23,536 research outputs found
Confining the Electroweak Model to a Brane
We introduce a simple scenario where, by starting with a five-dimensional
SU(3) gauge theory, we end up with several 4-D parallel branes with localized
fermions and gauge fields. Similar to the split fermion scenario, the
confinement of fermions is generated by the nontrivial topological solution of
a SU(3) scalar field. The 4-D fermions are found to be chiral, and to have
interesting properties coming from their 5-D group representation structure.
The gauge fields, on the other hand, are localized by loop corrections taking
place at the branes produced by the fermions. We show that these two confining
mechanisms can be put together to reproduce the basic structure of the
electroweak model for both leptons and quarks. A few important results are:
Gauge and Higgs fields are unified at the 5-D level; and new fields are
predicted: One left-handed neutrino with zero-hypercharge, and one massive
vector field coupling together the new neutrino with other left-handed leptons.
The hierarchy problem is also addressed.Comment: 9 pages, 8 figures; references added; version published in PR
Meron-cluster simulation of the quantum antiferromagnetic Heisenberg model in a magnetic field in one- and two-dimensions
Motivated by the numerical simulation of systems which display quantum phase
transitions, we present a novel application of the meron-cluster algorithm to
simulate the quantum antiferromagnetic Heisenberg model coupled to an external
uniform magnetic field both in one and in two dimensions. In the infinite
volume limit and at zero temperature we found numerical evidence that supports
a quantum phase transition very close to the critical values and
for the system in one and two dimensions, respectively. For the one
dimensional system, we have compared the numerical data obtained with
analytical predictions for the magnetization density as a function of the
external field obtained by scaling-behaviour analysis and Bethe Ansatz
techniques. Since there is no analytical solution for the two dimensional case,
we have compared our results with the magnetization density obtained by scaling
relations for small lattice sizes and with the approximated thermodynamical
limit at zero temperature guessed by scaling relations. Moreover, we have
compared the numerical data with other numerical simulations performed by using
different algorithms in one and two dimensions, like the directed loop method.
The numerical data obtained are in perfect agreement with all these previous
results, which confirms that the meron-algorithm is reliable for quantum Monte
Carlo simulations and applicable both in one and two dimensions. Finally, we
have computed the integrated autocorrelation time to measure the efficiency of
the meron algorithm in one dimension.Comment: 18 pages, 11 figure
The string swampland constraints require multi-field inflation
An important unsolved problem that affects practically all attempts to
connect string theory to cosmology and phenomenology is how to distinguish
effective field theories belonging to the string landscape from those that are
not consistent with a quantum theory of gravity at high energies (the "string
swampland"). It was recently proposed that potentials of the string landscape
must satisfy at least two conditions, the "swampland criteria", that severely
restrict the types of cosmological dynamics they can sustain. The first
criterion states that the (multi-field) effective field theory description is
only valid over a field displacement (in units where the Planck mass is 1), measured as a distance in the
target space geometry. A second, more recent, criterion asserts that, whenever
the potential is positive, its slope must be bounded from below, and
suggests . A recent analysis
concluded that these two conditions taken together practically rule out
slow-roll models of inflation. In this note we show that the two conditions
rule out inflationary backgrounds that follow geodesic trajectories in field
space, but not those following curved, non-geodesic, trajectories (which are
parametrized by a non-vanishing bending rate of the multi-field
trajectory). We derive a universal lower bound on (relative to the
Hubble parameter ) as a function of and the number of efolds
, assumed to be at least of order 60. If later studies confirm and
to be strictly , the bound implies strong turns with
. Slow-roll inflation in the landscape is not
ruled out, but it is strongly multi-field.Comment: v1: 15 pages; v2: 16 pages, references added, improved discussions,
version accepted for publication in JCA
Gauge-Higgs unification on the brane
From the quantum field theory point of view, matter and gauge fields are
generally expected to be localised around branes or topological defects
occurring in extra dimensions. Here I discuss a simple scenario where, by
starting with a five dimensional SU(3) gauge theory, we end up with several 4-D
parallel branes with localised "chiral" fermions and gauge fields to them. I
will show that it is possible to reproduce the electroweak model confined to a
single brane, allowing a simple and geometrical approach to the fermion
hierarchy problem. Some nice results of this construction are: Gauge and Higgs
fields are unified at the 5-D level; and new particles are predicted: a
left-handed neutrino of zero hypercharge, and a massive vector field coupling
together the new neutrino to other left-handed leptons.Comment: Contribution to the proceedings of the RTN workshop "The Quest for
Unification: Theory Confronts Experiment", Corfu, Greece, Sept 11-18, 200
Sensitivity-based multistep MPC for embedded systems
In model predictive control (MPC), an optimization problem is solved every sampling instant to determine an optimal control for a physical system. We aim to accelerate this procedure for fast systems applications and address the challenge of implementing the resulting MPC scheme on an embedded system with limited computing power. We present the sensitivity-based multistep MPC, a strategy which considerably reduces the computing requirements in terms of floating point operations (FLOPs), compared to a standard MPC formulation, while fulfilling closed- loop performance expectations. We illustrate by applying the method to a DC-DC converter model and show how a designer can optimally trade off closed-loop performance considerations with computing requirements in order to fit the controller into a resource-constrained embedded system
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