32,656 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 $b$ 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

### Dynamical Systems, Stability, and Chaos

In this expository and resources chapter we review selected aspects of the
mathematics of dynamical systems, stability, and chaos, within a historical
framework that draws together two threads of its early development: celestial
mechanics and control theory, and focussing on qualitative theory. From this
perspective we show how concepts of stability enable us to classify dynamical
equations and their solutions and connect the key issues of nonlinearity,
bifurcation, control, and uncertainty that are common to time-dependent
problems in natural and engineered systems. We discuss stability and
bifurcations in three simple model problems, and conclude with a survey of
recent extensions of stability theory to complex networks.Comment: 28 pages, 10 figures. 26/04/2007: The book title was changed at the
last minute. No other changes have been made. Chapter 1 in: J.P. Denier and
J.S. Frederiksen (editors), Frontiers in Turbulence and Coherent Structures.
World Scientific Singapore 2007 (in press

### Liberating the NHS; commissioning, outsourcing and a new politics debate

In the short months following the result of the UK 2010 General election,
a new Government White Paper has been released entitled: Equity and
Excellence: Liberating the NHS (Department of Health (DH), 2010a). It strives
to distance itself from previous health-care proposals (DH, 2009), yet if the
initiatives of this latest paper are combined against previous initiatives,
also using high impact declarative terms, such as competition and choice,
it is clear that little has changed and more important principles than saving
money are at risk

### Light-cone sum rules for $B \to \pi$ form factors revisited

We reconsider and update the QCD light-cone sum rules for $B\to \pi$ form
factors. The gluon radiative corrections to the twist-2 and twist-3 terms in
the correlation functions are calculated. The $\bar{MS}$ $b$-quark mass is
employed, instead of the one-loop pole mass used in the previous analyses. The
light-cone sum rule for $f^+_{B\pi}(q^2)$ is fitted to the measured
$q^2$-distribution in $B\to \pi l \nu_l$, fixing the input parameters with the
largest uncertainty: the Gegenbauer moments of the pion distribution amplitude.
For the $B\to \pi$ vector form factor at zero momentum transfer we predict
$f^+_{B\pi}(0)= 0.26^{+0.04}_{-0.03}$. Combining it with the value of the
product $|V_{ub}f^+_{B\pi}(0)|$ extracted from experiment, we obtain
$|V_{ub}|=(3.5\pm 0.4\pm 0.2\pm 0.1) \times 10^{-3}$. In addition, the scalar
and penguin $B\to \pi$ form factors $f^0_{B\pi}(q^2)$ and $f^T_{B\pi}(q^2)$ are
calculated.Comment: 33 pages, 7 figures, one figure and a few comments added, version to
appear in JHE

### The C-Theorem and Chiral Symmetry Breaking in Asymptotically Free Vectorlike Gauge Theories

We confront Cardy's suggested c-function for four-dimensional field theories
with the spontaneous breaking of chiral symmetries in asymptotically free
vectorlike gauge theories with fermions transforming according to different
representations under the gauge group. Assuming that the infrared limit of the
c-function is determined by the dimension of the associated Goldstone manifold,
we find that this c-function always decreases between the ultraviolet and
infrared fixed points.Comment: 8 pages, no figures, a few references adde

### Operator Relations for SU(3) Breaking Contributions to K and K* Distribution Amplitudes

We derive constraints on the asymmetry a1 of the momentum fractions carried
by quark and antiquark in K and K* mesons in leading twist. These constraints
follow from exact operator identities and relate a1 to SU(3) breaking
quark-antiquark-gluon matrix elements which we determine from QCD sum rules.
Comparing our results to determinations of a1 from QCD sum rules based on
correlation functions of quark currents, we find that, for a1^\parallel(K*) the
central values agree well and come with moderate errors, whereas for a1(K) and
a1^\perp(K*) the results from operator relations are consistent with those from
quark current sum rules, but come with larger uncertainties. The consistency of
results confirms that the QCD sum rule method is indeed suitable for the
calculation of a1. We conclude that the presently most accurate predictions for
a1 come from the direct determination from QCD sum rules based on correlation
functions of quark currents and are given by: a1(K) = 0.06\pm 0.03,
a1^\parallel(K*) = 0.03\pm 0.02, a1^\perp(K*) = 0.04\pm 0.03.Comment: 21 page

### B_s^0 - bar{B}_s^0 Mixing and the B_s -> J/psi phi Asymmetry in Supersymmetric Models

We analyse supersymmetric contributions to B_s mixing and their impact on
mixing-induced CP asymmetries, using the mass insertion approximation. We
discuss in particular the correlation of SUSY effects in the CP asymmetries of
B_s -> J/psi phi and B_d -> phi K_S and find that the mass insertions dominant
in B_s mixing and B_d -> phi K_S are (delta_{23}^d)_{LL, RR} and
(delta_{23}^d)_{LR, RL}, respectively. We show that models with dominant
(delta_{23}^d)_{LR, RL} can accomodate a negative value of S_{phi K_S}, in
agreement with the BELLE measurement of that observable, but yield a B_s mixing
phase too small to be observed. On the other hand, models with dominant
(delta_{23}^d)_{LL, RR} predict sizeable SUSY contributions to both Delta M_s
and the mixing phase, but do not allow the asymmetry in B_d -> phi K_S to
become negative, except for small values of the average down squark mass,
which, in turn, entail a value of Delta M_s too large to be observed at the
Tevatron and the LHC. We conclude that the observation of B_s mixing at hadron
machines, together with the confirmation of a negative value of S_{phi K_S},
disfavours models with a single dominant mass insertion.Comment: 17 pages, 5 figure

### Strong "quantum" chaos in the global ballooning mode spectrum of three-dimensional plasmas

The spectrum of ideal magnetohydrodynamic (MHD) pressure-driven (ballooning) modes in strongly nonaxisymmetric toroidal systems is difficult to analyze numerically owing to the singular nature of ideal MHD caused by lack of an inherent scale length. In this paper, ideal MHD is regularized by using a k-space cutoff, making the ray tracing for the WKB ballooning formalism a chaotic Hamiltonian billiard problem. The minimum width of the toroidal Fourier spectrum needed for resolving toroidally localized ballooning modes with a global eigenvalue code is estimated from the Weyl formula. This phase-space-volume estimation method is applied to two stellarator cases

### Non-parametric models in the monitoring of engine performance and condition: Part 2: non-intrusive estimation of diesel engine cylinder pressure and its use in fault detection

An application of the radial basis function model, described in Part 1, is demonstrated on a four-cylinder DI diesel engine with data from a wide range of speed and load settings. The prediction capabilities of the trained model are validated against measured data and an example is given of the application of this model to the detection of a slight fault in one of the cylinders

### Models for Light-Cone Meson Distribution Amplitudes

Leading-twist distribution amplitudes (DAs) of light mesons like pi,rho etc.
describe the leading nonperturbative hadronic contributions to exclusive QCD
reactions at large energy transfer, for instance electromagnetic form factors.
They also enter B decay amplitudes described in QCD factorisation, in
particular nonleptonic two-body decays. Being nonperturbative quantities, DAs
cannot be calculated from first principles, but have to be described by models.
Most models for DAs rely on a fixed order conformal expansion, which is
strictly valid for large factorisation scales, but not always sufficient in
phenomenological applications. We derive models for DAs that are valid to all
orders in the conformal expansion and characterised by a small number of
parameters which are related to experimental observables.Comment: 19 pages, 10 figure

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