100 research outputs found
Higgs Enhancement for the Dark Matter Relic Density
We consider the long-range effect of the Higgs on the density of
thermal-relic dark matter. While the electroweak gauge boson and gluon exchange
have been previously studied, the Higgs is typically thought to mediate only
contact interactions. We show that the Sommerfeld enhancement due to a 125 GeV
Higgs can deplete TeV-scale dark matter significantly, and describe how the
interplay between the Higgs and other mediators influences this effect. We
discuss the importance of the Higgs enhancement in the Minimal Supersymmetric
Standard Model, and its implications for experiments.Comment: 6 pages, 5 figures; references and minor comments added; matches
published versio
Asymmetric thermal-relic dark matter: Sommerfeld-enhanced freeze-out, annihilation signals and unitarity bounds
Dark matter that possesses a particle-antiparticle asymmetry and has
thermalised in the early universe, requires a larger annihilation cross-section
compared to symmetric dark matter, in order to deplete the dark antiparticles
and account for the observed dark matter density. The annihilation
cross-section determines the residual symmetric component of dark matter, which
may give rise to annihilation signals during CMB and inside haloes today. We
consider dark matter with long-range interactions, in particular dark matter
coupled to a light vector or scalar force mediator. We compute the couplings
required to attain a final antiparticle-to-particle ratio after the thermal
freeze-out of the annihilation processes in the early universe, and then
estimate the late-time annihilation signals. We show that, due to the
Sommerfeld enhancement, highly asymmetric dark matter with long-range
interactions can have a significant annihilation rate, potentially larger than
symmetric dark matter of the same mass with contact interactions. We discuss
caveats in this estimation, relating to the formation of stable bound states.
Finally, we consider the non-relativistic partial-wave unitarity bound on the
inelastic cross-section, we discuss why it can be realised only by long-range
interactions, and showcase the importance of higher partial waves in this
regime of large inelasticity. We derive upper bounds on the mass of symmetric
and asymmetric thermal-relic dark matter for s-wave and p-wave annihilation,
and exhibit how these bounds strengthen as the dark asymmetry increases.Comment: 31 pages, 8 figures. V2: small corrections, added discussion and
reference
Signals from dark atom formation in halos
We consider indirect detection signals of atomic dark matter, with a massive
dark photon which mixes kinetically with hypercharge. In significant regions of
parameter space, dark matter remains at least partially ionized today, and dark
atom formation can occur efficiently in dense regions, such as the centers of
galactic halos. The formation of dark atoms is accompanied by emission of a
dark photon, which can subsequently decay into Standard Model particles. We
discuss the expected signal strength and compare it to that of annihilating
dark matter. As a case study, we explore the possibility that dark atom
formation can account for the observed 511 keV line and outline the relevant
parameter space.Comment: 14 pages, 10 figure
Self-interacting asymmetric dark matter coupled to a light massive dark photon
Dark matter (DM) with sizeable self-interactions mediated by a light species
offers a compelling explanation of the observed galactic substructure;
furthermore, the direct coupling between DM and a light particle contributes to
the DM annihilation in the early universe. If the DM abundance is due to a dark
particle-antiparticle asymmetry, the DM annihilation cross-section can be
arbitrarily large, and the coupling of DM to the light species can be
significant. We consider the case of asymmetric DM interacting via a light (but
not necessarily massless) Abelian gauge vector boson, a dark photon. In the
massless dark photon limit, gauge invariance mandates that DM be
multicomponent, consisting of positive and negative dark ions of different
species which partially bind in neutral dark atoms. We argue that a similar
conclusion holds for light dark photons; in particular, we establish that the
multi-component and atomic character of DM persists in much of the parameter
space where the dark photon is sufficiently light to mediate sizeable DM
self-interactions. We discuss the cosmological sequence of events in this
scenario, including the dark asymmetry generation, the freeze-out of
annihilations, the dark recombination and the phase transition which gives mass
to the dark photon. We estimate the effect of self-interactions in DM haloes,
taking into account this cosmological history. We place constraints based on
the observed ellipticity of large haloes, and identify the regimes where DM
self-scattering can affect the dynamics of smaller haloes, bringing theory in
better agreement with observations. Moreover, we estimate the cosmological
abundance of dark photons in various regimes, and derive pertinent bounds.Comment: v3: published versio
Dark-matter bound states from Feynman diagrams
If dark matter couples directly to a light force mediator, then it may form
bound states in the early universe and in the non-relativistic environment of
haloes today. In this work, we establish a field-theoretic framework for the
computation of bound-state formation cross-sections, de-excitation and decay
rates, in theories with long-range interactions. Using this formalism, we carry
out specific computations for scalar particles interacting either via a light
scalar or vector mediator. At low relative velocities of the interacting
particles, the formation of bound states is enhanced by the Sommerfeld effect.
For particle-antiparticle pairs, we show that bound-state formation can be
faster than annihilation into radiation in the regime where the Sommerfeld
effect is important. The field-theoretic formalism outlined here can be
generalised to compute bound-state formation cross-sections in a variety of
theories, including theories featuring non-Abelian (albeit non-confining)
interactions, such as the electroweak interactions.Comment: 36 pages + appendices + references, 9 figures, 1 table; v2: published
versio
Collider signatures of sterile neutrinos in models with a gauge-singlet Higgs
Sterile neutrinos have been invoked to explain the observed neutrino masses,
but they can also have significant implications for cosmology and accelerator
experiments. We explore the collider signatures of a simple extension of the
Standard Model, where sterile neutrinos acquire their mass after electroweak
symmetry breaking, via their coupling to a real singlet Higgs. In this model,
heavy sterile neutrinos can be produced in accelerators from decays of the
Higgs bosons. Their own decay can yield distinct signals, suggesting both the
presence of an extended Higgs sector and the coupling of the singlet fermions
to the latter. In the same scenario, a relic matter abundance arises from the
decay of the singlet Higgs into weakly coupled keV sterile neutrinos. The
coupling of the Higgs doublet to particles outside the Standard Model relaxes
the current experimental bounds on its mass.Comment: v2: JHEP accepted version, 19 pages, 9 figure
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