2,082 research outputs found
Quark distributions in QCD sum rules: unexpected features and paradoxes
Some very unusual features of the hadron structure functions, obtained in the
generalized QCD sum rules, like the surprisingly strong difference between
longitudinally and transversally polarized mesons structure functions
and the strong suppression of the gluon sea in longitudinally polarized
mesons are discussed. Also the problem of exact zero contribution of gluon
condensates to pion and longitudinally polarized meson quark
distributions is discussed.Comment: 9 pages, 5 fig
Improved Calculations of Quark Distributions in Hadrons: the case of pion
The earlier introduced method of calculation of quark distributions in
hadrons, based on QCD sum rules, is improved. The imaginary part of the virtual
photon forward scattering amplitude on some hadronic current is considered in
the case, when initial and final virtualities of the current , and
are different, . The operator product expansion (OPE)
in , is performed. The sum rule for quark distribution is
obtained using double dispersion representation of the amplitude on one side in
terms of calculated in QCD OPE and on the other side in terms of physical
states contributions. Double Borel transformation in , is
applied to the sum rule, killing background non-diagonal transition terms,
which deteriorated the accuracy in previous calculations. The case of valence
quark distribution in pion is considered, which was impossible to treat by the
previous method. OPE up to dimension 6 operators is performed and leading order
perturbative corrections are accounted. Valence -quark distribution in
was found at intermediate , and normalization point
. These results may be used as input for evolution equations.Comment: 29 pages, LaTeX 2e, 13 eps figures include
Geometric variational problems of statistical mechanics and of combinatorics
We present the geometric solutions of the various extremal problems of
statistical mechanics and combinatorics. Together with the Wulff construction,
which predicts the shape of the crystals, we discuss the construction which
exhibits the shape of a typical Young diagram and of a typical skyscraper.Comment: 10 page
Pentaquark decay is suppressed by chirality conservation
It is shown, that if the pentaquark baryon can be
represented by the local quark current , its decay is forbidden in the limit of chirality conservation. The
decay width is proportional to , where , is quark condensate, and,
therefore, is strongly suppressed. Also the polarization operator of the
pentaquark current with isospin 1 is calculated using the operator product
expansion and estimation for it mass is obtained .Comment: 4 pages, 1 fig, typos correcte
Gluon Polarization from QCD Sum Rules
The gluon polarization in a nucleon can be defined in a gauge
invariant way as the integral over the Ioffe-time distribution of polarized
gluons. We argue that for sufficiently regular polarized gluon distributions
is dominated by contributions from small and moderate values of the
Ioffe-time z < 10. As a consequence can be estimated with 20%
accuracy from the first two even moments of the polarized gluon distribution,
and its behavior at small values of Bjorken x or, equivalently, at large
Ioffe-times z. We employ this idea and compute the first two moments of the
polarized gluon distribution within the framework of QCD sum rules. Combined
with the color coherence hypothesis we obtain an upper limit for at a typical scale .Comment: 12 pages, Latex, 2 figures include
Performance analysis of an interacting quantum dot thermoelectric system
We analyze the nanocaloritronic performance of an interacting quantum dot
that is subject to an applied bias and an applied temperature gradient. It is
now well known that, in the absence of phonon contribution, a weakly coupled
non-interacting quantum dot can operate at thermoelectric efficiencies
approaching the Carnot limit. However, it has also been recently pointed out
that such peak efficiencies can only be achieved when operated in the
reversible limit, with a vanishing current and hence a vanishing power output.
In this paper, we point out three fundamental results affecting the
thermoelectric performance due to the inclusion of Coulomb interactions: a) The
reversible operating point carries zero efficiency, b) operation at finite
power output is possible even at peak efficiencies approaching the Carnot
value, and c) the evaluated trends of the the maximum efficiency deviate
considerably from the conventional {\it{figure of merit}} based result.
Finally, we also analyze our system for thermoelectric operation at maximum
power output.Comment: 10 pages, 6 figures, Resubmission- to be published in Phys. Rev.
Spontaneous violation of chiral symmetry in QCD vacuum is the origin of baryon masses and determines baryon magnetic moments and their other static properties
A short review is presented of the spontaneous violation of chiral symmetry
in QCD vacuum. It is demonstrated, that this phenomenon is the origin of baryon
masses in QCD. The value of nucleon mass is calculated as well as the masses of
hyperons and some baryonic resonances and expressed mainly through the values
of quark condensates -- -- the vacuum
expectation values (v.e.v.) of quark field. The concept of vacuum expectation
values induced by external fields is introduced. It is demonstrated that such
v.e.v. induced by static electromagnetic field results in quark condensate
magnetic susceptibility, which plays the main role in determination of baryon
magnetic moments. The magnetic moments of proton, neutron and hyperons are
calculated. The results of calculation of baryon octet -decay constants
are also presented.Comment: 13 pades, 5 figures. Dedicated to 85-birthday of acad. S.T.Belyaev.
To be published in Phys.At.Nucl. Few references are correcte
Calculation of the pentaquark width by QCD sum rule
The pentaquark width is calculated in QCD sum rules. Result for
show, that can vary in the region less than
1. The main conclusion is, that if pentaquark is genuine states then sum
rules really predict the narrow width of pentaquark , and the
suppression of the width is both parametrical and numerical.Comment: 8 Ppages, 3 figures,the numerical error was corrected, two figures
are modified. In the limit of errors the result did not change significantl
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