[Paper Review] Models of decaying FIMP Dark Matter: potential links with the Neutrino Sector

Laura Covi, Avirup Ghosh+2 more|arXiv (Cornell University)|Aug 28, 2020

Dark Matter and Cosmic Phenomena56 references4 citations

TL;DR

This paper proposes a framework for long-lived, decaying FIMP dark matter (DM) without requiring absolute stability, using suppressed Yukawa couplings or higher-dimensional operators via a gauged $U(1)_{L_\mu - L_\tau}$ symmetry. It establishes a direct link between neutrino mass generation and DM decay dynamics, enabling natural suppression of DM decay rates and consistent relic abundance via freeze-in, while satisfying cosmological and collider constraints.

ABSTRACT

The absolute stability of a dark matter (DM) particle is not a binding requirement. Here we suggest a few scenarios where the DM particle is liable to decay via extremely feeble interactions. This can happen via inexplicably small Yukawa couplings in the simplest conjectures. After setting down such a model, we go beyond it, thus treading onto scenarios where the spontaneous breakdown of some gauged $U(1)$ symmetry may lead to intermediate scales, and suitably suppressed effective operators which allow the DM particle to decay slowly. The constraints from particle physics as well as cosmology are taken into account in each case. The last and more involved scenario, studied in detail, suggest a link between the model parameters that govern neutrino physics on one side, and the dynamics of a quasi-stable DM particle on the other.

Motivation & Objective

• To construct a viable model of decaying FIMP dark matter without relying on absolute stability or discrete symmetries.
• To explain the extreme smallness of DM decay couplings naturally, avoiding fine-tuning.
• To establish a direct connection between the neutrino sector and the dynamics of a quasi-stable dark matter candidate.
• To ensure cosmological consistency, anomaly freedom, and compatibility with current neutrino and Higgs data.
• To explore collider and direct detection signatures of the model, particularly via vector-like fermion decays and electron scattering.

Proposed method

• Introduce a spin-1/2 Majorana fermion DM candidate as the lightest of three SM singlet Majorana fermions.
• Implement a gauged $U(1)_{L_\mu - L_\tau}$ symmetry under which the DM and vector-like fermion doublet $F$ are charged, while SM leptons carry $L_\mu - L_\tau$ charge.
• Construct dimension-5 and dimension-6 effective operators suppressed by the Planck scale, invariant under both SM and $U(1)_{L_\mu - L_\tau}$ gauge groups.
• Spontaneously break the $U(1)$ symmetry via a scalar $\phi$ at $\sim 10^8$ GeV, generating tiny mixing terms between SM leptons and the dark sector.
• Use the resulting suppressed couplings to mediate both FIMP production (via $F \to \psi + \phi$) and DM decay (via $\psi \to \ell + \ell' + \ell''$).
• Compute constraints on Wilson coefficients from DM lifetime, relic density, and neutrino mass sum, while checking for anomalies and collider signals.

Experimental results

Research questions

• RQ1 Can a long-lived FIMP dark matter candidate be realized without requiring absolute stability or discrete symmetries?
• RQ2 How can the extreme smallness of DM decay couplings be naturally explained without fine-tuning?
• RQ3 What is the role of a $U(1)_{L_\mu - L_\tau}$ gauge symmetry in linking neutrino mass generation and FIMP dark matter dynamics?
• RQ4 Can the model simultaneously satisfy cosmological constraints (relic density, neutrino mass sum), collider phenomenology, and DM decay observables?
• RQ5 What are the implications for future experiments, such as those searching for neutrinoless double beta decay or electron scattering in DM direct detection?

Key findings

• The $U(1)_{L_\mu - L_\tau}$-inspired model generates naturally suppressed DM decay and FIMP production via non-renormalizable operators, avoiding fine-tuning of Wilson coefficients.
• The effective operators contribute to the neutrino mass matrix, allowing the sum of neutrino masses to be below the Planck constraint while still enabling a sizable rate for neutrinoless double beta decay.
• For the lowest $v_D$ scale, a cancellation in the neutrino mass matrix correlates CP phases and coupling phases, restricting the allowed range of the effective Majorana mass in $0\nu\beta\beta$ decay to relatively large values.
• The model is cosmologically consistent and anomaly-free, with a viable FIMP production mechanism via decay of the $L_\mu - L_\tau$-charged scalar $\phi$.
• DM direct detection via electron scattering and contributions to the electron electric dipole moment are predicted to be below the sensitivity of future experiments, making the model viable.
• The model predicts observable signatures at colliders through the decay of the vector-like fermion doublet $F$, offering a testable path for future searches.