1,012 research outputs found
Sum Rules for Heavy Flavor Transitions in the SV Limit
We show how sum rules for the weak decays of heavy flavor hadrons can be
derived as the moments of spectral distributions in the small velocity (SV)
limit. This systematic approach allows us to determine corrections to these sum
rules, to obtain new sum rules and it provides us with a transparent physical
interpretation; it also opens a new perspective on the notion of the heavy
quark mass. Applying these sum rules we derive a lower bound on the deviation
of the exclusive form factor from unity at zero recoil; likewise
we give a field-theoretical derivation of a previously formulated inequality
between the expectation value for the kinetic energy operator of the heavy
quark and for the chromomagnetic operator. We analyze how the known results on
nonperturbative corrections must be understood when one takes into account the
normalization point dependence of the low scale parameters. Relation between
the field-theoretic derivation of the sum rules and the quantum-mechanical
approach is elucidated.Comment: 72 pages, 5 figures (uuencoded ps files added using figures option),
Revised version. Paper is considerably extended to include some new results.
New section on quantum mechanical approach is adde
On the Motion of Heavy Quarks inside Hadrons: Universal Distributions and Inclusive Decays
In previous papers we have pointed out that there exists a QCD analog of the
phenomenological concept of the so called Fermi motion for the heavy quark
inside a hadron. Here we show in a more detailed way how this comes about and
we analyze the limitations of this concept. Non-perturbative as well as
perturbative aspects are included. We emphasize both the similarities and the
differences to the well-known treatment of deep inelastic lepton-nucleon
scattering. We derive a model-independent {\em lower} bound on the kinetic
energy of the heavy quark inside the hadron.Comment: 34 pages, Latex, 4 figures. Some clarifying comments and postscript
files of figures are adde
Hadronic Light-by-Light Scattering in the Muonium Hyperfine Splitting
We consider an impact of hadronic light-by-light scattering on the muonium
hyperfine structure. A shift of the hyperfine interval is calculated with the light-by-light scattering approximated
by exchange of pseudoscalar and pseudovector mesons. Constraints from the
operator product expansion in QCD are used to fix parameters of the model
similar to the one used earlier for the hadronic light-by-light scattering in
calculations of the muon anomalous magnetic moment. The pseudovector exchange
is dominant in the resulting shift, . Although the effect is tiny it is useful in understanding
the level of hadronic uncertainties.Comment: 16 pages, 7 figures, a reference adde
Exact Results in Gauge Theories: Putting Supersymmetry to Work. The 1999 Sakurai Prize Lecture
Powerful methods based on supersymmetry allow one to find exact solutions to
certain problems in strong coupling gauge theories. The inception of some of
these methods (holomorphy in the gauge coupling and other chiral parameters, in
conjunction with instanton calculations) dates back to the 1980's. I describe
the early exact results -- the calculation of the beta function and the gluino
condensate -- and their impact on the subsequent developments. A brief
discussion of the recent breakthrough discoveries where these results play a
role is given.Comment: Based on the talk at the Centennial Meeting of The American Physical
Society, March 20-26, Atlanta, GA. LaTex (uses sprocl.sty), 36 pages, 5 eps
figures include
Tunneling-assisted impact ionization fronts in semiconductors
We propose a novel type of ionization front in layered semiconductor
structures. The propagation is due to the interplay of band-to-band tunneling
and impact ionization. Our numerical simulations show that the front can be
triggered when an extremely sharp voltage ramp () is
applied in reverse direction to a Si structure that is connected in
series with an external load. The triggering occurs after a delay of 0.7 to 0.8
ns. The maximal electrical field at the front edge exceeds .
The front velocity is 40 times faster than the saturated drift velocity
. The front passes through the base with a thickness of
within approximately 30 ps, filling it with dense electron-hole plasma. This
passage is accompanied by a voltage drop from 8 kV to dozens of volts. In this
way a voltage pulse with a ramp up to can be applied to the
load. The possibility to form a kilovolt pulse with such a voltage rise rate
sets new frontiers in pulse power electronics.Comment: 12 pages, 6 figure
Rapid dissipation of magnetic fields due to Hall current
We propose a mechanism for the fast dissipation of magnetic fields which is
effective in a stratified medium where ion motions can be neglected. In such a
medium, the field is frozen into the electrons and Hall currents prevail.
Although Hall currents conserve magnetic energy, in the presence of density
gradients, they are able to create current sheets which can be the sites for
efficient dissipation of magnetic fields. We recover the frequency,
, for Hall oscillations modified by the presence of density
gradients. We show that these oscillations can lead to the exchange of energy
between different components of the field. We calculate the time evolution and
show that magnetic fields can dissipate on a timescale of order
. This mechanism can play an important role for magnetic
dissipation in systems with very steep density gradients where the ions are
static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.
Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo
The possibility of the magnetic flux expulsion from the Galaxy in the
superbubble (SB) explosions, important for the Alpha-Omega dynamo, is
considered. Special emphasis is put on the investigation of the downsliding of
the matter from the top of the shell formed by the SB explosion which is able
to influence the kinematics of the shell. It is shown that either Galactic
gravity or the development of the Rayleigh-Taylor instabilities in the shell
limit the SB expansion, thus, making impossible magnetic flux expulsion. The
effect of the cosmic rays in the shell on the sliding is considered and it is
shown that it is negligible compared to Galactic gravity. Thus, the question of
possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure
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