42 research outputs found
Spectral representation and QCD sum rules for nucleon at finite temperature
We examine the problem of constructing spectral representations for two point
correlation functions, needed to write down the QCD sum rules in the medium. We
suggest constructing them from the Feynman diagrams for the correlation
functions. As an example we use this procedure to write the QCD sum rules for
the nucleon current at finite temperature
Decay constants, light quark masses and quark mass bounds from light quark pseudoscalar sum rules
The flavor and pseudoscalar correlators are investigated using
families of finite energy sum rules (FESR's) known to be very accurately
satisfied in the isovector vector channel. It is shown that the combination of
constraints provided by the full set of these sum rules is sufficiently strong
to allow determination of both the light quark mass combinations ,
and the decay constants of the first excited pseudoscalar mesons in
these channels. The resulting masses and decay constants are also shown to
produce well-satisfied Borel transformed sum rules, thus providing non-trivial
constraints on the treatment of direct instanton effects in the FESR analysis.
The values of and obtained are in good agreement with the
values implied by recent hadronic decay analyses and the ratios obtained
from ChPT. New light quark mass bounds based on FESR's involving weight
functions which strongly suppress spectral contributions from the excited
resonance region are also presented.Comment: 28 pages, 10 figure
QCD Sum Rules for Hyperons in Nuclear Matter
Within finite-density QCD sum-rule approach we investigate the self-energies
of hyperons propagating in nuclear matter from a correlator of
interpolating fields evaluated in the nuclear matter ground state. We
find that the Lorentz vector self-energy of the is similar to the
nucleon vector self-energy. The magnitude of Lorentz scalar self-energy of the
is also close to the corresponding value for nucleon; however, this
prediction is sensitive to the strangeness content of the nucleon and to the
assumed density dependence of certain four-quark condensate. The scalar and
vector self-energies tend to cancel, but not completely. The implications for
the couplings of to the scalar and vector mesons in nuclear matter and
for the spin-orbit force in a finite nucleus are discussed.Comment: 20 pages in revtex, 6 figures available under request as ps files,
UMD preprint #94--11
Pade-Improvement of QCD Running Coupling Constants, Running Masses, Higgs Decay Rates, and Scalar Channel Sum Rules
We discuss Pad\'e-improvement of known four-loop order results based upon an
asymptotic three-parameter error formula for Pad\'e-approximants. We derive an
explicit formula estimating the next-order coefficient from the previous
coefficients in a series . We show that such an
estimate is within 0.18% of the known five-loop order term in the O(1)
-function, and within 10% of the known five-loop term in the O(1)
anomalous mass-dimension function . We apply the same formula to
generate a [22] Pad\'e-summation of the QCD -function and anomalous
mass dimension in order to demonstrate both the relative insensitivity of the
evolution of and the running quark masses to higher order
corrections, as well as a somewhat increased compatibility of the present
empirical range for with the range anticipated via evolution
from the present empirical range for . For
we demonstrate that positive zeros of any [22] Pad\'e-summation estimate of
the all-orders -function which incorporates known two-, three-, and
four-loop contributions necessarily correspond to ultraviolet fixed points,
regardless of the unknown five-loop term. Pad\'e-improvement of higher-order
perturbative expressions is presented for the decay rates of the Higgs into two
gluons and into a pair, and is used to show the relative
insensitivity of these rates to higher order effects. However,
Pad\'e-improvement of the purely-perturbative component of scalar/pseudoscalar
current correlation functions is indicative of large theoretical uncertainties
in QCD sum rules for these channels, particularly if the continuum-threshold
parameter is near 1 GeV.Comment: latex, 22 pages, 8 figures, references correcte
QCD Sum Rules and Applications to Nuclear Physics
Applications of QCD sum-rule methods to the physics of nuclei are reviewed,
with an emphasis on calculations of baryon self-energies in infinite nuclear
matter. The sum-rule approach relates spectral properties of hadrons
propagating in the finite-density medium, such as optical potentials for
quasinucleons, to matrix elements of QCD composite operators (condensates). The
vacuum formalism for QCD sum rules is generalized to finite density, and the
strategy and implementation of the approach is discussed. Predictions for
baryon self-energies are compared to those suggested by relativistic nuclear
physics phenomenology. Sum rules for vector mesons in dense nuclear matter are
also considered.Comment: 92 pages, ReVTeX, 9 figures can be obtained upon request (to Xuemin
Jin
QCD sum rules at finite temperature
We derive thermal QCD sum rules for the correlation function of two vector
currents in the rho-meson channel. It takes into account the leading
non-perturbative corrections from the additional operators, which appear due to
the breakdown of Lorentz invariance at finite temperature. The mixing of the
new operators has a drastic effect on their coefficients. The thermal average
of all the operators can be related to that of the quark condensate and the
energy density. The sum rules then yield the temperature dependence of the
parameters of the -meson, namely its mass and coupling to the vector
current. Our result is that these parameters are practically independent of
temperature at least up to a temperature of 125 MeV.Comment: 11 pages, revtex, 2 figure
QCD Analysis of Hadronic Decays Revisited
The calculation of perturbative corrections to the spectral moments
observable in hadronic decays is reconsidered. The exact
order- results and the resummation procedure of Le~Diberder and
Pich are compared with a partial resummation of the perturbative series based
on the analysis of so-called renormalon chains. The perturbative analysis is
complemented by a model-independent description of power corrections. For the
contributions of dimension four and six in the OPE, it is demonstrated how
infrared renormalon ambiguities in the definition of perturbation theory can be
absorbed by a redefinition of nonperturbative parameters. We find that previous
determinations of QCD parameters from a measurement of spectral moments in
decays have underestimated the theoretical uncertainties. Given the
present understanding of the asymptotic behaviour of perturbation theory, the
running coupling constant can be measured at best with a theoretical
uncertainty , and the gluon condensate
with an uncertainty of order its magnitude. Two weighted integrals of the
hadronic spectral function are constructed, which can be used to test the
absence of dimension-two operators and to measure directly the gluon
condensate.Comment: 36 pages LaTeX, uses epsf, 5 figures contained in separate file
figures.u