[Paper Review] Freeze-in production of sterile neutrino dark matter in a gauged U$(1)^\prime$ model with inverse seesaw

Arindam Das, Sreetama Goswami+2 more|arXiv (Cornell University)|Apr 28, 2021

Particle physics theoretical and experimental studies124 references4 citations

TL;DR

This paper proposes a U(1)′ model with inverse seesaw mechanism to simultaneously explain light neutrino masses and sterile neutrino dark matter (DM) via the freeze-in mechanism. The DM, produced through Z′-mediated scattering of SM particles, can account for the observed relic abundance and, if its mass exceeds ~1 MeV and Z′ is heavy, explain the Galactic 511 keV gamma-ray line via DM decay into positrons.

ABSTRACT

We consider a general, anomaly free U$(1)^\prime$ extension of the Standard Model (SM) where the neutrino mass is generated at the tree level via the inverse seesaw mechanism. The model contains three right handed neutrinos, three additional singlet fermions, one extra complex scalar and a neutral gauge boson $(Z^\prime)$. Instead of resorting to a specific $U(1)$ extension, we consider a class of models by taking the $U(1)^\prime$ charges of the scalars to be free parameters. Here, we assign one pair of the pseudo-Dirac degenerate sterile neutrinos as Dark Matter (DM) candidates which are produced by the freeze-in mechanism. Considering different mass regimes of the DM, $Z^\prime$ and reheating temperature, we obtain constraints on the $U(1)^\prime$ charges giving the correct relic abundance. We have also obtained constraints on $Z^\prime$ mass and coupling from consideration of relic density as well as high energy collider experiments like ATLAS in case of heavy $Z^\prime$ or in intensity and lifetime frontier experiments like DUNE, FASERs, and ILC beam dump which are looking for light $Z^\prime$. Additionally, in this model, the decay of pseudo-Dirac DM into active neutrinos can explain the 511 keV line observed by the INTEGRAL satellite.

Motivation & Objective

• To explain the observed dark matter relic abundance and tiny light neutrino masses within a single framework.
• To address the limitations of WIMP DM candidates, which face stringent direct detection bounds.
• To explore non-WIMP DM candidates—specifically sterile neutrinos—produced via the freeze-in mechanism.
• To test whether sterile neutrino DM can simultaneously explain the 511 keV gamma-ray line from the Galactic center.
• To constrain model parameters using DM relic density, vacuum stability, perturbativity, and LHC searches for Z′ bosons.

Proposed method

• Introduces a U(1)′ extension of the SM with three right-handed neutrinos and three gauge singlet Majorana fermions to realize the inverse seesaw mechanism for light neutrino masses.
• Assigns a degenerate pair of sterile neutrinos as the dark matter candidate, produced via freeze-in from SM particle scattering mediated by the Z′ gauge boson.
• Calculates the freeze-in production rate using the Z′-mediated scattering amplitude, with the DM yield depending on the Z′ mass, U(1)′ gauge coupling, and sterile-active mixing angles.
• Derives the decay width of the sterile neutrino DM into νe⁺e⁻ using Fermi's golden rule, incorporating contributions from off-shell W and Z bosons.
• Evaluates the 511 keV gamma-ray flux from DM decay and derives the required mixing angle |Vαi|² ≈ 5.44×10⁻²⁴ for m_N = 1 MeV.
• Applies constraints from LHC searches for heavy Z′, vacuum stability, perturbativity, and astrophysical bounds on DM and neutrino decay.

Experimental results

Research questions

• RQ1 Can a U(1)′ model with inverse seesaw generate both light neutrino masses and a viable sterile neutrino dark matter candidate?
• RQ2 Can the freeze-in mechanism produce the observed dark matter relic density without requiring thermal equilibrium?
• RQ3 Can the decay of sterile neutrino DM into e⁺e⁻ explain the 511 keV gamma-ray line observed by INTEGRAL?
• RQ4 How do constraints from LHC searches for heavy Z′, vacuum stability, and perturbativity shape the viable parameter space?
• RQ5 To what extent can future experiments like SHiP and FASER probe the parameter space allowed by DM relic density in the light Z′ regime?

Key findings

• The sterile neutrino DM with mass ≳1 MeV can explain the 511 keV gamma-ray line from the Galactic center if the Z′ is heavier than the DM and the mixing angle satisfies |Vαi|² ≈ 5.44×10⁻²⁴ for m_N = 1 MeV.
• For m_N ≈ 1 MeV, the required mixing angle is consistent with existing experimental bounds (e.g., from X-ray surveys and neutrino oscillation experiments) for 1 MeV ≤ m_N ≤ 70 MeV.
• The model parameters—U(1)′ gauge coupling g′, Z′ mass M_Z′, and U(1)′ charges x_h and x_ϕ—are constrained by the need to satisfy DM relic density, vacuum stability, and perturbativity.
• In the case of a TeV-scale Z′, LHC searches for Z′ resonances significantly restrict the parameter space, especially for large g′.
• For light Z′ (10⁻⁴–100 GeV), the parameter space consistent with DM relic density is partially probed by future lifetime frontier experiments SHiP and FASER, particularly in the U(1)_{B-L} model.
• The freeze-in mechanism leads to a DM yield that increases with coupling strength, contrasting with the freeze-out mechanism, and allows for a viable DM scenario even with very weak couplings.