54 research outputs found
White dwarfs constraints on dark sector models with light particles
The white dwarf luminosity function is well understood in terms of standard
model physics and leaves little room for exotic cooling mechanisms related to
the possible existence of new weakly interacting light particles. This puts
significant constraints on the parameter space of models that contain a massive
dark photon and light dark sector particles.Comment: CETUP*2013 proceeding
Singlet Scalar Dark Matter: monochromatic gamma rays and metastable vacua
We calculate the pair-annihilation cross section of real scalar singlet dark
matter into two mono-energetic photons. We derive constraints on the theory
parameter space from the Fermi limits on gamma-ray lines, and we compare with
current limits from direct dark matter detection. We show that the new limits,
albeit typically relevant only when the dark matter mass is close to half the
Standard Model Higgs mass, rule out regions of the theory parameter space that
are otherwise not constrained by other observations or experiments. In
particular, the new excluded regions partly overlap with the parameter space
where real scalar singlet dark matter might explain the anomalous signals
observed by CDMS. We also calculate the lifetime of unstable vacuum
configurations in the scalar potential, and show that the gamma-ray limits are
quite relevant in regions where the electro-weak vacuum is meta-stable with a
lifetime longer than the age of the universe.Comment: 21 pages, 6 figures; references added, minor additions to text and
figures, version accepted for publication in Phys. Rev.
Probing Dark Matter with AGN Jets
We study the possibility of detecting a signature of particle dark matter in
the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting
from the scattering of high-energy particles in the AGN jet off of dark matter
particles. We consider particle dark matter models in the context of both
supersymmetry and universal extra-dimensions (UED), and we present the complete
lowest-order calculation for processes where a photon is emitted in dark
matter-electron and/or dark matter-proton scattering, where electrons and
protons belong to the AGN jet. We find that the process is dominated by a
resonance whose energy is dictated by the particle spectrum in the dark matter
sector (neutralino and selectron for the case of supersymmetry, Kaluza-Klein
photon and electron for UED). The resulting gamma-ray spectrum exhibits a very
characteristic spectral feature, consisting of a sharp break to a hard
power-law behavior. Although the normalization of the gamma-ray flux depends
strongly on assumptions on both the AGN jet geometry, composition and particle
spectrum as well as on the particle dark matter model and density distribution,
we show that for realistic parameters choices, and for two prominent nearby
AGNs (Centaurus A and M87), the detection of this effect is in principle
possible. Finally, we compare our predictions and results with recent gamma-ray
observations from the Fermi, H.E.S.S. and VERITAS telescopes.Comment: 23 pages, 13 figures, submitte
Dynamics of Relaxed Inflation
The cosmological relaxation of the electroweak scale has been proposed as a
mechanism to address the hierarchy problem of the Standard Model. A field, the
relaxion, rolls down its potential and, in doing so, scans the squared mass
parameter of the Higgs, relaxing it to a parametrically small value. In this
work, we promote the relaxion to an inflaton. We couple it to Abelian gauge
bosons, thereby introducing the necessary dissipation mechanism which slows
down the field in the last stages. We describe a novel reheating mechanism,
which relies on the gauge-boson production leading to strong electromagnetic
fields, and proceeds via the vacuum production of electron-positron pairs
through the Schwinger effect. We refer to this mechanism as Schwinger
reheating. We discuss the cosmological dynamics of the model and the
phenomenological constraints from CMB and other experiments. We find that a
cutoff close to the Planck scale may be achieved. In its minimal form, the
model does not generate sufficient curvature perturbations and additional
ingredients, such as a curvaton field, are needed.Comment: 39 pages, 3 figures, appendices D and E added, published in JHE
Is the Relaxion an Axion?
We consider the recently proposed cosmological relaxation mechanism where the
hierarchy problem is ameliorated, and the electroweak scale is dynamically
selected by a slowly rolling axion field. We argue that, in its simplest form,
the construction breaks a gauge symmetry that always exists for
pseudo-Nambu-Goldstone bosons (in particular the axion). The small parameter in
the relaxion model is therefore not technically natural as it breaks a gauge
symmetry rather than global symmetries only. The consistency of the theory
generically implies that the cutoff must lie around the electroweak scale, but
not qualitatively higher. We discuss several ways to evade the above
conclusion. Some of them may be sufficient to increase the cutoff to the
few-TeV range (and therefore may be relevant for the little-hierarchy problem).
To demonstrate the ideas in a concrete setting we consider a model with a
familon, the Nambu-Goldstone boson of a spontaneously broken chiral flavor
symmetry. The model has some interesting collider-physics aspects and contains
a viable weakly interacting dark matter candidate.Comment: some typos fixed, clarifications adde
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