Very Heavy and Composite Dark Matter: Theory and Experimental Searches

Abstract

Dark matter much heavier than the weak scale remains a comparatively unexplored frontier. This review surveys theoretical and experimental developments on very heavy dark matter, including composite and dissipative formation mechanisms, multiscatter detection, and astrophysical searches.

Figures (6)

• Figure 1: Composite formation pathways for dark matter. Top: hierarchical radiative dark nucleosynthesis, where two-body bound states form through mediator emission and merge into larger composites, radiating a scalar $\phi$ at each fusion step. Middle: dissipative formation and fragmentation of dark compact objects, adapted from Fig. 1 of Bramante:2024pyc, which shows a sequence of dark halo virialization, radiative cooling, fragmentation, dilution, and low-redshift compact remnant formation. Bottom: composite formation in a first-order phase transition, where shrinking bubbles of the high temperature phase trap and compress dark constituents.
• Figure 2: Multiscatter phenomenology of very heavy dark matter. Left: schematic of multiple nuclear recoils aligned along a single linear trajectory for $m_X\!\gg\!m_N$, producing a correlated light or charge track Bramante:2018tos. Right: exclusion region from the DEAP-3600 multiscatter search DEAPCollaboration:2021raj, constraining dark matter masses up to $10^{19}\,\mathrm{GeV}$. The enclosed region is bounded below by the minimum cross-section required to produce a detectable multiscatter track using the pulse-timing discrimination analysis at DEAP-3600, above by energy attenuation in the Earth's overburden, and on the right by the local dark matter flux. Variations in the terminus of the right boundary depend on DEAP-3600 sensitivity to different energy thresholds/high-velocity phase space of dark matter.
• Figure 3: Schematic regimes of composite dark matter scattering in detectors. Four conceptual panels are shown from left to right: (1.) Whole-composite boundary acceleration: an incoming dark composite interacts coherently with a nucleus, transferring momentum to the entire bound state and producing nuclear recoil radiation; (2.) Soft boundary recoils and Migdal emission: partial or glancing interactions excite electrons and emit soft bremsstrahlung photons, leading to correlated electronic signals along the track; (3.) Reflection or saturation regime: for large effective mediator field or large $|\langle\varphi\rangle|$, the scattering becomes surface-dominated and the momentum transfer saturates near the geometric limit; (4.) Loosely bound composite regime: the probe resolves individual constituents within an extended dark bound state, producing incoherent constituent-level scattering as described in Sec. \ref{['subsec:looselybound']}. Adapted from Fig. 1 of Acevedo:2021kly, with the final panel illustrating the loosely bound composite regime explored in Acevedo:2024lyr.
• Figure 4: Some multiscatter and high-mass dark matter limits from terrestrial detectors. Left: Chicago EJ-301 coincidence search Cappiello:2020lbk; Right: LZ multiscatter search LZ:2024psa. The relatively flat lower bound of the EJ-301 search is determined by a threshold for total nuclear recoil deposition in a liquid scintillator detector, while the LZ SR1 multiscatter search has a more characteristic $1/ m_X$ scaling of a single-recoil sensitive detector.
• Figure 5: Etched plastic and ancient mica constraints on very heavy or composite dark matter. Excavated ancient mica limits for the per-nucleon (left) and nucleon-independent (right) parameterizations are shown in blue and red Acevedo:2021tbl alongside etched-plastic limits in gray (labeled Skylab and Ohya) from Bhoonah:2020fys. Both studies incorporated overburden attenuation and detector thresholds - the features apparent in the red mica bounds result from a detailed Monte Carlo of dark matter attenuation through the Earth's crust Acevedo:2021tbl, whereas the plastic etch bounds have more uniform overburden material Bhoonah:2020fys. A broader overview of mineral-based searches is given in Baum:2023cct.