Magnetic Dipole Portal Vector Dark Matter at Fixed-Targets

TL;DR

This work develops a sub-GeV vector dark matter model based on a new SU(2)_D that is broken by a scalar doublet and triplet, yielding DM candidates X^± and a mediator X^0, with an inverse mass hierarchy relative to the Z' via a dimension-5 non-Abelian kinetic-mixing portal. Dark matter interacts with the Standard Model through magnetic-dipole-like couplings to the photon and Z, enabling off-shell production channels at fixed-target experiments when m_{Z'} < 2 m_{DM}. The relic abundance is computed across freeze-out regimes, including forbidden dark-sector annihilation and direct SM annihilation, while a broad suite of collider, direct-detection, EW precision, BBN, and CMB constraints are incorporated. The analysis highlights fixed-target sensitivity at LDMX and NA64, with invisible vector-meson decays providing particularly strong reach, and finds substantial viable parameter space, notably for g_D ~ 10^{-2} and very small sin ζ, though BBN and direct-detection limits carve away much of the low-coupling region. Overall, the paper demonstrates complementary probing channels across cosmology, laboratory experiments, and astrophysics for a non-Abelian dark sector with an inverted mass hierarchy, offering concrete targets for upcoming fixed-target runs.

Abstract

We present a model featuring a sub-GeV vector dark matter by augmenting the Standard Model with a new non-Abelian dark $SU(2)_D$, spontaneously broken by the vacuum expectation values of a scalar doublet and a triplet. Interactions between the dark and visible sectors arise through a dimension-5 non-Abelian avatar of kinetic mixing portal, inducing effective magnetic dipole couplings of the dark matter, with the photon and Z boson. The resulting spectrum of the dark gauge bosons naturally exhibits an inverse mass hierarchy between the dark matter and the $Z^\prime$, leading to interesting phenomenology at fixed-target experiments such as LDMX through dark off-shell bremsstrahlung, dark Higgs-strahlung, invisible vector meson decay, and visible decays. We compute the thermal relic abundance across sub-GeV dark matter masses, with regions of freeze-out proceeding via forbidden annihilation into dark sector states or direct annihilation into Standard Model states. Bounds from a broad set of laboratory probes, along with cosmological and astrophysical observations, are incorporated in our analysis. Among them, the most restrictive bounds originate from direct detection experiments, Big Bang Nucleosynthesis, collider searches, and the CMB. Our results demonstrate that a sizeable region of the parameter space remains consistent with the observed relic abundance and the existing experimental results.

Figures (7)

• Figure 1: DM annihilation processes that set the relic abundance. The left diagram is dominant for small $\zeta$, and the right for $\zeta$ close to 1.
• Figure 2: Direct and indirect detection detection processes. Left: DM-electron scattering that induces electronic excitations at direct detection experiments. Right: DM annihilation that inject energy into the CMB through velocity independent thermally averaged cross sections.
• Figure 3: The number of DM events expected at LDMX Phase II, with a beam energy of 8 GeV and $10^{16}$ EOT, for two DM production channels: dark bremsstrahlung (in solid) and invisible vector meson decay (in dashed). The model parameters are set to: $\zeta = 10^{-3}$, $g_D = 0.01$, and the scalar masses are $m_{\Sigma_D^{\pm}}=m_{\Sigma_D^{0}}=m_{\Phi_D^0} = 1.2 \: m_{\text{DM}}$.
• Figure 4: The dominant off-shell dark bremsstrahlung processes generating DM that occur at fixed target experiments, where $N$ represents the target nucleus. Left: DM production diagrams off the initial or final state beam electron. Right: Bethe-Heitler trident diagrams for DM production.
• Figure 5: Examples of dark Higgs-strahlung processes providing additional missing energy/momentum signatures.