14,106 research outputs found
Loops, Cutoffs and Anomalous Gauge Boson Couplings
We discuss several issues regarding analyses which use loop calculations to
put constraints on anomalous trilinear gauge boson couplings (TGC's). Many such
analyses give far too stringent bounds. This is independent of questions of
gauge invariance, contrary to the recent claims of de Rujula et. al., since the
lagrangians used in these calculations ARE gauge invariant, but the SU(2)_L X
U(1)_Y symmetry is nonlinearly realized. The real source of the problem is the
incorrect use of cutoffs -- the cutoff dependence of a loop integral does not
necessarily reflect the true dependence on the heavy physics scale M. If done
carefully, one finds that the constraints on anomalous TGC's are much weaker.
We also compare effective lagrangians in which SU(2)_L X U(1)_Y is realized
linearly and nonlinearly, and discuss the role of custodial SU(2) in each
formulation.Comment: talk presented at the XXVI International Conference on High Energy
Physics, Dallas, USA, August 1992, plain TeX, 12 pages, 3 figures (not
included), UdeM-LPN-TH-105, McGill-92/3
Probing New Physics via an Angular Analysis of B --> V1 V2 decays
We show that an angular analysis of B --> V1 V2 decays yields numerous tests
for new physics in the decay amplitudes. Unlike direct CP asymmetries, many of
these new-physics observables are nonzero even if the strong phase differences
vanish. For certain observables, neither time-dependent measurements nor
tagging is necessary. Should a signal for new physics be found, one can place a
lower limit on the size of the new-physics parameters, as well as on their
effect on the measurement of the phase of B0--Bbar0 mixing.Comment: 9 pages, plain latex, no figures. Title modified slightly. Paragraph
added about viability of method. Conclusions unchanged. To be published in
Europhysics Letter
All-optical reconstruction of atomic ground-state population
The population distribution within the ground-state of an atomic ensemble is
of large significance in a variety of quantum optics processes. We present a
method to reconstruct the detailed population distribution from a set of
absorption measurements with various frequencies and polarizations, by
utilizing the differences between the dipole matrix elements of the probed
transitions. The technique is experimentally implemented on a thermal rubidium
vapor, demonstrating a population-based analysis in two optical pumping
examples. The results are used to verify and calibrate an elaborated numerical
model, and the limitations of the reconstruction scheme which result from the
symmetry properties of the dipole matrix elements are discussed.Comment: 6 pages, 4 figure
Vortex Penetration into a Type II Superconductor due to a Mesoscopic External Current
Applying the London theory we study curved vortices produced by an external
current near and parallel to the surface of a type II superconductor. By
minimizing the energy functional we find the contour describing the hard core
of the flux line, and predict the threshold current for entrance of the first
vortex. We assume that the vortex entrance is allowed due to surface defects,
despite the Bean-Livingston barrier. Compared to the usual situation with a
homogeneous magnetic field, the main effect of the present geometry is that
larger magnetic fields can be applied locally before vortices enter the
superconducting sample. It is argued that this effect can be further enhanced
in anisotropic superconductors.Comment: 9 pages, 14 figure
Can one detect new physics in I=0 and/or I=2 contributions to the decays B --> pi pi?
We study the effects of new-physics contributions to B --> pi pi decays,
which can be parametrized as four new complex quantities. A simple analysis is
provided by utilizing the reparametrization invariance of the decay amplitudes.
We find that six quantities can be reabsorbed into the definitions of Standard
Model-like parameters. As a result, the usual isospin analysis provides only
two constraints on new physics which are independent of estimates for the
Standard Model contributions. In particular, we show that one is not sensitive
to new physics affecting the I=0 amplitudes. On the other hand, I=2 new physics
can be detected, and its parameters can be measured by using independent
determinations of the weak phases. We obtain constraints on these new-physics
parameters through a fit to the current experimental data.Comment: 8 pages, RevTe
Effects of disorder on the vortex charge
We study the influence of disorder on the vortex charge, both due to random
pinning of the vortices and due to scattering off non-magnetic impurities. In
the case when there are no impurities present, but the vortices are randomly
distributed, the effect is very small, except when two or more vortices are
close by. When impurities are present, they have a noticeable effect on the
vortex charge. This, together with the effect of temperature, changes
appreciably the vortex charge. In the case of an attractive impurity potential
the sign of the charge naturally changes.Comment: 10 pages, 16 figures. Accepted in Phys. Rev.
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