Velocity Dependence from Resonant Self-Interacting Dark Matter

Abstract

The dark matter density distribution in small-scale astrophysical objects may indicate that dark matter is self-interacting, while observations from clusters of galaxies suggest that the corresponding cross section depends on the velocity. Using a model-independent approach, we show that resonant self-interacting dark matter (RSIDM) can naturally explain such a behavior. In contrast to what is often assumed, this does not require a light mediator. We present explicit realizations of this mechanism and discuss the corresponding astrophysical constraints.Dark matter (DM) makes up more than 80% of the matter in the Universe today and played a crucial role in forming stars and galaxies, and hence us. Yet its nature is unknown. Currently the best pieces of information come from astrophysical observations. N-body simulations of collisionless DM predict astrophysical halos with DM density following a universal profile that scales as ρ ∝ r −3 in its outskirts but exhibits a central cusp, ρ ∝ r −β , with β 1, referred to as the Navarro-Frenk-White (NFW) profile [1][2][3]. Nevertheless, many studies show hints of a DM mass deficit in the inner regions of certain halos. Notably, observations indicate that numerous dwarf galaxies [4-6] and some low-surface-brightness spiral galaxies [7-9] have a shallower central DM density, better described by a core of constant density, i.e., by β 0. This is known as the core-vs-cusp problem. Although it is more pressing in small-scale objects, shallower DM density profiles -with a slope of β 0.5-have been reported for certain galaxy clusters [10,11]. Moreover, the DM mass deficit also manifests itself in halos that are less dense than what simulations suggest if they host the galaxies that we observe. This is the too-bigto-fail problem, observed for the subhalos of the Milky Way [12], Andromeda [13] and the Local Group [14].Several explanations for these discrepancies have been discussed in the literature. The systematic uncertainties introduced in deriving DM distributions from observations of luminous objects are one of them. Most importantly, the motions of HI gas and stars may not be faithful tracers of the DM circular velocity [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Baryonic processes are another conceivable explanation for the discrepancies, since the aforementioned simulations only include collisionless DM. Solutions along this line include supernova-driven baryonic winds [30][31][32][33], DM heating due to star formation [34], infalling baryonic clumps [35][36][37][38] as well as active galactic nuclei or black holes [39]. Nonetheless, there is no consensus on why systematic uncertainties or baryonic processes lead to a seemingly universal mass deficit at various scales.A more exciting possibility consists of considering DM collisions in the inner regions of astrophysical objects [40]. This is known as self-interacting dark matter (SIDM). N-body simulations [41][42][43][44][45][46] confirm that DM scattering processes indeed reduce the central density of DM halos...

A practical and consistent parametrization of dark matter self-interactions

“…4, we conclude that GeV SIDM is associated with scattering lengths of several fm and that sub-GeV SIDM is either excluded by cluster observations or requires a cross section of around 1 cm 2 /g throughout all scales of interest. While some of these conclusions have been obtained in specific SIDM scenarios such as those involving a light mediator [35] or resonant SIDM [20], we would like to emphasize that these conclusions apply to any model where the effective-range approach applies. Fig.…”

“…The width in general varies with the energy in such way that Γ(E) ∝ E (2 +1)/2 (see e.g. [20]). Using the effectiverange approximation to the phase shift (Eq.…”

Baryogenesis from a dark first-order phase transition

“…address the small-scale structure problems if the resonant velocity is v R ∼ 100 km/s with a constant s-wave cross section with σ/m ∼ 0.1 cm 2 /g [30]. The possibility of this threshold resonance prefers m π − /m p ∼ 0.4, which is also desirable to permit larger dark photon masses.…”

Lightening gravity-mediated dark matter

“…In the case of scalar dark matter, there is an s-channel contribution with the spin-2 mediator too, but it is velocity-suppressed by the overall factor. On the other hand, for vector dark matter, the DM self-scattering cross section could be enhanced at a particular DM velocity due to the s-channel resonance [44,45], so it would be possible to accommodate the velocity-dependent self-interaction, being compatible with galaxy clusters such as Bullet Cluster [22][23][24].…”