757 research outputs found
The Origin of Spontaneous Symmetry Breaking in Theories with Large Extra Dimensions
We suggest that the electroweak Higgs particles can be identified with
extra-dimensional components of the gauge fields, which after compactification
on a certain topologically non-trivial background become tachyonic and
condense. If the tachyonic mass is a tree level effect, the natural scale of
the gauge symmetry breaking is set by the inverse radius of the internal space,
which, in case of the electroweak symmetry, must be around TeV. We
discuss the possibility of a vanishing tree level mass for the Higgs. In such a
scenario the tachyonic mass can be induced by quantum loops and can be
naturally smaller than the compactification scale. We give an example in which
this possibility can be realized. Starting from an Einstein--Yang--Mills theory
coupled to fermions in 10-dimensions, we are able to reproduce the spectrum of
the Standard Model like chiral fermions and Higgs type scalars in 4-dimensions
upon compactifying on . The existence of
a monopole solution on and a self dual U(1) instanton on
are essential in obtaining chiral fermions as well as
tachyonic or massless scalars in 4-dimensions. We give a simple rule which
helps us to identify the presence of tachyons on the monopole background on
.Comment: 33 pages. Version accepted for publication in Phys.Rev.
Relativistic predictions of spin observables for exclusive proton knockout reactions
Within the framework of the relativistic distorted wave impulse approximation
(DWIA), we investigate the sensitivity of complete sets of polarization
transfer observables for exclusive proton knockout from the 3s,
2d and 2d states in Pb, at an incident laboratory
kinetic energy of 202 MeV, and for coincident coplanar scattering angles
(, ), to different distorting optical potentials,
finite-range (FR) versus zero-range (ZR) approximations to the DWIA, as well as
medium-modified meson-nucleon coupling constants and meson masses. Results are
also compared to the nonrelativistic DWIA predictions based on the
Schr\"{o}dinger equation.Comment: Submitted for publication to Physicical Review C, 23 pages, 7 figure
Supersymmetry breaking on orbifolds from Wilson lines
We consider five dimensional theories compactified on the orbifold S^1/Z_2
and prove that spontaneous local supersymmetry breaking by Wilson lines and by
the Scherk-Schwarz mechanism are equivalent. Wilson breaking is triggered by
the SU(2)_R symmetry which is gauged in off-shell N=2 supergravity by auxiliary
fields. The super-Higgs mechanism disposes of the would-be Goldstinos which are
absorbed by the gravitinos to become massive. The breaking survives in the flat
limit, where we decouple all gravitational interactions, and the theory becomes
softly broken global supersymmetry.Comment: 9 pages, some comments in the discussion of the super-Higgs effect
and some references adde
Multiphoton Transitions in a Spin System Driven by Strong Bichromatic Field
EPR transient nutation spectroscopy is used to measure the effective field
(Rabi frequency) for multiphoton transitions in a two-level spin system
bichromatically driven by a transverse microwave (MW) field and a longitudinal
radio-frequency (RF) field. The behavior of the effective field amplitude is
examined in the case of a relatively strong MW field, when the derivation of
the effective Hamiltonian cannot be reduced to first-order perturbation theory
in w_{1} / w_{rf} (w_{1} is the microwave Rabi frequency, w_{rf} is the RF
frequency). Experimental results are consistently interpreted by taking into
account the contributions of second and third order in w_{1} / w_{rf} evaluated
by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously
broadened EPR line, the third-order correction modifies the nutation frequency,
while the second-order correction gives rise to a change in the nutation
amplitude due to a Bloch-Siegert shift.Comment: 7 pages, 6 figure
Light scalars in strongly-coupled extra-dimensional theories
The low-energy dynamics of five-dimensional Yang-Mills theories compactified
on S^1 can be described by a four-dimensional gauge theory coupled to a scalar
field in the adjoint representation of the gauge group. Perturbative
calculations suggest that the mass of this elementary scalar field is protected
against power divergences, and is controlled by the size of the extra dimension
R. As a first step in the study of this phenomenon beyond perturbation theory,
we investigate the phase diagram of a SU(2) Yang-Mills theory in five
dimensions regularized on anisotropic lattices and we determine the ratios of
the relevant physical scales. The lattice system shows a dimensionally reduced
phase where the four-dimensional correlation length is much larger than the
size of the extra dimension, but still smaller than the four-dimensional
volume. In this region of the bare parameter space, at energies below 1/R, the
non-perturbative spectrum contains a \emph{light} scalar state. This state has
a mass that is independent of the cut-off, and a small overlap with glueball
operators. Our results suggest that light scalar fields can be introduced in a
lattice theory using compactified extra dimensions, rather than fine tuning the
bare mass parameter.Comment: 38 pages (7 pages of Appendix), 10 tables, 21 figures. Minor
corrections. Version accepted for publication in JHE
Big Corrections from a Little Higgs
We calculate the tree-level expressions for the electroweak precision
observables in the SU(5)/SO(5) littlest Higgs model. The source for these
corrections are the exchange of heavy gauge bosons, explicit corrections due to
non-linear sigma-model dynamics and a triplet Higgs VEV. Weak isospin violating
contributions are present because there is no custodial SU(2) global symmetry.
The bulk of these weak isospin violating corrections arise from heavy gauge
boson exchange while a smaller contribution comes from the triplet Higgs VEV. A
global fit is performed to the experimental data and we find that throughout
the parameter space the symmetry breaking scale is bounded by f > 4 TeV at 95%
C.L. Stronger bounds on f are found for generic choices of the high energy
gauge couplings. We find that even in the best case scenario one would need
fine tuning of less than a percent to get a Higgs mass as light as 200 GeV.Comment: 20 pages, 5 figures included, typos fixed, comments on the effects of
extra vector-like heavy fermions adde
Magnetic translation groups in an n-dimensional torus
A charged particle in a uniform magnetic field in a two-dimensional torus has
a discrete noncommutative translation symmetry instead of a continuous
commutative translation symmetry. We study topology and symmetry of a particle
in a magnetic field in a torus of arbitrary dimensions. The magnetic
translation group (MTG) is defined as a group of translations that leave the
gauge field invariant. We show that the MTG on an n-dimensional torus is
isomorphic to a central extension of a cyclic group Z_{nu_1} x ... x
Z_{nu_{2l}} x T^m by U(1) with 2l+m=n. We construct and classify irreducible
unitary representations of the MTG on a three-torus and apply the
representation theory to three examples. We shortly describe a representation
theory for a general n-torus. The MTG on an n-torus can be regarded as a
generalization of the so-called noncommutative torus.Comment: 29 pages, LaTeX2e, title changed, re-organized, to be published in
Journal of Mathematical Physic
Standard Model Higgs from Higher Dimensional Gauge Fields
We consider the possibility that the standard model Higgs fields may
originate from extra components of higher dimensional gauge fields. Theories of
this type considered before have had problems accommodating the standard model
fermion content and Yukawa couplings different from the gauge coupling.
Considering orbifolds based on abelian discrete groups we are lead to a 6
dimensional G_2 gauge theory compactified on T^2/Z_4. This theory can naturally
produce the SM Higgs fields with the right quantum numbers while predicting the
value of the weak mixing angle sin^2 theta_W = 0.25 at the tree-level, close to
the experimentally observed one. The quartic scalar coupling for the Higgs is
generated by the higher dimensional gauge interaction and predicts the
existence of a light Higgs. We point out that one can write a quadratically
divergent counter term for Higgs mass localized to the orbifold fixed point.
However, we calculate these operators and show that higher dimensional gauge
interactions do not generate them at least at one loop. Fermions are introduced
at orbifold fixed points, making it easy to accommodate the standard model
fermion content. Yukawa interactions are generated by Wilson lines. They may be
generated by the exchange of massive bulk fermions, and the fermion mass
hierarchy can be obtained. Around a TeV, the first KK modes would appear as
well as additional fermion modes localized at the fixed point needed to cancel
the quadratic divergences from the Yukawa interactions. The cutoff scale of the
theory could be a few times 10 TeV.Comment: 29 pages, 1 figure, LaTeX. v2: Section on the absence of tadpole at
all order removed. Typos corrected. Refs. added. Final version appeared in
PR
Electrical tuning of elastic wave propagation in nanomechanical lattices at MHz frequencies
Nanoelectromechanical systems (NEMS) that operate in the megahertz (MHz) regime allow energy transducibility between different physical domains. For example, they convert optical or electrical signals into mechanical motions and vice versa. This coupling of different physical quantities leads to frequency-tunable NEMS resonators via electromechanical non-linearities. NEMS platforms with single- or low-degrees of freedom have been employed to demonstrate quantum-like effects, such as mode cooling, mechanically induced transparency, Rabi oscillation, two-mode squeezing and phonon lasing. Periodic arrays of NEMS resonators with architected unit cells enable fundamental studies of lattice-based solid-state phenomena, such as bandgaps, energy transport, non-linear dynamics and localization, and topological properties, directly transferrable to on-chip devices. Here we describe one-dimensional, non-linear, nanoelectromechanical lattices (NEML) with active control of the frequency band dispersion in the radio-frequency domain (10â30âMHz). The design of our systems is inspired by NEMS-based phonon waveguides and includes the voltage-induced frequency tuning of the individual resonators. Our NEMLs consist of a periodic arrangement of mechanically coupled, free-standing nanomembranes with circular clamped boundaries. This design forms a flexural phononic crystal with a well-defined bandgap, 1.8âMHz wide. The application of a d.c. gate voltage creates voltage-dependent on-site potentials, which can significantly shift the frequency bands of the device. Additionally, a dynamic modulation of the voltage triggers non-linear effects, which induce the formation of a phononic bandgap in the acoustic branch, analogous to Peierls transition in condensed matter. The gating approach employed here makes the devices more compact than recently proposed systems, whose tunability mostly relies on materialsâ compliance and mechanical non-linearities
Dilatonic quantum multi-brane-worlds
d5 dilatonic gravity action with surface counterterms motivated by AdS/CFT
correspondence and with contributions of brane quantum CFTs is considered
around AdS-like bulk. The effective equations of motion are constructed. They
admit two (outer and inner) or multi-brane solutions where brane CFTs may be
different. The role of quantum brane CFT is in inducing of complicated brane
dilatonic gravity. For exponential bulk potentials the number of AdS-like bulk
spaces is found in analytical form.The correspondent flat or curved (de Sitter
or hyperbolic) dilatonic two branes are created, as a rule, thanks to quantum
effects. The observable early Universe may correspond to inflationary brane.
The found dilatonic quantum two brane-worlds usually contain the naked
singularity but in couple explicit examples the curvature is finite and horizon
(corresponding to wormhole-like space) appears.Comment: LaTeX file, 25 pages, discussion is enlarge
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