Composite Inelastic Dark Matter
3/24/2009
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
The DAMA/LIBRA seasonal modulation signal was a long-running anomaly that I was initially skeptical of. But it persisted through many years and survived additional scrutiny. Neal Weiner and David Tucker-Smith had proposed inelastic dark matter as an explanation: a model that could produce the DAMA signal while simultaneously evading every other direct detection experiment. As WIMP dark matter became increasingly constrained by conventional experiments, more complex dark matter scenarios became worth taking seriously.
I remember a public lecture Neal Weiner gave at the Aspen Center for Physics where he introduced the analogy of “Mr. Dark Matter,” a person made of dark matter who constituted 80% of the universe’s mass and was trying to discover the other 20% of missing mass (to him). Mr. Dark Matter would probably build simple models with 1 or 2 particles. He’d be astonished to learn there were 92 different species of his missing matter, all the different elements. Now flip that around: why should we assume that 80% of the universe is all one particle?
That motivated me to think about a natural way to get the small energy splittings that inelastic dark matter requires. Hyperfine splittings in a dark quark sector, where the dark matter is a heavy meson, could produce exactly the right O(100 keV) scale from the spin-spin interactions of the constituent fermions rather than from parameter tuning. Daniele Alves and Siavosh Behbahani were my graduate students, and Philip Schuster was a postdoc I had advocated for the SLAC theory group to hire. We worked on finding the right parameters and making sure we weren’t missing anything for quite a few months. The paper led to several outgrowths over the following years: the cosmology paper, the parity violation analysis, and ultimately the intellectual thread that connected composite dark sectors to the SIMP miracle.