Abstract
Indirect searches for dark matter (DM) have conventionally been applied to the products of DM annihilation or decay. If DM couples to light force carriers, however, it can be captured into bound states via dissipation of energy that may yield detectable signals. We extend the indirect searches to DM bound state formation and transitions between bound levels, and constrain the emission of unstable dark photons. Our results significantly refine the predicted signal flux that could be observed in experiments. As a concrete example, we use Fermi-LAT dwarf spheroidal observations to obtain constraints in terms of the dark photon mass and energy which we use to search for the formation of stable or unstable bound states.
Highlights
The radiative formation of dark matter (DM) bound states, as well as exothermic level transitions between bound levels, provide novel sources of signals that can be probed via indirect searches
We employed indirect searches to derive constraints on the formation of DM bound states that occurs with emission of a dark photon kinetically mixed with hypercharge
While the radiated energy in DM annihilation is of the order of the DM mass, Bound-state formation (BSF) occurs with dissipation of a smaller amount of energy that depends on the underlying dynamics
Summary
Most of the dark matter (DM) research in the past decades has focused on DM with contacttype interactions, i.e. interactions mediated by particles of similar or larger mass than the DM itself, mmed mDM. Stable bound states arise typically either due to confining forces (hadronic bound states), and/or due to weak forces in models of asymmetric DM The latter scenario hypothesizes that the DM relic density is, analogously to ordinary matter, due to an excess of dark particles over antiparticles [42,43,44] that cannot be annihilated in the early Universe even if the DM annihilation cross section is very large. The BSF cross sections in galactic environments may exceed the so-called canonical annihilation cross section, σvrel ≈ 3 × 10−26cm3/s, by orders of magnitude due to different reasons These include a large Sommerfeld enhancement at low velocities, and possibly the associated parametric resonances in the case of massive mediators [59], as well as, in the case of asymmetric DM, a larger DM-mediator coupling than that required to attain the observed DM density via freeze-out in the symmetric limit [2].
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