1,741 research outputs found
Doublet-Triplet Fermionic Dark Matter
We extend the Standard Model (SM) by adding a pair of fermionic
SU(2)-doublets with opposite hypercharge and a fermionic SU(2)-triplet with
zero hypercharge. We impose a discrete Z_2-symmetry that distinguishes the SM
fermions from the new ones. Then, gauge invariance allows for two
renormalizable Yukawa couplings between the new fermions and the SM Higgs
field, as well as for direct masses for the doublet (M_D) and the triplet
(M_T). After electroweak symmetry breaking, this model contains, in addition to
SM particles, two charged Dirac fermions and a set of three neutral Majorana
fermions, the lightest of which contributes to Dark Matter (DM). We consider a
case where the lightest neutral fermion is an equal admixture of the two
doublets with mass M_D close to the Z-boson mass. This state remains stable
under radiative corrections thanks to a custodial SU(2)-symmetry and is
consistent with the experimental data from oblique electroweak corrections.
Moreover, the amplitudes relevant to spin-dependent or independent nucleus-DM
particle scattering cross section both vanish at tree level. They arise at one
loop at a level that may be observed in near future DM direct detection
experiments. For Yukawa couplings comparable to the top-quark, the DM particle
relic abundance is consistent with observation, not relying on co-annihilation
or resonant effects and has a mass at the electroweak scale. Furthermore, the
heavier fermions decay to the DM particle and to electroweak gauge bosons
making this model easily testable at the LHC. In the regime of interest, the
charged fermions suppress the Higgs decays to diphoton by 45-75 % relative to
SM prediction.Comment: 40 pages, v2: discussion and references on dark matter direct
detection expanded, matches published version, v3: formulae in Appendix A
correcte
Anatomy of the Higgs boson decay into two photons in the unitary gauge
In this work, we review and clarify computational issues about the W-gauge
boson one-loop contribution to the H -> gamma gamma decay amplitude, in the
unitary gauge and in the Standard Model. We find that highly divergent
integrals depend upon the choice of shifting momenta with arbitrary vectors.
One particular combination of these arbitrary vectors reduces the superficial
divergency down to a logarithmic one. The remaining ambiguity is then fixed by
exploiting gauge invariance and the Goldstone Boson Equivalence Theorem. Our
method is strictly realised in four-dimensions. The result for the amplitude
agrees with the "famous" one obtained using dimensional regularisation (DR) in
the limit d-> 4, where d is the number of spatial dimensions in Euclidean
space. At the exact equality d=4, a three-sphere surface term appears that
renders the Ward Identities and the equivalence theorem inconsistent. We also
examined a recently proposed four-dimensional regularisation scheme and found
agreement with the DR outcome.Comment: 1+20 pages, 2 figures, v2 few references added, Appendix A expanded,
v3 matches published versio
Properties of extra-planar HI clouds in the outer part of the Milky Way
There is mounting evidence for an extra-planar gas layer around the Milky Way
disk, similar to the anomalous HI gas detected in a few other galaxies. As much
as 10% of the gas may be in this phase. We analyze HI clouds located in the
disk-halo interface outside the solar circle to probe the properties of the
extra-planar HI gas, which is following Galactic rotation. We use the
Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for HI clouds which
take part in the rotation of the Galactic plane, but are located above the disk
layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of
the HI halo clouds are studied in detail for their small scale structure using
the Westerbork Synthesis Radio Telescope (WSRT). Data from the 100m telescope
allow for the parameterization of 25 distinct HI halo clouds at Galactocentric
radii 10 kpc <R<15 kpc and heights 1 kpc <z<5 kpc. The clouds have a median
temperature of 620 K, column densities of NH~10E19 cm^-2, and most of them are
surrounded by an extended envelope of warmer HI gas. Interferometer
observations for two selected regions resolve the HI clouds into several
arc-minute sized cores. These cores show narrow line widths (FWHM ~3 km/s),
they have volume densities of n > 1.3 cm^-3, masses up to 24 M_{sol}, and are
on average in pressure equilibrium with the surrounding envelopes. Pressures
and densities fall within the expectations from theoretical phase diagrams (P
vs ). The HI cores tend to be unstable if one assumes a thermally
bistable medium, but are in better agreement with models that predict thermal
fragmentation driven by a turbulent flow.Comment: 9 pages, 5 figures, 3 tables, Accepted for publication in A&
Radiative Light Dark Matter
We present a Peccei-Quinn (PQ)-symmetric two-Higgs doublet model that
naturally predicts a fermionic singlet dark matter in the mass range 10 keV-1
GeV. The origin of the smallness of the mass of this light singlet fermion
arises predominantly at the one-loop level, upon soft or spontaneous breakdown
of the PQ symmetry via a complex scalar field in a fashion similar to the
so-called Dine-Fischler-Sredniki-Zhitnitsky axion model. The mass generation of
this fermionic Radiative Light Dark Matter (RLDM) requires the existence of two
heavy vector-like SU(2) isodoublets, which are not charged under the PQ
symmetry. We show how the RLDM can be produced via the freeze-in mechanism,
thus accounting for the missing matter in the Universe. Finally, we briefly
discuss possible theoretical and phenomenological implications of the RLDM
model for the strong CP problem and the CERN Large Hadron Collider (LHC).Comment: 17 pages, v2: typos corrected, matches published versio
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