776 research outputs found
Rho and Sigma Mesons in Unitarized Thermal ChPT
We present our recent results for the rho and sigma mesons considered as
resonances in pion-pion scattering in a thermal bath. We use chiral
perturbation theory to fourth order in p for the low energy behaviour, then
extend the analysis via the unitarization method of the Inverse Amplitude into
the resonance region. The width of the rho broadens about twice the amount
required by phase space considerations alone, its mass staying practically
constant up to temperatures of order 150 MeV. The sigma meson behaves in
accordance to chiral symmetry restoration expectations.Comment: Proc. Workshop Strong and Electroweak Matter 02, Heidelberg, German
Two-meson cloud contribution to the baryon antidecuplet self-energy
We study the self-energy of the SU(3) antidecuplet coming from two-meson
virtual clouds. Assuming that the exotic Theta+ belongs to an antidecuplet
representation with N(1710) as nucleon partner, we derive effective Lagrangians
that describe the decay of N(1710) into N pi pi with two pions in s- or p-wave.
It is found that the self-energies for all members of the antidecuplet are
attractive, and the larger strangeness particle is more bound. From two-meson
cloud, we obtain about 20 % of the empirical mass splitting between states with
different strangeness.Comment: 4 pages, 2 figures, 1 table, Talk given at the 10th International
Conference on Baryons (Baryons04), Palaiseau (France), October 25-29, 200
Heavy Quark Fluorescence
Heavy hadrons containing heavy quarks (for example, Upsilon-mesons) feature a
scale separation between the heavy quark mass (about 4.5 GeV for the b-quark)
and the QCD scale (about 0.3 GeV}) that controls effective masses of lighter
constituents. Therefore, as in ordinary molecules, the de-excitation of the
lighter, faster degrees of freedom leaves the velocity distribution of the
heavy quarks unchanged, populating the available decay channels in
qualitatively predictable ways. Automatically an application of the
Franck-Condon principle of molecular physics explains several puzzling results
of Upsilon(5S) decays as measured by the Belle collaboration, such as the high
rate of Bs*-anti Bs* versus Bs*-anti Bs production, the strength of three-body
B-anti B + pion decays, or the dip in B momentum shown in these decays. We
argue that the data is showing the first Sturm-Liouville zero of the
Upsilon(5S) quantum mechanical squared wavefunction, and providing evidence for
a largely b-anti b composition of this meson.Comment: 4 pages, 4 figures, Figure 2 updated and some typos corrected. To be
published in Physical Review Letter
Dynamically induced scalar quark confinement
We employ a functional approach to investigate the confinement problem in
quenched Landau gauge QCD. We demonstrate analytically that a linear rising
potential between massive quarks is generated by infrared singularities in the
dressed quark-gluon vertex. The selfconsistent mechanism that generates these
singularities is driven by the scalar Dirac amplitudes of the full vertex and
the quark propagator. These can only be present when chiral symmetry is broken.
We have thus uncovered a novel mechanism that directly links chiral symmetry
breaking with confinement.Comment: 12 pages, 2 figures; v2: clarifications added and typos corrected,
version to be published by MPL
Minimum of and the phase transition of the Linear Sigma Model in the large-N limit
We reexamine the possibility of employing the viscosity over entropy density
ratio as a diagnostic tool to identify a phase transition in hadron physics to
the strongly coupled quark-gluon plasma and other circumstances where direct
measurement of the order parameter or the free energy may be difficult.
It has been conjectured that the minimum of eta/s does indeed occur at the
phase transition. We now make a careful assessment in a controled theoretical
framework, the Linear Sigma Model at large-N, and indeed find that the minimum
of eta/s occurs near the second order phase transition of the model due to the
rapid variation of the order parameter (here the sigma vacuum expectation
value) at a temperature slightly smaller than the critical one.Comment: 22 pages, 19 figures, v2, some references and several figures added,
typos corrected and certain arguments clarified, revised for PR
The BES f_0(1810): a new glueball candidate
We analyze the f_0(1810) state recently observed by the BES collaboration via
radiative J/\psi decay to a resonant \phi\omega spectrum and confront it with
DM2 data and glueball theory. The DM2 group only measured \omega\omega decays
and reported a pseudoscalar but no scalar resonance in this mass region. A
rescattering mechanism from the open flavored KKbar decay channel is considered
to explain why the resonance is only seen in the flavor asymmetric \omega\phi
branch along with a discussion of positive C parity charmonia decays to
strengthen the case for preferred open flavor glueball decays. We also
calculate the total glueball decay width to be roughly 100 MeV, in agreement
with the narrow, newly found f_0, and smaller than the expected estimate of
200-400 MeV. We conclude that this discovered scalar hadron is a solid glueball
candidate and deserves further experimental investigation, especially in the
K-Kbar channel. Finally we comment on other, but less likely, possible
assignments for this state.Comment: 11 pages, 4 figures. Major substantive additions, including an
ab-initio, QCD-based computation of the glueball inclusive decay width,
evaluation of final state effects, and enhanced discussion of several
alternative possibilities. Our conclusions are unchanged: the BES f_0(1810)
is a promising glueball candidat
Local Two-Photon Couplings and the J=0 Fixed Pole in Real and Virtual Compton Scattering
The local coupling of two photons to the fundamental quark currents of a
hadron gives an energy-independent contribution to the Compton amplitude
proportional to the charge squared of the struck quark, a contribution which
has no analog in hadron scattering reactions. We show that this local
contribution has a real phase and is universal, giving the same contribution
for real or virtual Compton scattering for any photon virtuality and skewness
at fixed momentum transfer squared t. The t-dependence of this J=0 fixed Regge
pole is parameterized by a yet unmeasured even charge-conjugation form factor
of the target nucleon. The t=0 limit gives an important constraint on the
dependence of the nucleon mass on the quark mass through the Weisberger
relation. We discuss how this 1/x form factor can be extracted from high energy
deeply virtual Compton scattering and examine predictions given by models of
the H generalized parton distribution.Comment: 20 pages, 15 figure
Fermion family recurrences in the Dyson-Schwinger formalism
We study the multiple solutions of the truncated propagator Dyson-Schwinger
equation for a simple fermion theory with Yukawa coupling to a scalar field.
Upon increasing the coupling constant , other parameters being fixed, more
than one non-perturbative solution breaking chiral symmetry becomes possible
and we find these numerically. These ``recurrences'' appear as a mechanism to
generate different fermion generations as quanta of the same fundamental field
in an interacting field theory, without assuming any composite structure. The
number of recurrences or flavors is reduced to a question about the value of
the Yukawa coupling, and has no special profound significance in the Standard
Model. The resulting mass function can have one or more nodes and the
measurement that potentially detects them can be thought of as a collider-based
test of the virtual dispersion relation for the charged
lepton member of each family. This requires three independent measurements of
the charged lepton's energy, three-momentum and off-shellness. We illustrate
how this can be achieved for the (more difficult) case of the tau lepton
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