3,456 research outputs found
Light Stop Searches at the LHC in Events with two b-Jets and Missing Energy
We propose a new method to discover light top squarks (stops) in the
co-annihilation region at the Large Hadron Collider (LHC). The bino-like
neutralino is the lightest supersymmetric particle (LSP) and the lighter stop
is the next-to-LSP. Such scenarios can be consistent with electroweak
baryogenesis and also with dark matter constraints. We consider the production
of two stops in association with two b-quarks, including pure QCD as well as
mixed electroweak-QCD contributions. The stops decay into a charm quark and the
LSP. For a higgsino-like light chargino the electroweak contributions can
exceed the pure QCD prediction. We show the size of the electroweak
contributions as a function of the stop mass and present the LHC discovery
reach in the stop-neutralino mass plane.Comment: 12 pages, 10 figure
Development of the Universe and New Cosmology
Cosmology is undergoing an explosive period of activity, fueled both by new,
accurate astrophysical data and by innovative theoretical developments.
Cosmological parameters such as the total density of the Universe and the rate
of cosmological expansion are being precisely measured for the first time, and
a consistent standard picture of the Universe is beginning to emerge. Recent
developments in cosmology give rise the intriguing possibility that all
structures in the Universe, from superclusters to planets, had a
quantum-mechanical origin in its earliest moments. Furthermore, these ideas are
not idle theorizing, but predictive, and subject to meaningful experimental
test. We review the concordance model of the development of the Universe, as
well as evidence for the observational revolution that this field is going
through. This already provides us with important information on particle
physics, which is inaccessible to accelerators.Comment: 9 pages; The German translation of this mini review can be uploaded
from http://www.cc.ethz.ch/bulletin/ New references are added with respective
minor changes of tex
Isocurvature bounds on axions revisited
The axion is one of the best motivated candidates for particle dark matter.
We study and update the constraints imposed by the recent CMB and LSS
experiments on the mass of axions produced by the misalignment mechanism, as a
function of both the inflationary scale and the reheating temperature. Under
some particular although not unconventional assumptions, the axionic field
induces too large isocurvature perturbations. Specifically, for inflation
taking place at intermediate energy scales, we derive some restrictive limits
which can only be evaded by assuming an efficient reheating mechanism, with
T>10^{11} GeV. Chaotic inflation with a quadratic potential is still compatible
with the axion scenario, provided that the Peccei-Quinn scale f_a is close to
10^{10} or 10^{11} GeV. Isocurvature bounds eliminate the possibility of a
larger f_a and a small misalignment angle. We find that isocurvature
constraints on the axion scenario must be taken into account whenever the scale
of inflation is above 10^{12} GeV; below this scale, axionic isocurvature modes
are too small to be probed by current observations.Comment: 12 pages, 3 figures; more concise version, new figures, accepted in
PR
CP Violation in Fourth Generation Quark Decays
We show that, if a fourth generation is discovered at the Tevatron or LHC,
one could study CP violation in b' \to s decays. Asymmetries could reach 30%
for b'\to sZ for m_{b'} \lesssim 350 GeV, while it could be greater than 50%
for b'\to s\gamma and extend to higher m_{b'}. Branching ratios are
10^{-3}--10^{-5}, and CPV measurement requires tagging. Once measured, however,
the CPV phase can be extracted with little theoretical uncertainty.Comment: 4 pages, 7 eps figure
Single Field Baryogenesis
We propose a new variant of the Affleck-Dine baryogenesis mechanism in which
a rolling scalar field couples directly to left- and right-handed neutrinos,
generating a Dirac mass term through neutrino Yukawa interactions. In this
setup, there are no explicitly CP violating couplings in the Lagrangian. The
rolling scalar field is also taken to be uncharged under the quantum
numbers. During the phase of rolling, scalar field decays generate a
non-vanishing number density of left-handed neutrinos, which then induce a net
baryon number density via electroweak sphaleron transitions.Comment: 4 pages, LaTe
Phase transition in a supersymmetric axion model
In a supersymmetric axion model where the scale for both supersymmetry
breaking and Peccei-Quinn symmetry breaking is around GeV, we find
that there is a reasonable parameter space for a strongly first order phase
transition at the scale.Comment: 5 pages, 1 figur
Leptogenesis from Supersymmetry Breaking
We show that soft supersymmetry breaking terms involving the heavy sneutrinos
can lead to sneutrino-antisneutrino mixing and to new sources of CP violation,
which are present even if a single generation is considered. These terms are
naturally present in supersymmetric versions of leptogenesis scenarios, and
they induce indirect CP violation in the decays of the heavy sneutrinos,
eventually generating a baryon asymmetry. This new contribution can be
comparable to or even dominate over the asymmetry produced in traditional
leptogenesis scenarios.Comment: 4 pages; An improved discussion of the relevant numerical range of
the soft breaking terms (in agreement with hep-ph/0308031
Squeezed States and Affleck Dine Baryogenesis
Quantum fluctuations in the post inflationary Affleck-Dine baryogenesis model
are studied. The squeezed states formalism is used to give evolution equations
for the particle and anti-particle modes in the early universe. The role of
expansion and parametric amplification of the quantum fluctuations on the
baryon asymmetry produced is investigated.Comment: 8 pages 9 figure
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