728 research outputs found
First-Order Electroweak Phase Transition in the Standard Model with a Low Cutoff
We study the possibility of a first-order electroweak phase transition (EWPT)
due to a dimension-six operator in the effective Higgs potential. In contrast
with previous attempts to make the EWPT strongly first-order as required by
electroweak baryogenesis, we do not rely on large one-loop thermally generated
cubic Higgs interactions. Instead, we augment the Standard Model (SM) effective
theory with a dimension-six Higgs operator. This addition enables a strong
first-order phase transition to develop even with a Higgs boson mass well above
the current direct limit of 114 GeV. The phi^6 term can be generated for
instance by strong dynamics at the TeV scale or by integrating out heavy
particles like an additional singlet scalar field. We discuss conditions to
comply with electroweak precision constraints, and point out how future
experimental measurements of the Higgs self couplings could test the idea.Comment: 5 pages, 4 figures. v2: corrected typos, improved discussion of the
case lambda<0 and added references. To be published in PR
Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach
Gravitational wave production from bubble collisions was calculated in the
early nineties using numerical simulations. In this paper, we present an
alternative analytic estimate, relying on a different treatment of
stochasticity. In our approach, we provide a model for the bubble velocity
power spectrum, suitable for both detonations and deflagrations. From this, we
derive the anisotropic stress and analytically solve the gravitational wave
equation. We provide analytical formulae for the peak frequency and the shape
of the spectrum which we compare with numerical estimates. In contrast to the
previous analysis, we do not work in the envelope approximation. This paper
focuses on a particular source of gravitational waves from phase transitions.
In a companion article, we will add together the different sources of
gravitational wave signals from phase transitions: bubble collisions,
turbulence and magnetic fields and discuss the prospects for probing the
electroweak phase transition at LISA.Comment: 48 pages, 14 figures. v2 (PRD version): calculation refined; plots
redone starting from Fig. 4. Factor 2 in GW energy spectrum corrected. Main
conclusions unchanged. v3: Note added at the end of paper to comment on the
new results of 0901.166
Dirac Neutrino Dark Matter
We investigate the possibility that dark matter is made of heavy Dirac
neutrinos with mass in the range [O(1) GeV- a few TeV] and with suppressed but
non-zero coupling to the Standard Model Z as well as a coupling to an
additional Z' gauge boson. The first part of this paper provides a
model-independent analysis for the relic density and direct detection in terms
of four main parameters: the mass, the couplings to the Z, to the Z' and to the
Higgs. These WIMP candidates arise naturally as Kaluza-Klein states in
extra-dimensional models with extended electroweak gauge group SU(2)_L* SU(2)_R
* U(1). They can be stable because of Kaluza-Klein parity or of other discrete
symmetries related to baryon number for instance, or even, in the low mass and
low coupling limits, just because of a phase-space-suppressed decay width. An
interesting aspect of warped models is that the extra Z' typically couples only
to the third generation, thus avoiding the usual experimental constraints. In
the second part of the paper, we illustrate the situation in details in a
warped GUT model.Comment: 35 pages, 25 figures; v2: JCAP version; presentation and plots
improved, results unchange
Elastic Scattering and Direct Detection of Kaluza-Klein Dark Matter
Recently a new dark matter candidate has been proposed as a consequence of
universal compact extra dimensions. It was found that to account for
cosmological observations, the masses of the first Kaluza-Klein modes (and thus
the approximate size of the extra dimension) should be in the range 600-1200
GeV when the lightest Kaluza-Klein particle (LKP) corresponds to the
hypercharge boson and in the range 1 - 1.8 TeV when it corresponds to a
neutrino. In this article, we compute the elastic scattering cross sections
between Kaluza-Klein dark matter and nuclei both when the lightest Kaluza-Klein
particle is a KK mode of a weak gauge boson, and when it is a neutrino. We
include nuclear form factor effects which are important to take into account
due to the large LKP masses favored by estimates of the relic density. We
present both differential and integrated rates for present and proposed
Germanium, NaI and Xenon detectors. Observable rates at current detectors are
typically less than one event per year, but the next generation of detectors
can probe a significant fraction of the relevant parameter space.Comment: 23 pages, 11 figures; v2,v3: Ref. added, discussion improved,
conclusions unchanged. v4: Introduction was expanded to be more appropriate
for non experts. Various clarifications added in the text. Version to be
published in New Journal of Physic
Indirect search for Dark Matter with H.E.S.S
Observations of the Galactic center region with the H.E.S.S. telescopes have
established the existence of a steady, extended source of gamma-ray emission
coinciding with the position of the super massive black hole Sgr A*. This is a
remarkable finding given the expected presence of dense self-annihilating Dark
Matter in the Galactic center region. The self-annihilation process is giving
rise to gamma-ray production through hadronization including the production of
neutral pions which decay into gamma-rays but also through (loop-suppressed)
annihilation into final states of almost mono-energetic photons. We study the
observed gamma-ray signal (spectrum and shape) from the Galactic center in the
context of Dark Matter annihilation and indicate the prospects for further
indirect Dark matter searches with H.E.S.S.Comment: 9 pages, 3 figures. Accepted for Publication in Advances is Space
Research, COSPAR meeting Beijing (2006
Observation of vortex coalescence in the anisotropic spin-triplet superconductor SrRuO
We present direct imaging of magnetic flux structures in the anisotropic,
spin-triplet superconductor SrRuO using a scanning SQUID
microscope. Individual quantized vortices were seen at low magnetic fields.
Coalescing vortices forming flux domains were revealed at intermediate fields.
Based on our observations we suggest that a mechanism intrinsic to the material
stabilizes the flux domains against the repulsive vortex-vortex interaction.
Topological defects like domain walls can provide this, implying proof for
unconventional chiral superconductivity.Comment: submitted to PR
The Indirect Search for Dark Matter with IceCube
We revisit the prospects for IceCube and similar kilometer-scale telescopes
to detect neutrinos produced by the annihilation of weakly interacting massive
dark matter particles (WIMPs) in the Sun. We emphasize that the astrophysics of
the problem is understood; models can be observed or, alternatively, ruled out.
In searching for a WIMP with spin-independent interactions with ordinary
matter, IceCube is only competitive with direct detection experiments if the
WIMP mass is sufficiently large. For spin-dependent interactions IceCube
already has improved the best limits on spin-dependent WIMP cross sections by
two orders of magnitude. This is largely due to the fact that models with
significant spin-dependent couplings to protons are the least constrained and,
at the same time, the most promising because of the efficient capture of WIMPs
in the Sun. We identify models where dark matter particles are beyond the reach
of any planned direct detection experiments while being within reach of
neutrino telescopes. In summary, we find that, even when contemplating recent
direct detection results, neutrino telescopes have the opportunity to play an
important as well as complementary role in the search for particle dark matter.Comment: 17 pages, 10 figures, published in the New Journal of Physics 11
105019 http://www.iop.org/EJ/abstract/1367-2630/11/10/105019, new version
submitted to correct Abstract in origina
Baryon Number in Warped GUTs : Model Building and (Dark Matter Related) Phenomenology
In the past year, a new non-supersymmetric framework for electroweak symmetry
breaking (with or without Higgs) involving SU(2)_L * SU(2)_R * U(1)_{B-L} in
higher dimensional warped geometry has been suggested. In this work, we embed
this gauge structure into a GUT such as SO(10) or Pati-Salam. We showed
recently (in hep-ph/0403143) that in a warped GUT, a stable Kaluza-Klein
fermion can arise as a consequence of imposing proton stability. Here, we
specify a complete realistic model where this particle is a weakly interacting
right-handed neutrino, and present a detailed study of this new dark matter
candidate, providing relic density and detection predictions. We discuss
phenomenological aspects associated with the existence of other light (<~ TeV)
KK fermions (related to the neutrino), whose lightness is a direct consequence
of the top quark's heaviness. The AdS/CFT interpretation of this construction
is also presented. Most of our qualitative results do not depend on the nature
of the breaking of the electroweak symmetry provided that it happens near the
TeV brane.Comment: 61 pages, 12 figures; v2: minor changes; v3: Two additional diagrams
in Fig. 10; a numerical factor corrected in section 16.1 (baryogenesis
section), corresponding discussion slightly modified but qualitative results
unchange
Indirect Detection of Kaluza-Klein Dark Matter from Latticized Universal Dimensions
We consider Kaluza-Klein dark matter from latticized universal dimensions. We
motivate and investigate two different lattice models, where the models differ
in the choice of boundary conditions. The models reproduce relevant features of
the continuum model for Kaluza-Klein dark matter. For the model with simple
boundary conditions, this is the case even for a model with only a few lattice
sites. We study the effects of the latticization on the differential flux of
positrons from Kaluza-Klein dark matter annihilation in the galactic halo. We
find that for different choices of the compactification radius, the
differential positron flux rapidly converges to the continuum model results as
a function of the number of lattice sites. In addition, we consider the
prospects for upcoming space-based experiments such as PAMELA and AMS-02 to
probe the latticization effect.Comment: 25 pages, 9 figures, LaTeX. Final version published in JCA
- …