2,904 research outputs found
Finite-width effects in the near-threshold ZZZ and ZWW production at ILC
We calculate the cross-section of the near-threshold off-shell and
production at the International Linear Collider taking into account
their instability and the principal part of NLO corrections. The calculations
are performed in the framework of the model of unstable particles with smeared
mass-shell. We show that the contribution of the finite and widths
(their instability) is large in the Higgs resonance range and should be taken
into account in the Higgs boson searches at future colliders.Comment: 5 pages, 6 figure
Effect of the Pauli principle on photoelectron spin transport in GaAs
In p+ GaAs thin films, the effect of photoelectron degeneracy on spin
transport is investigated theoretically and experimentally by imaging the spin
polarization profile as a function of distance from a tightly-focussed light
excitation spot. Under degeneracy of the electron gas (high concentration, low
temperature), a dip at the center of the polarization profile appears with a
polarization maximum at a distance of about from the center. This
counterintuitive result reveals that photoelectron diffusion depends on spin,
as a direct consequence of the Pauli principle. This causes a concentration
dependence of the spin stiffness while the spin dependence of the mobility is
found to be weak in doped material. The various effects which can modify spin
transport in a degenerate electron gas under local laser excitation are
considered. A comparison of the data with a numerical solution of the coupled
diffusion equations reveals that ambipolar coupling with holes increases the
steady-state photo-electron density at the excitation spot and therefore the
amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents
are predicted to depend on spin under degeneracy (spin Soret currents), but
these currents are negligible except at very high excitation power where they
play a relatively small role. Coulomb spin drag and bandgap renormalization are
negligible due to electrostatic screening by the hole gas
Flipped SU(5), see-saw scale physics and degenerate vacua
We investigate the requirement of the existence of two degenerate vacua of
the effective potential as a function of the Weinberg-Salam Higgs scalar field
norm, as suggested by the multiple point principle, in an extension of the
Standard Model including see-saw scale physics. Results are presented from an
investigation of an extension of the Standard Model to the gauge symmetry group
SU(3)_C\times SU(2)_L\times U(1)'\times \tilde U(1), where two groups U(1)' and
\tilde U(1) originate at the see-saw scale M_{SS}, when heavy (right-handed)
neutrinos appear. The consequent unification of the group SU(3)_C\times
SU(2)_L\times U(1)' into the flipped SU(5) at the GUT scale leads to the group
SU(5)\times \tilde U(1). We assume the position of the second minimum of the
effective potential coincides with the fundamental scale, here taken to be the
GUT scale. We solve the renormalization group equations in the one-loop
approximation and obtain a top-quark mass of 171\pm 3 GeV and a Higgs mass of
129\pm 4 GeV, in the case when the Yukawa couplings of the neutrinos are less
than half that of the top quark at the GUT scale.Comment: 12 pages and 3 Figures; Presented at 'Planck 05' conference, ICT
Hint for axial-vector contact interactions in the data on e+e- -> e+e-(gamma) at centre-of-mass energies 192-208 GeV
For the first time the experiments ALEPH, DELPHI, L3 and OPAL have presented
preliminary results for fermion-pair production in e+e- collisions on the full
data set above the Z pole. A combined analysis of the Bhabha scattering
measurements is performed to search for effects of contact interactions. In the
case of two axial-vector (AA) currents the best fit to the data is 2.6 standard
deviations away from the Standard Model expectation, corresponding to an energy
scale TeV for contact interactions. For other
models no statistically significant deviations are observed, and the data are
used to set lower limits at 95 % confidence level on the contact interaction
scales ranging from 8.2 to 21.3 TeV, depending on the helicity structure.Comment: 6 pages, 1 table, 3 figure
Radiative quantum efficiency in an InAs/AlSb intersubband transition
The quantum efficiency of an electroluminescent intersubband emitter based on
InAs/AlSb has been measured as a function of the magnetic field up to 20T. Two
series of oscillations periodic in 1/B are observed, corresponding to the
elastic and inelastic scattering of electrons of the upper state of the
radiative transitions. Experimental results are accurately reproduced by a
calculation of the excited state lifetime as a function of the applied magnetic
field. The interpretation of these data gives an exact measure of the relative
weight of the scattering mechanisms and allows the extraction of material
parameters such as the energy dependent electron effective mass and the optical
phonon energy.Comment: 4 pages, 5 figure
Constraints on the parameters of the Left Right Mirror Model
We study some phenomenological constraints on the parameters of a left right
model with mirror fermions (LRMM) that solves the strong CP problem. In
particular, we evaluate the contribution of mirror neutrinos to the invisible Z
decay width (\Gamma_Z^{inv}), and we find that the present experimental value
on \Gamma_Z^{inv}, can be used to place an upper bound on the Z-Z' mixing angle
that is consistent with limits obtained previously from other low-energy
observables. In this model the charged fermions that correspond to the standard
model (SM) mix with its mirror counterparts. This mixing, simultaneously with
the Z-Z' one, leads to modifications of the \Gamma(Z --> f \bar{f}) decay
width. By comparing with LEP data, we obtain bounds on the standard-mirror
lepton mixing angles. We also find that the bottom quark mixing parameters can
be chosen to fit the experimental values of R_b, and the resulting values for
the Z-Z' mixing angle do not agree with previous bounds. However, this
disagreement disappears if one takes the more recent ALEPH data.Comment: 7 pages, 2 figures, REVTe
CP-violating Z-gamma-gamma and top-quark electric dipole couplings in gamma gamma -> t bar{t}
An effective anomalous CP-violating Z-gamma-gamma coupling can give rise to
observable CP-odd effects in gamma gamma -> t bar{t}. We study certain
asymmetries in the decay lepton distributions in gamma gamma -> t bar{t}
arising from top decay in the presence of a CP-violating Z-gamma-gamma coupling
as well as a top-quark electric dipole coupling. We find that a photon linear
collider with geometric luminosity of 20 fb^{-1} can put limits of the order of
0.1 on the imaginary part of the CP-violating anomalous Z-gamma-gamma coupling
using these asymmetries.Comment: 10 pages, latex, two figures included, Version accepted for
publication in PL
Tau and Charm physics highlights
In tau physics, we are at the frontier between the completion of the LEP
program and the start of analyses from b-factories, which are expected to
produce results in the coming years. Nice results from CLEO are steadily
delivered in the meantime. For charm, impressive progress have been achieved by
fixed target experiments in the search for CP violation and D^0 - \bar D^0
oscillations. First results from b-factories demonstrate the power of these
facilities in such areas. The novel measurement of the D* width by CLEO happens
to be rather different from current expectations. The absence of a charm
factory explains the lack or the very slow progress in the absolute scale
determinations for charm decays.Comment: "Typos corrected and references added
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