John March-Russell

Abstract:

We argue for the existence of bounds on the tensions of p-branes in de Sitter space in terms of the Hubble rate and the strength of a class of Chern-Simons-like couplings. The world-volume couplings involve Abelian 1-form gauge fields in the bulk and possibly field strengths intrinsic to the brane. In many cases these couplings are the D-brane Chern-Simons terms present in string theory, while in other cases they are the interactions of axion domain walls with U(1) fields. Our arguments use the same logic and assumptions as the recent Festina Lente proposal (thus utilizing the properties of Nariai de Sitter black holes) and generalize it to extended objects, thereby providing a bottom-up set of constraints independent of any particular UV completion. We compare these bounds to the properties of (wrapped) D-branes in Type II string theory in the weak coupling limit, under the assumption that these properties are not modified significantly in de Sitter constructions. We find that all constraints are satisfied by D-branes, providing further evidence for the Festina Lente conjecture. For the particular case of 2-branes with Chern-Simons interactions we obtain a bound, which however can be evaded if the theory contains a light axion. Similarly, we find the bounds do not apply to axion domain walls due to the presence of the axion.

Abstract:

Direct detection experiments aimed at uncovering the elusive nature of dark matter (DM) have made significant progress in probing ever lower cross-sections for DM-nucleon interactions. At the same time, an upper limit in the cross-section sensitivity region is present due to DM scattering in the Earth and atmosphere and as a result never reaching the detector. We investigate the impact of this effect for both spin-dependent and spin-independent interactions. In contrast to previous studies that assume a straight line path for DM scattering we employ a semi-analytic diffusion model that takes into account the impact of potentially large angle deviations prevalent for light DM masses. We find that for sufficiently low energy thresholds, this difference in modelling impacts the DM interaction cross-section sensitivity. This study evaluates the impact in the context of the QUEST-DMC experiment, which utilises surface-based detectors with superfluid Helium-3 bolometers to search for sub-GeV DM exploiting low energy threshold. At masses below 1 GeV/c^2 the deviation between the two frameworks becomes pronounced. The ceiling sensitivity limit for QUEST-DMC on spin-dependent DM-neutron cross-sections is ∼ 3 × 10^-24cm^2 using the diffusive framework and approximately doubles with the straight-line path DM scattering. Similarly, for spin-independent DM-nucleon cross-sections, the ceiling limit is ∼ 7.5 × 10^-27cm^2 under the diffusive framework and also increases about a factor of two with the straight-line path approximation, within the mass range of 0.025–5 GeV/c^2.

Abstract:

We study the dynamics and interactions of the solitonic domain walls that occur in realistic axion electrodynamics models including the Chern-Simons interaction, aϵμνλσFμνFλσ, between an axion a(x) of mass ma, and a massless U(1) gauge field, e.g. EM, interacting with strength α = e2/4π with charged matter, e.g. electron-positron pairs. In particular, in the presence of a U(1) gauge-and-matter relativistic thermal plasma we study the friction experienced by the walls due to the Chern-Simons term. Utilizing the linear response method we include the collective effects of the plasma, as opposed to purely particle scattering across the wall (as is done in previous treatments) which is valid only in the thin wall regime that is rarely applicable in realistic cases. We show that the friction depends on the Lorentz-γ-factor-dependent inverse thickness of the wall in the plasma frame, ℓ−1 ~ γma, compared to the three different plasma scales, the temperature T, the Debye mass mD ~ αT, and the damping rate Γ ~ α2T, and elucidate the underlying physical intuition for this behavior. (For friction in the thin-wall-limit we correct previous expressions in the literature.) We further consider the effects of long-range coherent magnetic fields that are possibly present in the early universe and compare their effect with that of thermal magnetic fields. We comment on the changes to our results that likely apply in the thermal deconfined phase of a non-Abelian gauge theory. Finally, we briefly discuss the possible early universe consequences of our results for domain wall motion and network decay, stochastic gravitational wave production from domain wall networks, and possible primordial black hole production from domain wall collapse, though a more complete discussion of these topics is reserved for a companion paper.