1,545 research outputs found
Model independent constraints from vacuum and in-medium QCD Sum Rules
We discuss QCD sum rule constraints based on moments of vector meson spectral
distributions in the vacuum and in a nuclear medium. Sum rules for the two
lowest moments of these spectral distributions do not suffer from uncertainties
related to QCD condensates of dimension higher than four. We exemplify these
relations for the case of the omega meson and discuss the issue of in-medium
mass shifts from this viewpoint.Comment: 6 pages, 2 figures, to appear in Eur. Phys. J.
Information on the structure of the rho meson from the pion form-factor
The electromagnetic pion form-factor is calculated in a Bethe-Salpeter
approach which accounts for pion rescattering. In the scattering kernel the
pion-pion contact interaction from lowest-order chiral perturbation theory is
considered together with an optional vector meson in the s-channel.
Correspondingly the virtual photon couples to a two-pion state and optionally
to the vector meson. It is shown that for reasonable ranges of input parameters
the experimentally observed pion form-factor cannot be described by the
iterated pion-pion contact interaction alone, i.e. without an elementary vector
meson. The inclusion of an elementary vector meson allows for an excellent
description. This completes a recent study (``Information on the structure of
the a_1 from tau decay'') where it has been shown that the a_1 meson can be
well understood as a rescattering process of rho meson and pion. Here it is
demonstrated that within the same formalism the rho meson cannot be understood
as a pion-pion rescattering process. This suggests that the chiral partners a_1
and rho are not only different in mass, but also different in nature.Comment: 12 page
Estimates for X(4350) Decays from the Effective Lagrangian Approach
The strong and electromagnetic decays of with quantum numbers and have been studied by using the effective Lagrangian
approach. The coupling constant between and
is determined with the help of the compositeness condition which means that
is a bound state of . Other coupling
constants applied in the calculation are determined phenomenologically. Our
numerical results show that, using the present data within the present model,
the possibility that is a molecule can not
be ruled out.Comment: 15 pages, 4 eps figure
Effect of spectral modification of on shear viscosity of a pion gas
We evaluate the shear viscosity of a pion gas in the relativistic kinetic
theory approach. The in-medium propagator of the meson at finite
temperature is used to evaluate the scattering amplitude in the
medium. The real and imaginary parts of the self-energy calculated from
one-loop diagrams are seen to have noticeable effects on the scattering
cross-section. The consequences on temperature dependence of the shear
viscosity evaluated in the Chapman-Enskog and relaxation time approximations
are studied
Pion transition form factor in the Regge approach and incomplete vector-meson dominance
The concept of incomplete vector-meson dominance and Regge models is applied
to the transition form factor of the pion. First, we argue that variants of the
chiral quark model fulfilling the chiral anomaly may violate the Terazawa-West
unitarity bounds, as these bounds are based on unverified assumptions for the
real parts of the amplitudes, precluding a possible presence of polynomial
terms. A direct consequence is that the transition form factor need not
necessarily vanish at large values of the photon virtuality. Moreover, in the
range of the BaBar experiment, the Terazawa-West bound is an order of magnitude
above the data, thus is of formal rather than practical interest. Then we
demonstrate how the experimental data may be properly explained with incomplete
vector-meson dominance in a simple model with one state, as well as in more
sophisticated Regge models. Generalizations of the simple Regge model along the
lines of Dominguez result in a proper description of the data, where one may
adjust the parameters in such a way that the Terazawa-West bound is satisfied
or violated. We also impose the experimental constraint from the Z -> pi0 gamma
decay. Finally, we point out that the photon momentum asymmetry parameter may
noticeably influence the precision analysis.Comment: 11 pages, 7 figure
The role of the N*(1535) resonance and the pi^- p --> KY amplitudes in the OZI forbidden pi N --> phi N reaction
We study the pi N --> phi N reaction close to the phi N threshold within the
chiral unitary approach, by combining the pi^- p --> K^+ Sigma^-, pi^- p -->
K^0 Sigma^0 and pi^- p --> K^0 Lambda amplitudes with the coupling of the phi
to the K components of the final states of these reactions via quantum loops.
We obtain a good agreement with experiment when the dominant pi^- p --> K^0
Lambda amplitude is constrained with its experimental cross section. We also
evaluate the coupling of the N*(1535) to phi N and find a moderate coupling as
a consequence of partial cancellation of the large KY components of the
N*(1535). We also show that the N*(1535) pole approximation is too small to
reproduce the measured cross section for the pi N --> phi N reaction.Comment: 10 pages, 6 figure
reaction near threshold
We analyze the total cross section data for near threshold
measured recently at SATURNE. Using an effective range approximation for the
on-shell S-wave final state interaction we extract from these data the
modulus fm of the threshold transition amplitude
. We present a calculation of various (tree-level) meson exchange
diagrams contributing to . It is essential that -emission from
the anomalous -vertex interferes destructively with
-emission from the proton lines. The contribution of scalar
-meson exchange to turns out to be negligibly small. Without
introducing off-shell meson-nucleon form factors the experimental value
fm can be reproduced with an -coupling constant
of . The results of the present approach agree qualitatively
with the J\"ulich model. We also perform a combined analysis of the reactions
and near threshold.Comment: Latex-file 6 pages, 2 Figure
Phases of QCD, Thermal Quasiparticles and Dilepton Radiation from a Fireball
We calculate dilepton production rates from a fireball adapted to the
kinematical conditions realized in ultrarelativistic heavy ion collisions over
a broad range of beam energies. The freeze-out state of the fireball is fixed
by hadronic observables. We use this information combined with the initial
geometry of the collision region to follow the space-time evolution of the
fireball. Assuming entropy conservation, its bulk thermodynamic properties can
then be uniquely obtained once the equation of state (EoS) is specified. The
high-temperature (QGP) phase is modelled by a non-perturbative quasiparticle
model that incorporates a phenomenological confinement description, adapted to
lattice QCD results. For the hadronic phase, we interpolate the EoS into the
region where a resonance gas approach seems applicable, keeping track of a
possible overpopulation of the pion phase space. In this way, the fireball
evolution is specified without reference to dilepton data, thus eliminating it
as an adjustable parameter in the rate calculations. Dilepton emission in the
QGP phase is then calculated within the quasiparticle model. In the hadronic
phase, both temperature and finite baryon density effects on the photon
spectral function are incorporated. Existing dilepton data from CERES at 158
and 40 AGeV Pb-Au collisions are well described, and a prediction for the
PHENIX setup at RHIC for sqrt(s) = 200 AGeV is given.Comment: 31 pages, 15 figures, final versio
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