89,751 research outputs found
Sum rules in the heavy quark limit of QCD
In the leading order of the heavy quark expansion, we propose a method within
the OPE and the trace formalism, that allows to obtain, in a systematic way,
Bjorken-like sum rules for the derivatives of the elastic Isgur-Wise function
in terms of corresponding Isgur-Wise functions of transitions to
excited states. A key element is the consideration of the non-forward
amplitude, as introduced by Uraltsev. A simplifying feature of our method is to
consider currents aligned along the initial and final four-velocities. As an
illustration, we give a very simple derivation of Bjorken and Uraltsev sum
rules. On the other hand, we obtain a new class of sum rules that involve the
products of IW functions at zero recoil and IW functions at any . Special
care is given to the needed derivation of the projector on the polarization
tensors of particles of arbitrary integer spin. The new sum rules give further
information on the slope and also on the curvature
, and imply, modulo a very natural assumption, the
inequality , and therefore the absolute bound
.Comment: 64 pages, Late
Critical Analysis of Theoretical Estimates for to Light Meson Form Factors and the Data
We point out that current estimates of form factors fail to explain the
non-leptonic decays and that the combination of data
on the semi-leptonic decays and on the non-leptonic
decays (in particular recent po\-la\-ri\-za\-tion
data) severely constrain the form (normalization and dependence) of the
heavy-to-light meson form factors, if we assume the factorization hypothesis
for the latter. From a simultaneous fit to \bpsi and \dk data we find that
strict heavy quark limit scaling laws do not hold when going from to
and must have large corrections that make softer the dependence on the masses.
We find that should increase slower with \qq than .
We propose a simple parametrization of these corrections based on a quark
model or on an extension of the \hhs laws to the \hl case, complemented with an
approximately constant . We analyze in the light of these data and
theoretical input various theoretical approaches (lattice calculations, QCD sum
rules, quark models) and point out the origin of the difficulties encountered
by most of these schemes. In particular we check the compatibility of several
quark models with the heavy quark scaling relations.Comment: 48 pages, DAPNIA/SPP/94-24, LPTHE-Orsay 94/1
New challenges for Adaptive Optics: Extremely Large Telescopes
The performance of an adaptive optics (AO) system on a 100m diameter ground
based telescope working in the visible range of the spectrum is computed using
an analytical approach. The target Strehl ratio of 60% is achieved at 0.5um
with a limiting magnitude of the AO guide source near R~10, at the cost of an
extremely low sky coverage. To alleviate this problem, the concept of
tomographic wavefront sensing in a wider field of view using either natural
guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use
3 or 4 reference sources and up to 3 deformable mirrors, which increase up to
8-fold the corrected field size (up to 60\arcsec at 0.5 um). Operation with
multiple NGS is limited to the infrared (in the J band this approach yields a
sky coverage of 50% with a Strehl ratio of 0.2). The option of open-loop
wavefront correction in the visible using several bright NGS is discussed. The
LGS approach involves the use of a faint (R ~22) NGS for low-order correction,
which results in a sky coverage of 40% at the Galactic poles in the visible.Comment: 11 pages, 9 figures, 4 tables. Accepted for publication in MNRA
New Baryons in the Delta eta and Delta omega Channels
The decays of excited nonstrange baryons into the final states Delta eta and
Delta omega are examined in a relativized quark pair creation model. The
wavefunctions and parameters of the model are fixed by previous calculations of
N pi and N pi pi, etc., decays through various quasi-two body channels
including N eta and N omega. Our results show that the combination of
thresholds just below the region of interest and the isospin selectivity of
these channels should allow the discovery of several new baryons in such
experiments.Comment: 10 pages, RevTe
Valence Quark Spin Distribution Functions
The hyperfine interactions of the constituent quark model provide a natural
explanation for many nucleon properties, including the Delta-N splitting, the
charge radius of the neutron, and the observation that the proton's quark
distribution function ratio d(x)/u(x)->0 as x->1. The hyperfine-perturbed quark
model also makes predictions for the nucleon spin-dependent distribution
functions. Precision measurements of the resulting asymmetries A_1^p(x) and
A_1^n(x) in the valence region can test this model and thereby the hypothesis
that the valence quark spin distributions are "normal".Comment: 16 pages, 2 Postscript figure
Forensics in Industrial Control System: A Case Study
Industrial Control Systems (ICS) are used worldwide in critical
infrastructures. An ICS system can be a single embedded system working
stand-alone for controlling a simple process or ICS can also be a very complex
Distributed Control System (DCS) connected to Supervisory Control And Data
Acquisition (SCADA) system(s) in a nuclear power plant. Although ICS are widely
used to-day, there are very little research on the forensic acquisition and
analyze ICS artefacts. In this paper we present a case study of forensics in
ICS where we de-scribe a method of safeguarding important volatile artefacts
from an embedded industrial control system and several other source
Statistics of fermions in a -dimensional box near a hard wall
We study noninteracting fermions in a domain bounded by a hard wall
potential in dimensions. We show that for large , the
correlations at the edge of the Fermi gas (near the wall) at zero temperature
are described by a universal kernel, different from the universal edge kernel
valid for smooth potentials. We compute this dimensional hard edge kernel
exactly for a spherical domain and argue, using a generalized method of images,
that it holds close to any sufficiently smooth boundary. As an application we
compute the quantum statistics of the position of the fermion closest to the
wall. Our results are then extended in several directions, including non-smooth
boundaries such as a wedge, and also to finite temperature.Comment: 5 pages + 14 pages (Supp. Mat.), 6 figure
Weak local rules for planar octagonal tilings
We provide an effective characterization of the planar octagonal tilings
which admit weak local rules. As a corollary, we show that they are all based
on quadratic irrationalities, as conjectured by Thang Le in the 90s.Comment: 23 pages, 6 figure
Baryon Self-Energy With QQQ Bethe-Salpeter Dynamics In The Non-Perturbative QCD Regime: n-p Mass Difference
A qqq BSE formalism based on DB{\chi}S of an input 4-fermion Lagrangian of
`current' u,d quarks interacting pairwise via gluon-exchange-propagator in its
{\it non-perturbative} regime, is employed for the calculation of baryon
self-energy via quark-loop integrals. To that end the baryon-qqq vertex
function is derived under Covariant Instantaneity Ansatz (CIA), using Green's
function techniques. This is a 3-body extension of an earlier q{\bar q}
(2-body) result on the exact 3D-4D interconnection for the respective BS wave
functions under 3D kernel support, precalibrated to both q{\bar q} and qqq
spectra plus other observables. The quark loop integrals for the neutron (n) -
proton (p) mass difference receive contributions from : i) the strong SU(2)
effect arising from the d-u mass difference (4 MeV); ii) the e.m. effect of the
respective quark charges. The resultant n-p difference comes dominantly from
d-u effect (+1.71 Mev), which is mildly offset by e.m.effect (-0.44), subject
to gauge corrections. To that end, a general method for QED gauge corrections
to an arbitrary momentum dependent vertex function is outlined, and on on a
proportionate basis from the (two-body) kaon case, the net n-p difference works
out at just above 1 MeV. A critical comparison is given with QCD sum rules
results.Comment: be 27 pages, Latex file, and to be published in IJMPA, Vol 1
Size distributions of shocks and static avalanches from the Functional Renormalization Group
Interfaces pinned by quenched disorder are often used to model jerky
self-organized critical motion. We study static avalanches, or shocks, defined
here as jumps between distinct global minima upon changing an external field.
We show how the full statistics of these jumps is encoded in the
functional-renormalization-group fixed-point functions. This allows us to
obtain the size distribution P(S) of static avalanches in an expansion in the
internal dimension d of the interface. Near and above d=4 this yields the
mean-field distribution P(S) ~ S^(-3/2) exp(-S/[4 S_m]) where S_m is a
large-scale cutoff, in some cases calculable. Resumming all 1-loop
contributions, we find P(S) ~ S^(-tau) exp(C (S/S_m)^(1/2) -B/4 (S/S_m)^delta)
where B, C, delta, tau are obtained to first order in epsilon=4-d. Our result
is consistent to O(epsilon) with the relation tau = 2-2/(d+zeta), where zeta is
the static roughness exponent, often conjectured to hold at depinning. Our
calculation applies to all static universality classes, including random-bond,
random-field and random-periodic disorder. Extended to long-range elastic
systems, it yields a different size distribution for the case of contact-line
elasticity, with an exponent compatible with tau=2-1/(d+zeta) to
O(epsilon=2-d). We discuss consequences for avalanches at depinning and for
sandpile models, relations to Burgers turbulence and the possibility that the
above relations for tau be violated to higher loop order. Finally, we show that
the avalanche-size distribution on a hyper-plane of co-dimension one is in
mean-field (valid close to and above d=4) given by P(S) ~ K_{1/3}(S)/S, where K
is the Bessel-K function, thus tau=4/3 for the hyper plane.Comment: 34 pages, 30 figure
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