119 research outputs found
Variation of the cross section for e+e- --> W+H- in the Minimal Supersymmetric Standard Model
We study the loop-induced process e+e- --> W+H- in the Minimal Supersymmetric
Standard Model (MSSM). This process allows the charged Higgs boson to be
produced in e+e- collisions when its mass is larger than half the
center-of-mass energy, so that e+e- --> H+H- is kinematically forbidden. By
scanning over the MSSM parameters subject to experimental constraints we
examine the range of values possible for this cross section. We find that, in
regions of parameter space where this cross section is large enough to be of
interest, the contributions from supersymmetric particles typically increase
the cross section by 50-100% compared to the non-supersymmetric two Higgs
doublet model result. Choosing a few typical MSSM parameter sets, we show the
regions in the m_{H^{\pm}}-tan(beta) plane in which at least 10 W^{\pm}H^{\mp}
events would be produced at the e+e- collider for m_{H^{\pm}} >= sqrt(s)/2. We
also show that including radiative corrections to the MSSM Higgs sector has
only a small effect on the cross section.Comment: 5 pages, 4 figures, v2: minor changes; v3: extensive changes to text
and figures, version to appear in PR
Mean-field results on the Anderson impurity model out of equilibrium
We investigate the mean-field phase diagram of the Anderson impurity model
out of equilibrium. Generalising the unrestricted Hartree-Fock approach to the
non-equilibrium situation we derive and analyse the system of equations
defining the critical surface separating the magnetic regime from the
non-magnetic one. An exact analytic solution for the phase boundary as a
function of the applied voltage is found in the symmetric case. Surprisingly,
we find that as soon as there is an asymmetry, even small, between the
contacts, no finite voltage is able to destroy the magnetic regime which
persists at arbitrary high voltages.Comment: 4 pages, 2 figures (eps files); to appear in PRB Brief Report
Oscillations of the magnetic polarization in a Kondo impurity at finite magnetic fields
The electronic properties of a Kondo impurity are investigated in a magnetic
field using linear response theory. The distribution of electrical charge and
magnetic polarization are calculated in real space. The (small) magnetic field
does not change the charge distribution. However, it unmasks the Kondo cloud.
The (equal) weight of the d-electron components with their magnetic moment up
and down is shifted and the compensating s-electron clouds don't cancel any
longer (a requirement for an experimental detection of the Kondo cloud). In
addition to the net magnetic polarization of the conduction electrons an
oscillating magnetic polarization with a period of half the Fermi wave length
is observed. However, this oscillating magnetic polarization does not show the
long range behavior of Rudermann-Kittel-Kasuya-Yosida oscillations because the
oscillations don't extend beyond the Kondo radius. They represent an internal
electronic structure of the Kondo impurity in a magnetic field. PACS: 75.20.Hr,
71.23.An, 71.27.+
Smoking-gun signatures of little Higgs models
Little Higgs models predict new gauge bosons, fermions and scalars at the TeV
scale that stabilize the Higgs mass against quadratically divergent one-loop
radiative corrections. We categorize the many little Higgs models into two
classes based on the structure of the extended electroweak gauge group and
examine the experimental signatures that identify the little Higgs mechanism in
addition to those that identify the particular little Higgs model. We find that
by examining the properties of the new heavy fermion(s) at the LHC, one can
distinguish the structure of the top quark mass generation mechanism and test
the little Higgs mechanism in the top sector. Similarly, by studying the
couplings of the new gauge bosons to the light Higgs boson and to the Standard
Model fermions, one can confirm the little Higgs mechanism and determine the
structure of the extended electroweak gauge group.Comment: 59 pages, 10 figures. v2: refs added, typos fixed, JHEP versio
Magnetic properties of the three-dimensional Hubbard model at half filling
We study the magnetic properties of the 3d Hubbard model at half-filling in
the TPSC formalism, previously developed for the 2d model. We focus on the
N\'eel transition approached from the disordered side and on the paramagnetic
phase. We find a very good quantitative agreement with Dynamical Mean-Field
results for the isotropic 3d model. Calculations on finite size lattices also
provide satisfactory comparisons with Monte Carlo results up to the
intermediate coupling regime. We point out a qualitative difference between the
isotropic 3d case, and the 2d or anisotropic 3d cases for the double occupation
factor. Even for this local correlation function, 2d or anisotropic 3d cases
are out of reach of DMF: this comes from the inability of DMF to account for
antiferromagnetic fluctuations, which are crucial.Comment: RevTex, 9 pages +10 figure
The Numerical Renormalization Group Method for correlated electrons
The Numerical Renormalization Group method (NRG) has been developed by Wilson
in the 1970's to investigate the Kondo problem. The NRG allows the
non-perturbative calculation of static and dynamic properties for a variety of
impurity models. In addition, this method has been recently generalized to
lattice models within the Dynamical Mean Field Theory. This paper gives a brief
historical overview of the development of the NRG and discusses its application
to the Hubbard model; in particular the results for the Mott metal-insulator
transition at low temperatures.Comment: 14 pages, 7 eps-figures include
QCD Corrections to t anti-b H^- Associated Production in e^+ e^- Annihilation
We calculate the QCD corrections to the cross section of e^+ e^- -> t anti-b
H^- and its charge-conjugate counterpart within the minimal supersymmetric
extension of the Standard Model. This process is particularly important if m_t
b H^+ and e^+ e^- -> H^+ H^- are
not allowed kinematically. Large logarithmic corrections that arise in the
on-mass-shell scheme of quark mass renormalization, especially from the t
anti-b H^- Yukawa coupling for large values of tan(beta), are resummed by
adopting the modified minimal-subtraction scheme, so that the convergence
behavior of the perturbative expansion is improved. The inclusion of the QCD
corrections leads to a significant reduction of the theoretical uncertainties
due to scheme and scale dependences.Comment: 21 pages (Latex), 8 figures (Postscript); detailed discussion of
scheme and scale dependences adde
Metal-insulator transition in two-dimensional disordered systems with power-law transfer terms
We investigate a disordered two-dimensional lattice model for noninteracting
electrons with long-range power-law transfer terms and apply the method of
level statistics for the calculation of the critical properties. The
eigenvalues used are obtained numerically by direct diagonalization. We find a
metal-insulator transition for a system with orthogonal symmetry. The exponent
governing the divergence of the correlation length at the transition is
extracted from a finite size scaling analysis and found to be . The critical eigenstates are also analyzed and the distribution of the
generalized multifractal dimensions is extrapolated.Comment: 4 pages with 4 figures, printed version: PRB, Rapid Communication
Evidence for covert attention switching from eye-movements. Reply to commentaries on Liechty et al., 2003
We argue that our research objectives in Liechty, Pieters, and Wedel (2003) are to provide generalizable insights into covert visual attention to complex, multimodal stimuli in their natural context, through inverse inference from eye-movement data. We discuss the most important issues raised by Feng (2003) and Reichle and Nelson (2003), in particular the task definition, inclusion of ad features, object-based versus space-based attention and the evidence for the where and what streams.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45760/1/11336_2005_Article_BF02295611.pd
Density of States in the Magnetic Ground State of the Friedel-Anderson Impurity
By applying a magnetic field whose Zeeman energy exceeds the Kondo energy by
an order of magnitude the ground state of the Friedel-Anderson impurity is a
magnetic state. In recent years the author introduced the Friedel Artificially
Inserted Resonance (FAIR) method to investigate impurity properties. Within
this FAIR approach the magnetic ground state is derived. Its full excitation
spectrum and the composition of the excitations is calculated and numerically
evaluated. From the excitation spectrum the electron density of states is
calculated. Majority and minority d-resonances are obtained. The width of the
resonances is about twice as wide as the mean field theory predicts. This
broadening is due to the fact that any change of the occupation of the d-state
in one spin band changes the eigenstates in the opposite spin band and causes
transitions in both spin bands. This broadening reduces the height of the
resonance curve and therefore the density of states by a factor of two. This
yields an intuitive understanding for a previous result of the FAIR approach
that the critical value of the Coulomb interaction for the formation of a
magnetic moment is twice as large as the mean field theory predicts
- …