Searching for ways to probe pseudo-Nambu-Goldstone-boson Dark Matter

Introduction

The existence of dark matter (DM) is one of the most compelling pieces of evidence for physics beyond the Standard Model (SM). The gravitational effects of DM are well established, as evidenced by the rotation curves of galaxies and the large-scale structure of the universe. However, its particle nature remains elusive, with no direct detection of DM particles to date. Among several viable candidates, the weakly interacting massive particle (WIMP) scenario has long served as a benchmark due to its natural consistency with thermal freeze-out production. Nevertheless, null results from direct detection experiments have imposed increasingly stringent constraints on WIMP models, motivating the search for alternative mechanisms that naturally evade such limits.

A compelling class of candidates arises from pseudo-Nambu–Goldstone bosons (pNGBs), which emerge from the spontaneous and soft breaking of global symmetries. Models of pNGB DM are particularly attractive, since their derivative-dominated interactions (in a non-linear representation of scalars) suppress scattering amplitudes at low momentum transfer, thereby remaining consistent with the latest bounds from direct detection experiments. At the same time, these models retain a sufficiently large annihilation cross section into SM particles, $\langle \sigma v\rangle_\text{ann}^{} \simeq 10^{-26}~\text{cm}^3 \text{s}^{-1}$, to account for the observed relic density $\Omega_{\text{DM}} h^2 = 0.12 \pm 0.001$.

Lying dormant for decades, the idea of boosted dark matter (BDM) has recently gained traction as a promising avenue for exploring the nature of DM. The underlying concept of BDM is that DM particles constituting the halo of the Milky Way can scatter with nuclei inside a massive celestial body (such as the Sun or the Earth) if their orbits pass through it. If their velocity after scattering is smaller than the escape velocity of the body, they become gravitationally bound and start orbiting. Through additional scatterings they sink toward the center and accumulate, building up a local DM overdensity concentrated in a relatively small volume. Various mechanisms, such as semi-annihilation or annihilation of heavier particles into lighter ones, can then lead to collisions among DM particles and produce highly energetic, boosted DM.

Goals

• Construct a multi-component pseudo-Nambu–Goldstone boson (pNGB) dark matter framework where a heavier state can efficiently produce a boosted lighter component through annihilation and, if viable, decay.
• Quantify the boosted dark matter (BDM) flux from the Sun and the Earth and compute elastic and deep inelastic BDM–nucleon scattering cross sections, focusing on the discovery reach of large-volume neutrino detectors such as Super-Kamiokande, Hyper-Kamiokande, IceCube/DeepCore and DUNE.
• Identify regions of parameter space that simultaneously satisfy relic abundance, direct detection, CMB and collider constraints, and assess whether observable BDM signals are possible in realistic pNGB models or whether a no-go type conclusion is implied.

Current status

• Previous work¹ established a pNGB dark matter model that naturally suppresses direct detection signals and avoids the domain wall problem, but semi-annihilation based BDM production was found to yield only mild boosts ($\gamma \simeq 1.25$) and extremely small scattering cross sections, far below the sensitivity of current neutrino detectors.
• Building on this, we are developing a multi-component pNGB DM setup, aiming to generate a significantly larger boost and an observable BDM signal while maintaining consistency with cosmological and experimental bounds.