2024cgr for the benchmark runs listed in Tables \ref{['tab:FCCee']} and \ref{['tab:CEPC']}.
• Figure 2: Feynman diagrams illustrating the pair production of heavy neutrinos $N$, and their associated production with a light neutrino $\nu$.
• Figure 3: Left -- Polar angle distribution of the heavy neutrino in $NN$ production via $Z$, $Z_{LR}$ and $W_R$ exchange in blue, as well as in $N\nu$ associated production in red. Results are shown for $\sqrt{s}=240\,\text{GeV}$ (solid) and $1\,\text{TeV}$ (dashed). We additionally display the polar-angle distribution of the $\Delta$ state produced in association with a $Z$ boson ($e^+e^-\!\to\Delta Z$, purple), as discussed in the following subsection. Right -- Normalised transverse momentum distributions for the same processes.
• Figure 4: Illustrative Feynman diagram for associated $Zh$ and $Z\Delta$ production.
• Figure 5: Representative Feynman diagram for heavy neutrino production via vector-boson fusion (left) and doubly-charged scalar-boson fusion (right) in the LRSM.