Composite Inelastic Dark Matter

197 citations (174 excluding self-citations). An early paper connecting composite dark matter to direct detection anomalies, and a precursor to the SIMP program’s exploration of strongly interacting dark sectors.

The Problem

The DAMA/LIBRA experiment had reported an annual modulation signal consistent with dark matter for over a decade, peaking in June as expected from the Earth’s motion through the galactic dark matter halo. The signal strongly suggested inelastic dark matter scattering — dark matter transitioning to an excited state split by roughly 100 keV. But generating a 100 keV splitting in a fundamental particle requires fine-tuning. The question was whether a natural mechanism could produce exactly this splitting.

The Key Idea

If dark matter is a meson of a QCD-like hidden sector, the spin-spin interactions between its constituent fermions naturally break the degeneracy of the ground state, exactly like the hyperfine splitting in hydrogen or the pion-rho mass splitting in QCD. The resulting O(100 keV) splitting is a natural consequence of the strong dynamics, not a tuned parameter. An axially coupled U(1) gauge boson kinetically mixed with hypercharge mediates the inelastic transitions. The model connects the mass splitting, the scattering cross-section, and the annual modulation phase within a single framework.

Impact

The paper was part of a broader effort to take the DAMA signal seriously by constructing concrete dark matter models that could explain it while remaining consistent with null results from other experiments. The composite dark matter framework developed here, with dark mesons, hyperfine splittings, and kinetically mixed mediators, became a building block for the later SIMP program, which explored a different dynamical regime of the same class of theories.

Recollections

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• DOI
• arXiv:0903.3945
• INSPIRE