19,290 research outputs found
Light-cone sum rules for form factors revisited
We reconsider and update the QCD light-cone sum rules for form
factors. The gluon radiative corrections to the twist-2 and twist-3 terms in
the correlation functions are calculated. The -quark mass is
employed, instead of the one-loop pole mass used in the previous analyses. The
light-cone sum rule for is fitted to the measured
-distribution in , fixing the input parameters with the
largest uncertainty: the Gegenbauer moments of the pion distribution amplitude.
For the vector form factor at zero momentum transfer we predict
. Combining it with the value of the
product extracted from experiment, we obtain
. In addition, the scalar
and penguin form factors and are
calculated.Comment: 33 pages, 7 figures, one figure and a few comments added, version to
appear in JHE
A Review of the Agronomic Characteristics of Endophyte-Free and Endophyte-Infected Tall Fescue
Agronomic differences between endophyte- (Acremonium coenophialum Morgan Jones and Gams) free and endophyte-infected tall fescue (Festuca arundinacea Schreb.) exist, and should be considered when implementing a management strategy. Although dry matter yield of tall fescue does not generally appear to be influenced by endophyte infection status, endophyte infection has been shown to improve seedling performance and survival, is associated with insect and nematode resistance, drought resistance, improved nitrogen assimilation, and higher seed set. Considering all biologically valuable characters of the endophyte-tall fescue relationship, survival of endophyte-infected tall fescue is probably better than that of endophyte-free tall fescue, especially in drought- or heat-stressed environments. Despite problems which growers have reported in establishing stand; of endophyte-free tall fescue varieties, there is still a decided advantage to seeding endophyte-free tall fescue because of improved livestock performance. However, greater attention to management is needed, particularly during the establishment year. Good seedbed preparation, including fertility improvements, should be stressed for endophyte-free tall fescue. Since endophyte-free tall fescue seedlings are not as vigorous as endophyte-infected tall fescue seedlings, using other grasses as nurse crops, or seeding with clovers, is not recommended when establishing new stands. Environmental or imposed stress on newly established endophyte-free tall fescue stands should be avoided by selecting optimum planting dates, and limiting livestock access. Top growth should not be grazed or clipped shorter than 3-4 in. (7-10 cm) during the first year of growth
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Exomars entry and descent science
The entry, descent and landing of ExoMars offer a rare (once-per-mission) opportunity to perform in situ investigation of the martian environment over a wide altitude range. We present an initial assessment of the atmospheric science that can be performed using sensors of the Entry, Descent and Landing System (EDLS), over and above the expected engineering information. This is intended to help fulfill the concept of an Atmospheric Parameters Package (APP), as mentioned in the ExoMars draft Science Management Plan [ESA, 2005].
Mars' atmosphere is highly variable in time and space, due to phenomena including inertio-gravity waves, thermal tide effects, dust, solar wind conditions, and diurnal, seasonal and topographic effects. Atmospheric profile measurements, drawing on heritage from the Huygens Atmospheric Structure Instrument (HASI), which encountered Titan's atmosphere
in 2005 [1], should allow us to address questions of the martian atmosphere's structure, dynamics and variability
Phenomenological Bounds on B to Light Semileptonic Form Factors
The form factors for the weak currents between B and light mesons are studied
by relating them to the corresponding D form factors at q^2_{max} according to
HQET, by evaluating them at q^2=0 by QCD sum rules, and by assuming a polar q^2
dependence. The results found are consistent with the information obtained from
exclusive non-leptonic two-body decays and, with the only exception of A_1,
with lattice calculations.Comment: 8 LaTeX pages + 2 figures. Will appear in Mod. Phys. Lett.
Nonperturbative QCD Contributions to the Semileptonic Decay Width of the B Meson
Nonperturbative QCD contributions to the inclusive semileptonic decay of the
B meson consist of the dynamic and kinematic components. We calculate the decay
width in an approach based on the light-cone expansion and the heavy quark
effective theory, which is able to include both components of nonperturbative
QCD contributions. The kinematic component results in the phase-space extension
and is shown to be quantitatively crucial, which could increase the decay width
significantly. We find that the semileptonic decay width is enhanced by
long-distance strong interactions by +(9\pm 6)%. This analysis is used to
determine the CKM matrix element |V_{cb}| with a controlled theoretical error.
Implications of the phase-space effects for the nonleptonic decay widths of b
hadrons are briefly discussed. The experimental evidence for the phase-space
effects is pointed out.Comment: 16 pages, Latex, 3 eps figures included, published version
(discussion extended, references updated, the main result unchanged
Progress in the Neural Network Determination of Polarized Parton Distributions
We review recent progress towards a determination of a set of polarized
parton distributions from a global set of deep-inelastic scattering data based
on the NNPDF methodology, in analogy with the unpolarized case. This method is
designed to provide a faithful and statistically sound representation of parton
distributions and their uncertainties. We show how the FastKernel method
provides a fast and accurate method for solving the polarized DGLAP equations.
We discuss the polarized PDF parametrizations and the physical constraints
which can be imposed. Preliminary results suggest that the uncertainty on
polarized PDFs, most notably the gluon, has been underestimated in previous
studies.Comment: 5 pages, 2 figures; to appear in the proceedings of DIS 2010, Firenz
Experimental evidence for the role of cantori as barriers in a quantum system
We investigate the effect of cantori on momentum diffusion in a quantum
system. Ultracold caesium atoms are subjected to a specifically designed
periodically pulsed standing wave. A cantorus separates two chaotic regions of
the classical phase space. Diffusion through the cantorus is classically
predicted. Quantum diffusion is only significant when the classical phase-space
area escaping through the cantorus per period greatly exceeds Planck's
constant. Experimental data and a quantum analysis confirm that the cantori act
as barriers.Comment: 19 pages including 9 figures, Accepted for publication in Physical
Review E in March 199
The treatment of the infrared region in perturbative QCD
We discuss the contribution coming from the infrared region to NLO matrix
elements and/or coefficient functions of hard QCD processes. Strictly speaking,
this contribution is not known theoretically, since it is beyond perturbative
QCD. For DGLAP evolution all the infrared contributions are collected in the
phenomenological input parton distribution functions (PDFs), at some relatively
low scale Q_0; functions which are obtained from a fit to the `global' data.
However dimensional regularization sometimes produces a non-zero result coming
from the infrared region. Instead of this conventional regularization
treatment, we argue that the proper procedure is to first subtract from the NLO
matrix element the contribution already generated at the same order in \alpha_s
by the LO DGLAP splitting function convoluted with the LO matrix element. This
prescription eliminates the logarithmic infrared divergence, giving a
well-defined result which is consistent with the original idea that everything
below Q_0 is collected in the PDF input. We quantify the difference between the
proposed treatment and the conventional approach using low-mass Drell-Yan
production and deep inelastic electron-proton scattering as examples; and
discuss the potential impact on the `global' PDF analyses. We present arguments
to show that the difference cannot be regarded as simply the use of an
alternative factorization scheme.Comment: 15 pages, 5 figures, title changed, text considerably modified to
improve presentation, and discussion section enlarge
Physics Opportunities with the FCC-hh Injectors
In this chapter we explore a few examples of physics opportunities using the
existing chain of accelerators at CERN, including potential upgrades. In this
context the LHC ring is also considered as a part of the injector system. The
objective is to find examples that constitute sensitive probes of New Physics
that ideally cannot be done elsewhere or can be done significantly better at
theCERN accelerator complex. Some of these physics opportunities may require a
more flexible injector complex with additional functionality than that just
needed to inject protons into the FCC-hh at the right energy, intensity and
bunch structure. Therefore it is timely to discuss these options concurrently
with the conceptual design of the FCC-hh injector system.Comment: 13 pages, chapter 5 in Physics at the FCC-hh, a 100 TeV pp collide
Structure of the axial-vector meson and the strong coupling constant with the light-cone QCD sum rules
In this article, we take the point of view that the charmed axial-vector
meson is the conventional meson and calculate the
strong coupling constant in the framework of the light-cone
QCD sum rules approach. The numerical values of strong coupling constants
and are very large, and support the
hadronic dressing mechanism. Just like the scalar mesons and
, the scalar meson and axial-vector meson
may have small kernels of the typical
meson size, the strong couplings to the hadronic channels (or the virtual
mesons loops) may result in smaller masses than the conventional
mesons in the constituent quark models, and enrich the pure states
with other components.Comment: 17 pages, 7 figures, revised version. In the first version, I take
the value in numerical calculation, in the
revised version, I take a small value , the
value of the strong coupling constant is also change
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