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Register a new userDark Matter Sommerfeld-enhanced annihilation and Bound-state decay at finite temperature
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Traditional computations of the dark matter (DM) relic abundance, for models where attractive self-interactions are mediated by light force-carriers and bound states exist, rely on the solution of a coupled system of classical on-shell Boltzmann equations. This idealized description misses important thermal effects caused by the tight coupling among force-carriers and other charged relativistic species. We develop for the first time a comprehensive ab-initio derivation for the description of DM long-range interactions in the presence of a hot and dense plasma background directly from non-equilibrium quantum field theory. Most importantly, the scattering and bound states get strongly mixed in the thermal plasma environment, representing a characteristic difference from a pure vacuum theory computation. The main result of this work is a novel differential equation for the DM number density, written down in a form which is manifestly independent under the choice of what one would interpret as a bound or a scattering state at finite temperature. The collision term, unifying the description of annihilation and bound state decay, turns out to have in general a non-quadratic dependence on the DM number density. This generalizes the form of the conventional Lee-Weinberg equation which is typically adopted to describe the freeze-out process. We prove that our general number density equation is consistent with previous literature results under certain limits.
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We point out a selection rule for enhancement (suppression) of odd (even) partial waves of dark matter coannihilation or annihilation using Sommerfeld effect. Using this, the usually velocity-suppressed p-wave annihilation can dominate the annihilation signals in the present Universe. The selection mechanism is a manifestation of the exchange symmetry of identical incoming particles, and generic for multi-state DM with off-diagonal long-range interactions. As a consequence, the relic and late-time annihilation rates are parametrically different and a distinctive phenomenology, with large but strongly velocity-dependent annihilation rates, is predicted.
The thermal decoupling description of dark matter (DM) and co-annihilating partners is reconsidered. If DM is realized at around the TeV-mass region or above, even the heaviest electroweak force carriers could act as long-range forces, leading to the existence of meta-stable DM bound states. The formation and subsequent decay of the latter further deplete the relic density during the freeze-out process on top of the Sommerfeld enhancement, allowing for larger DM masses. While so far the bound-state formation was described via the emission of an on-shell mediator ($W^{pm}$, $Z$, $H$, $g$, photon or exotic), we point out that this particular process does not have to be the dominant scattering-bound state conversion channel in general. If the mediator is coupled in a direct way to any relativistic species present in the Early Universe, the bound-state formation can efficiently occur through particle scattering, where a mediator is exchanged virtually. To demonstrate that such a virtually stimulated conversion process can dominate the on-shell emission even for all temperatures, we analyze a simplified model where DM is coupled to only one relativistic species in the primordial plasma through an electroweak-scale mediator. We find that the bound-state formation cross section via particle scattering can exceed the on-shell emission by up to several orders of magnitude.
We propose a novel mechanism to realize two-component asymmetric dark matter of very different mass scales through bound state formation and late freeze-in decay. Assuming a particle-antiparticle asymmetry is initially shared by SM baryons and two dark matter components, we demonstrate that the existence of bound states formed by the heavy component can efficiently transfer the asymmetry from the heavy to the light component via late decay. In this case, the energy densities of the two components can be comparable, and the correct relic density is reproduced.
We demonstrate a new model which uses an ADD type braneworld scenario to produce a multi-state theory of dark matter. Compactification of the extra dimensions onto a sphere leads to the association of a single complex scalar in the bulk with multiple Kaluza-Klein towers in an effective four-dimensional theory. A mutually interacting multi-state theory of dark matter arises naturally within which the dark matter states are identified with the lightest Kaluza-Klein particles of fixed magnetic quantum number. These states are protected from decay by a combination of a global U(1) symmetry and the continuous rotational symmetry about the polar axis of the spherical geometry. We briefly discuss the relic abundance calculation and investigate the spin-independent elastic scattering off nucleons of the lightest and next-to-lightest dark matter states.
We study symmetry restoration at finite temperature in the standard model during the electroweak phase transition in the presence of a weak magnetic field. We compute the finite temperature effective potential up to the contribution of ring diagrams, using the broken phase degrees of freedom, and keep track of the gauge parameter dependence of the results. We show that under these conditions, the phase transition becomes stronger first order.
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