626 research outputs found
Remarks on Form Factor Bounds
Improved model independent upper bounds on the weak transition form factors
are derived using inclusive sum rules. Comparison of the new bounds with the
old ones is made for the form factors h_{A_1} and h_V in B -> D* decays.Comment: 8 pages, 2 figures, title changed and typos corrected for journal
publicatio
Testing factorization in B -> D(*)X decays
In QCD the amplitude for B0 -> D(*)+pi- factorizes in the large Nc limit or
in the large energy limit Q >> Lambda_QCD where Q = {m_b, m_c, m_b-m_c}. Data
also suggests factorization in exclusive processes B-> D* pi+ pi- pi- pi0 and
B-> D* omega pi-, however by themselves neither large Nc nor large Q can
account for this. Noting that the condition for large energy release in B0-> D+
pi- is enforced by the SV limit, m_b, m_c >> m_b-m_c >> Lambda, we propose that
the combined large Nc and SV limits justify factorization in B -> D(*) X. This
combined limit is tested with the inclusive decay spectrum measured by CLEO. We
also give exact large Nc relations among isospin amplitudes for B -> D(*)X and
B -> D(*) D-bar(*)X, which can be used to test factorization through exclusive
or inclusive measurements. Predictions for the modes B-> D(*) pi pi, B-> D(*)K
K-bar and B-> D(*) D-bar(*) K are discussed using available data.Comment: 15 pages, 3 included .eps figures, minor change
Resumming the color-octet contribution to e+ e- -> J/psi + X
Recent observations of the spectrum of J/psi produced in e+ e- collisions at
the Upsilon(4S) resonance are in conflict with fixed-order calculations using
the Non-Relativistic QCD (NRQCD) effective field theory. One problem is that
leading order color-octet mechanisms predict an enhancement of the cross
section for J/psi with maximal energy that is not observed in the data.
However, in this region of phase space large perturbative corrections (Sudakov
logarithms) as well as enhanced nonperturbative effects are important. In this
paper we use the newly developed Soft-Collinear Effective Theory (SCET) to
systematically include these effects. We find that these corrections
significantly broaden the color-octet contribution to the J/psi spectrum. Our
calculation employs a one-stage renormalization group evolution rather than the
two-stage evolution used in previous SCET calculations. We give a simple
argument for why the two methods yield identical results to lowest order in the
SCET power counting.Comment: 27 pages, 7 figure
The minimum-error discrimination via Helstrom family of ensembles and Convex Optimization
Using the convex optimization method and Helstrom family of ensembles
introduced in Ref. [1], we have discussed optimal ambiguous discrimination in
qubit systems. We have analyzed the problem of the optimal discrimination of N
known quantum states and have obtained maximum success probability and optimal
measurement for N known quantum states with equiprobable prior probabilities
and equidistant from center of the Bloch ball, not all of which are on the one
half of the Bloch ball and all of the conjugate states are pure. An exact
solution has also been given for arbitrary three known quantum states. The
given examples which use our method include: 1. Diagonal N mixed states; 2. N
equiprobable states and equidistant from center of the Bloch ball which their
corresponding Bloch vectors are inclined at the equal angle from z axis; 3.
Three mirror-symmetric states; 4. States that have been prepared with equal
prior probabilities on vertices of a Platonic solid.
Keywords: minimum-error discrimination, success probability, measurement,
POVM elements, Helstrom family of ensembles, convex optimization, conjugate
states PACS Nos: 03.67.Hk, 03.65.TaComment: 15 page
B-->pi and B-->K transitions in standard and quenched chiral perturbation theory
We study the effects of chiral logs on the heavy-->light pseudoscalar meson
transition form factors by using standard and quenched chiral perturbation
theory combined with the static heavy quark limit. The resulting expressions
are used to indicate the size of uncertainties due to the use of the quenched
approximation in the current lattice studies. They may also be used to assess
the size of systematic uncertainties induced by missing chiral log terms in
extrapolating toward the physical pion mass. We also provide the coefficient
multiplying the quenched chiral log, which may be useful if the quenched
lattice studies are performed with very light mesons.Comment: 33 pages, 8 PostScript figures, version to appear in PR
A Gravitational Aharonov-Bohm Effect, and its Connection to Parametric Oscillators and Gravitational Radiation
A thought experiment is proposed to demonstrate the existence of a
gravitational, vector Aharonov-Bohm effect. A connection is made between the
gravitational, vector Aharonov-Bohm effect and the principle of local gauge
invariance for nonrelativistic quantum matter interacting with weak
gravitational fields. The compensating vector fields that are necessitated by
this local gauge principle are shown to be incorporated by the DeWitt minimal
coupling rule. The nonrelativistic Hamiltonian for weak, time-independent
fields interacting with quantum matter is then extended to time-dependent
fields, and applied to problem of the interaction of radiation with
macroscopically coherent quantum systems, including the problem of
gravitational radiation interacting with superconductors. But first we examine
the interaction of EM radiation with superconductors in a parametric oscillator
consisting of a superconducting wire placed at the center of a high Q
superconducting cavity driven by pump microwaves. We find that the threshold
for parametric oscillation for EM microwave generation is much lower for the
separated configuration than the unseparated one, which then leads to an
observable dynamical Casimir effect. We speculate that a separated parametric
oscillator for generating coherent GR microwaves could also be built.Comment: 25 pages, 5 figures, YA80 conference (Chapman University, 2012
The pressure of QCD at finite temperatures and chemical potentials
The perturbative expansion of the pressure of hot QCD is computed here to
order g^6ln(g) in the presence of finite quark chemical potentials. In this
process all two- and three-loop one-particle irreducible vacuum diagrams of the
theory are evaluated at arbitrary T and mu, and these results are then used to
analytically verify the outcome of an old order g^4 calculation of Freedman and
McLerran for the zero-temperature pressure. The results for the pressure and
the different quark number susceptibilities at high T are compared with recent
lattice simulations showing excellent agreement especially for the chemical
potential dependent part of the pressure.Comment: 35 pages, 6 figures; text revised, one figure replace
Chebyshev Solution of the Nearly-Singular One-Dimensional Helmholtz Equation and Related Singular Perturbation Equations: Multiple Scale Series and the Boundary Layer Rule-of-Thumb
The one-dimensional Helmholtz equation, Δ 2 u xx â u = f ( x ), arises in many applications, often as a component of three-dimensional fluids codes. Unfortunately, it is difficult to solve for ΔâȘ1 because the homogeneous solutions are expâ(± x /Δ), which have boundary layers of thickness O(1/Δ). By analyzing the asymptotic Chebyshev coefficients of exponentials, we rederive the OrszagâIsraeli rule [16] that Chebyshev polynomials are needed to obtain an accuracy of 1% or better for the homogeneous solutions. (Interestingly, this is identical with the boundary layer rule-of-thumb in [5], which was derived for singular functions like tanh([ x â1]/Δ).) Two strategies for small Δ are described. The first is the method of multiple scales, which is very general, and applies to variable coefficient differential equations, too. The second, when f ( x ) is a polynomial, is to compute an exact particular integral of the Helmholtz equation as a polynomial of the same degree in the form of a Chebyshev series by solving triangular pentadiagonal systems. This can be combined with the analytic homogeneous solutions to synthesize the general solution. However, the multiple scales method is more efficient than the Chebyshev algorithm when Δ is very, very tiny.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45436/1/11075_2004_Article_2865.pd
Resummations of free energy at high temperature
We discuss resummation strategies for free energy in quantum field theories
at nonzero temperatures T. We point out that resummations should be performed
for the short- and long-distance parts separately in order to avoid spurious
interference effects and double-counting. We then discuss and perform Pade
resummations of these two parts for QCD at high T. The resummed results are
almost invariant under variation of the renormalization and factorization
scales. We perform the analysis also in the case of the massless scalar
theory.Comment: 16 pages, revtex4, 15 eps-figures; minor typographic errors
corrected; the version as it appears in Phys.Rev.
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