Constraining the SMEFT Extended with Sterile Neutrinos at FCC-ee

TL;DR

This work develops and applies a comprehensive νSMEFT framework to constrain heavy neutral leptons at FCC-ee, using monophoton plus missing energy and displaced-vertex signatures for operators up to dimension $d\leq 7$. It combines detailed collider simulations (MadGraph/Feynrules) with robust cut-based analyses to bound active–sterile mixing $|V_{eN}|^2$ and a wide set of νSMEFT Wilson coefficients, translating those bounds into lower limits on the new-physics scale $\Lambda$ for both Majorana and Dirac HNLs. The results show that FCC-ee can substantially improve on LEP bounds, with different operator classes yielding strongest sensitivity in either mono-$\gamma$ or DV channels, and in some cases exceeding current EWPO projections. Overall, the study highlights FCC-ee’s potential to probe a broad νSMEFT landscape, including UV completions that generate tree-level $d\leq 7$ operators, and provides a map from collider limits to the SMEFT parameter space relevant for neutrino mass generation and beyond-Standard-Model physics.

Abstract

We investigate how extensions of the Standard Model (SM) involving heavy neutral leptons (HNLs) can be probed at FCC-ee, the proposed high-energy circular $e^+e^-$ collider. Using the effective field theory (EFT) approach, we determine the impact of new interactions on the production and decay of HNLs at FCC-ee. In particular, we consider $d\leq 7$ $ν$SMEFT operators which induce vector, scalar and tensor four-fermion and effective charged- and neutral-current interactions of HNLs, that may also mix with the active neutrinos of the SM. We consider sensitivities to the active-sterile mixing and EFT Wilson coefficients from monophoton searches and displaced vertex decay signatures. In both analyses, we consider the scenarios where HNLs are Majorana or Dirac fermions. We translate the upper bounds on the Wilson coefficients to lower limits on the scale of new physics.

Figures (18)

• Figure 1: Single and pair production of HNLs at FCC-ee via the EFT operators considered in this work: four-fermion operators (left) and effective $W^\pm$ (centre) and $Z$ (right) interactions. The active-sterile mixing $V_{e N}$ induces the $W^\pm$ and $Z$ diagrams, while $V_{\mu N}$ and $V_{\tau N}$ induce the $Z$ diagram only.
• Figure 2: A selection of decays of HNLs via the same EFT operators in Fig. \ref{['fig:feynman-diagrams']}. The active-sterile mixing $V_{\alpha N}$ also induces the $W^\pm$ and $Z$ diagrams.
• Figure 3: Normalised binned distributions in the outgoing photon angle $\cos\theta_\gamma$ (above) and energy $x_\gamma = 2E_\gamma/\sqrt{s}$ (below) for mono-$\gamma$ processes induced by the electron-flavour mixing $V_{eN}$ and SM background in the Dirac HNL scenario. Distributions are shown for $\sqrt{s} = 91.2$ GeV (left) and $\sqrt{s} = 240$ GeV (right). Solid lines indicate the scenario with $m_N = 10$ GeV. We also show the distributions for $m_N$ close to the kinematic threshold using dashed and dot-dashed lines.
• Figure 4: Sensitivity of mono-$\gamma$ plus $\slashed{E}$ searches at FCC-ee to the electron-flavour mixing strength as a function of the HNL mass at $90\%$ CL, for $\sqrt{s} = 91.2$ GeV (red) and $\sqrt{s} = 240$ GeV (black). Shown are the results of the exclusive signal analysis (solid), taking into account the probability of the HNL decaying outside the detector of length $L = 5$ m, and inclusive signal analysis (dashed), where this requirement is relaxed. The shaded regions correspond to the currently excluded regions of the parameter space.
• Figure 5: Normalised binned distributions in the cosine of the outgoing photon angle $\cos\theta_\gamma$ (above) and energy $x_\gamma = 2E_\gamma/\sqrt{s}$ (below) for mono-$\gamma$ processes induced by the four-fermion, effective $W^\pm$ and $Z$ interactions and SM background in the Dirac HNL scenario. Distributions are shown for $\sqrt{s} = 91.2$ GeV (left) and $\sqrt{s} = 240$ GeV (right). Solid lines indicate the scenario with $m_{N_2} = 10$ GeV. For the vector four-fermion operator, we also show the distributions for $m_{N_2}$ close to the kinematic threshold, see text for details.