10,707 research outputs found
Gauged linear sigma model and pion-pion scattering
A simple gauged linear sigma model with several parameters to take the
symmetry breaking and the mass differences between the vector meson and the
axial vector meson into account is considered here as a possibly useful
template for the role of a light scalar in QCD as well as for (at a different
scale) an effective Higgs sector for some recently proposed walking technicolor
models. An analytic procedure is first developed for relating the Lagrangian
parameters to four well established (in the QCD application) experimental
inputs. One simple equation distinguishes three different cases:1. QCD with
axial vector particle heavier than vector particle, 2. possible technicolor
model with vector particle heavier than the axial vector one, 3. the unphysical
QCD case where both the KSRF and Weinberg relations hold. The model is applied
to the s-wave pion-pion scattering in QCD. Both the near threshold region and
(with an assumed unitarization) theglobal region up to about 800 MeV are
considered. It is noted that there is a little tension between the choice of
bare sigma mass parameter for describing these two regions. If a reasonable
globa fit is made, there is some loss of precision in the near threshold
region.Comment: 19 pages, 9 figure
The pion-pion scattering amplitude. III: Improving the analysis with forward dispersion relations and Roy equations
We complete and improve the fits to experimental scattering
amplitudes, both at low and high energies, that we performed in the previous
papers of this series. We then verify that the corresponding amplitudes satisfy
analyticity requirements, in the form of partial wave analyticity at low
energies, forward dispersion relations (FDR) at all energies, and Roy equations
below threshold; the first by construction, the last two, inside
experimental errors. Then we repeat the fits including as constraints FDR and
Roy equations. The ensuing central values of the various scattering amplitudes
verify very accurately FDR and, especially, Roy equations, and change very
little from what we found by just fitting data, with the exception of the D2
wave phase shift, for which one parameter moves by . These improved
parametrizations therefore provide a reliable representation of pion-pion
amplitudes with which one can test various physical relations. We also present
a list of low energy parameters and other observables. In particular, we find
,
and .Comment: Plain TeX. 29 figures. Version to be published in PRD, with improved
P and F wave
Thermal rates for baryon and anti-baryon production
We use a form of the fluctuation-dissipation theorem to derive formulas
giving the rate of production of spin-1/2 baryons in terms of the fluctuations
of either meson or quark fields. The most general formulas do not assume
thermal or chemical equilibrium. When evaluated in a thermal ensemble we find
equilibration times on the order of 10 fm/c near the critical temperature in
QCD.Comment: 22 pages, 4 tables and 2 figures, REVTe
A MASTER FORMULA FOR CHIRAL SYMMETRY BREAKING
We derive a master formula for chiral breaking, based on
the Veltman-Bell equations and the Peierls-Dyson relation. Our approach does
not rely on the use of the soft pion limit or an expansion around the chiral
limit, and yields exact results for on-shell pions. Threshold theorems for , , ,
, and are recovered, and corrections to them are given. The
reactions , ,
and are also
discussed. A general formula for scattering and a new one loop
effective action are obtained. The new effective action reproduces the KSFR
relation, and yields specific estimates for the pion polarisabilities. A
detailed comparison with baryon-free chiral perturbation theory to one loop is
made. An extension of our effective action to two loops is outlined.Comment: 119 pages + 7 Figures available from (I.Zahed
Quark Mass dependence at Two Loops for Meson Properties
This talks contains a short introduction to Chiral Perturbation Theory and
the existing calculations to two-loop order in the mesonic sector. I include a
discussion on which quantities the expansion can be organized in. The present
best values of the Low-Energy-Constants as determined from continuum physics
are given as well as the assumptions underlying the fits to experimental data.
I present plots of masses, decay constants and in as a
function of quark or meson masses. The talk ends with a list of things for
which it would be extremely useful to have good results from lattice QCD
calculations.Comment: 15 pages, plenary talk at T he XXV International Symposium on Lattice
Field Theory, July 30 - August 4 2007, Regensburg, Germany. v2: some
clarifying statements adde
On the spectral functions of scalar mesons
In this work we study the spectral functions of scalar mesons in one- and
two-channel cases. When the propagators satisfy the K\"allen-Lehman
representation a normalized spectral function is obtained, allowing to take
into account finite-width effects in the evaluation of decay rates. In the
one-channel case, suitable to the light sigma and k mesons, the spectral
function can deviate consistently from a Breit-Wigner shape. In the two-channel
case with one subthreshold channel the evaluated spectral function is well
approximated by a Flatte' distribution; when applying the study to the a_0(980)
and f_0(980) mesons the tree-level forbidden KK decay is analysed.Comment: 13 pages, 7 figure
Parameters of scalar resonances from the combined analysis of data on processes and decays
A combined analysis of data on isoscalar S-wave processes
and on decays
from the DM2, Mark III and BESIII
collaborations is performed to study mesons. The method of analysis is
based on analyticity and unitarity and uses an uniformization procedure. In the
analysis limited only to the multi-channel -scattering data, two
possible sets of parameters of the were found: in both cases the
mass was about 700 MeV but the total width was either about 600 or 930 MeV. The
extension of the analysis using only the DM2 and Mark III data on the
decays does not allow to choose between these sets. However, the data from
BESIII on the di-pion mass distribution in the decay
clearly prefers the wider state. Spectroscopic implications from
results of the analysis are also discussed.Comment: the formalism is also described (text overlap) in arXiv:1108.3725;
new extended analysis of data; revised PRD versio
Further Analysis on \sigma-particle Properties
In a previous work, we have reanalyzed the I=0 S-wave \pi\pi-scattering
phase-shift through a new method of Interfering Breit-Wigner amplitudes, and
have shown the existence of a light scalar, \sigma particle, accompanied by a
negative "repulsive core"-type background phase-shift, whose origin may have
some correspondence to a ``compensating'' \pi\pi contact interaction. In this
work we make further analysis on the phase shift under KKbar threshold from the
same standpoint, to determine precise values of \sigma mass and width, so far
as present experimental data are concerned.Comment: Updated with recent progres
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