Magnetic Dipole Portal Vector Dark Matter Model Predicts Sub-GeV Interactions at Fixed-Target Experiments

The search for dark matter, the invisible substance comprising most of the universe’s mass, continues to challenge physicists, and a new model proposes a compelling pathway for detection. Avik Banerjee from the Tata Institute of Fundamental Research, alongside Riccardo Catena and Taylor R. Gray from Chalmers University of Technology, investigate a sub-GeV vector dark matter candidate interacting with ordinary matter through a novel “magnetic dipole portal”. This innovative framework predicts observable signals at fixed-target experiments, such as LDMX, arising from dark matter’s interactions with standard model particles, offering a promising avenue for directly detecting this elusive substance. By carefully considering the thermal history of the universe and incorporating constraints from a wide range of experiments and cosmological observations, the team demonstrate that a significant portion of the model’s parameter space remains viable, potentially bringing us closer to understanding the nature of dark matter.

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, 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′, 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. The team computes the thermal relic abundance.

Light Dark Matter and Dark Photon Searches

A comprehensive review of current research focuses on the search for light dark matter and dark photons, outlining theoretical frameworks and detailing numerous experimental efforts. Investigations explore dark photons, hypothetical particles associated with a hidden sector of the universe, and light dark matter candidates with masses below the GeV scale. Researchers are also examining complex dark sectors, consisting of multiple interacting particles, and models involving inelastic dark matter, where interactions with ordinary matter require energy exchange. Studies incorporate constraints from Big Bang Nucleosynthesis and the Cosmic Microwave Background, utilizing early universe observations to limit the properties of dark matter candidates.

A wide range of experiments are actively pursuing these searches. Beam dump experiments, such as NA64, LDMX, and SHiP, fire high-energy particle beams into dense targets, seeking evidence of dark matter and dark photons produced in the resulting interactions. Direct detection experiments, including PandaX, aim to observe dark matter particles directly through their interactions with detectors on Earth. Collider searches at facilities like Belle II also contribute by looking for dark matter signatures in high-energy collisions. This multifaceted approach combines various techniques to comprehensively probe the nature of dark matter.

Complementary constraints are derived from astrophysical observations, cosmic ray searches, and studies of galaxy clusters. Researchers analyze the distribution of galaxies and the cosmic microwave background to limit dark matter properties. Observations of colliding galaxy clusters, like the Bullet Cluster, provide insights into self-interacting dark matter. Recent efforts, including ongoing data analysis from NA64, LDMX, PandaX, and SHiP, continue to refine our understanding of this elusive substance. Theoretical work, such as studies of dark Higgs-strahlung at Belle II, further enhances the search for dark matter candidates.

Dark Matter Mediator Decays Visibly

Scientists have developed a new theoretical framework for light spin-1 dark matter, extending the Standard Model with a new non-Abelian gauge symmetry. This construction introduces massive gauge bosons, comprising the dark matter particle and a mediator, the Z’ boson, establishing a unique mass hierarchy where the mediator is lighter than twice the dark matter mass. The team introduced a novel dimension-5 portal operator to connect the dark and visible sectors, mediating interactions between dark matter, the Z’ boson, and known particles. This model predicts that the mediator cannot decay invisibly into dark matter pairs, suppressing its production rate compared to conventional scenarios.

Consequently, dark matter production proceeds via an off-shell mediator, while the mediator predominantly decays into visible final states, offering distinctive signatures for experimental searches. The analysis demonstrates that this reversed mass hierarchy leads to a suppressed rate of dark matter production, but simultaneously enhances the visibility of the mediator’s decay products. Researchers computed the thermal relic abundance across sub-GeV dark matter masses, finding regions where freeze-out annihilation proceeds via forbidden states or direct annihilation into Standard Model particles. Analysis incorporates bounds from direct detection experiments, Big Bang Nucleosynthesis, collider searches, and the cosmic microwave background. Results demonstrate a sizeable region of the parameter space remains consistent with observed relic abundance and existing experimental results. This framework provides distinctive signatures for experimental searches in the visible sector, offering a promising avenue for probing light dark matter candidates.

Sub-GeV Dark Matter and Kinetic Mixing

This research presents a model for sub-GeV dark matter, extending the Standard Model with a new dark sector and a spontaneously broken symmetry. Interactions between dark and visible matter occur through a specific mechanism involving kinetic mixing, resulting in magnetic dipole couplings between dark matter and known particles. The team investigated how much dark matter remains in the universe based on different masses, considering both annihilation into dark sector particles and direct annihilation into standard model particles. The analysis demonstrates that a significant portion of the model’s parameter space aligns with observed relic abundance and existing experimental results, despite constraints from direct detection experiments, cosmological observations, and collider searches. The researchers highlight the potential for future experiments, particularly the Light Dark Matter eXperiment (LDMX), to probe this model through several distinct signatures including dark bremsstrahlung, dark Higgs-strahlung, and invisible vector meson decay.