Radiative Dirac neutrino masses and dark matter in a $U(1)_{B-L}$ extended model

TL;DR

This work analyzes a gauged U(1)_{B-L} extension of the Standard Model in which Dirac neutrino masses arise radiatively at one loop and dark matter stability is guaranteed by a residual Z_6 symmetry. Neutrino masses are generated through loops involving new scalars and vector-like fermions, yielding realistic scales m_ u ~ 0.01 eV, while the dark sector hosts either a fermionic DM candidate Psi_1 or a scalar DM candidate S_1, with relic density and direct-detection constraints explored in detail. The paper also assesses charged lepton flavor violation, showing correlated cLFV signals across mu -> e gamma, mu -> 3e, and mu-e conversion that probe the model parameter space. Collider studies at the LHC and a future muon collider reveal promising prospects for observing the dark sector, particularly for fermionic DM, where clean 1l^+ l^- + MET final states can yield 3–5 sigma significance with modest luminosities, highlighting the model’s testability across complementary experimental frontiers.

Abstract

We study a $U(1)_{B-L}$ extension of the Standard Model (SM) in which Dirac neutrino masses are generated radiatively at the one-loop level through the exchange of new beyond the SM fields. This framework establishes a direct connection between neutrino mass generation and the dark sector, with the stability of the dark matter ensured by a residual discrete $Z_6$ symmetry arising from the spontaneous breaking of $U(1)_{B-L}$. We investigate the resulting charged lepton flavor violating processes and dark matter phenomenology, saturating relic observations and direct-detection constraints, and analyze the collider signatures of the dark sector at the Large Hadron Collider and at a future muon collider. We have identified excellent prospects for observing the considered dark matter candidates in these colliders, even with lower integrated luminosities than the proposed one.

Figures (16)

• Figure 1: One-loop Dirac mass generation for neutrinos where the BSM fields are running within the loop : vector-like isosinglet fermions $\Psi$, and the scalars from the extended scalar sector $\phi, \eta_1, \eta_2$, while the neutrino mass is generated via the vevs of $H$ and $\sigma$. The insertion on the $\Psi$ line denotes the bare mass term $M_\Psi$ for the exotic fermion.
• Figure 2: New diagrams involving the vector-like isosinglet fermion $\Psi_R$ and the singly charged BSM scalar $\phi^+$ contributing to radiative muon decay $\mu \to e\gamma$ process.
• Figure 3: New diagrams contributing to three-body decay of muon $\mu \rightarrow 3e$ process. In the upper panel, the Penguin diagrams are shown, while the lower panel corresponds to the box diagrams. These new contributions are arising due to the running of BSM particles $\Psi_R$ and $\phi^+$ running within the loop.
• Figure 4: New penguin diagrams contributing to coherent $\mu$--$e$ conversion in nuclei are shown, involving quarks $q$ of the relevant generation and the incoming muon. The diagrams also depict the presence of the BSM particles $\Psi_R$ and $\phi^+$ running in the loop.
• Figure 5: Correlations among cLFV observables obtained from the parameter scan. Left: $\text{BR}(\mu \to e\gamma)$ versus $\text{BR}(\mu \to 3e)$. Right: $\text{BR}(\mu \to e\gamma)$ versus $\text{CR}(\mu\,\text{Ti} \to e\,\text{Ti})$. Solid (dashed) lines indicate current (projected) experimental sensitivities.