35,290 research outputs found
B->gamma e nu Transitions from QCD Sum Rules
B->gamma e nu transitions have recently been studied in the framework of QCD
factorization. The attractiveness of this channel for such an analysis lies in
the fact that, at least in the heavy quark limit, the only hadron involved is
the B meson itself, so one expects a very simple description of the form factor
in terms of a convolution of the B meson distribution amplitude with a
perturbative kernel. This description, however, does not include contributions
suppressed by powers of the b quark mass. In this letter, we calculate
corrections to the factorized expression which are induced by the ``soft''
hadronic component of the photon. We demonstrate that the power-suppression of
these terms is numerically not effective for physical values of the quark
mass and that they increase the form factor by about 30% at zero momentum
transfer. We also derive a sum rule for lambda_B, the first negative moment of
the B meson distribution amplitude, and find lambda_B = 0.6 GeV (to leading
order in QCD).Comment: 13 pages, 5 figure
Weighted Hardy spaces: shift invariant and coinvariant subspaces, linear systems and operator model theory
The Sz.-Nagy--Foias model theory for contraction operators
combined with the Beurling-Lax theorem establishes a correspondence between any
two of four kinds of objects: shift-invariant subspaces, operator-valued inner
functions, conservative discrete-time input/state/output linear systems, and
Hilbert-space contraction operators. We discuss an analogue of
all these ideas in the context of weighted Hardy spaces over the unit disk and
an associated class of hypercontraction operators
On the Approximation of Isolated Eigenvalues of Ordinary Differential Operators
We extend a result of Stolz and Weidmann on the approximation of isolated
eigenvalues of singular Sturm-Liouville and Dirac operators by the eigenvalues
of regular operators.Comment: 4 page
An Investigation Into the Electrical Activity of Tender, Resting Paraspinal Muscles Using Surface Electromyography: A Pilot Study
Abnormal resting paraspinal muscle activity has been claimed to be responsible for changes in spinal tissue texture which are detectible by manual palpation. This pilot study investigated whether there was significant electrical activity in paraspinal musculature that was tender and that appeared to have altered tissue texture on palpation. Sixteen healthy volunteers between 18 and 35 years of age had their thoracic erector spinae mass palpated bilaterally from spinal levels T3 to T10 to identify paraspinal regions exhibiting altered tissue texture relative to the contralateral muscle mass. Surface electromyography (sEMG) was used to measure electrical activity in the muscle mass at the selected levels. No significant differences in electrical activity were observed between the tender and non-tender muscle masses, although a large difference existed in the one symptomatic subject. All muscle sites displayed EMG activity at rest, although the source of activity is not clear. A number of methodological problems with the EMG recording were encountered and are discussed. Future research is recommended using symptomatic participants
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