128 research outputs found
A Gauge and Lorentz covariant Approximation for the Quark Propagator in an arbitrary Gluon Field
We decompose the quark propagator in the presence of an arbitrary gluon field
with respect to a set of Dirac matrices. The four-dimensional integrals which
arise in first order perturbation theory are rewritten as line-integrals along
certain field lines, together with a weighted integration over the various
field lines. It is then easy to transform the propagator into a form involving
path ordered exponentials. The resulting expression is non-perturbative and has
the correct behavior under Lorentz transformations, gauge transformations and
charge conjugation. Furthermore it coincides with the exact propagator in first
order of the coupling g. No expansion with respect to the inverse quark mass is
involved, the expression can even be used for vanishing mass. For large mass
the field lines concentrate near the straight line connection and simple
results can be obtained immediately.Comment: 15 pages, 3 figures. Replaced with version to appear in Eur. Phys. J.
Spectroscopy of B_c Mesons in the Relativized Quark Model
We calculate the spectrum of the charm-beauty mesons using the relativized
quark model. Using the wavefunctions from this model we compute the radiative
widths of excited c\bar{b} states. The hadronic transition rates between
c\bar{b} states are estimated using the Kuang-Yan approach and are combined
with the radiative widths to give estimates of the relative branching ratios.
These results are combined with production rates at the Tevatron and the LHC to
suggest promising signals for excited B_c states. Our results are compared with
other models to gauge the reliability of the predictions and point out
differences.Comment: 15 pages, 1 fig. uses revtex4. References adde
The 1P quarkonium fine splittings at NLO
We calculate the 1P heavy quarkonium fine splittings at NLO and discuss the
impact of the calculation on the chi_b(1P) splittings.Comment: 10 pages, 3 figure
Quark-antiquark pair production in space-time dependent fields
Fermion-antifermion pair-production in the presence of classical fields is
described based on the retarded and advanced fermion propagators. They are
obtained by solving the equation of motion for the Dirac Green's functions with
the respective boundary conditions to all orders in the field. Subsequently,
various approximation schemes fit for different field configurations are
explained. This includes longitudinally boost-invariant forms. Those occur
frequently in the description of ultrarelativistic heavy-ion collisions in the
semiclassical limit. As a next step, the gauge invariance of the expression for
the expectation value of the number of produced fermion-antifermion pairs as a
functional of said propagators is investigated in detail. Finally, the
calculations are carried out for a longitudinally boost-invariant model-field,
taking care of the last issue, especially.Comment: 32 pages, 8 figures, revised versio
Nonperturbative QCD Vacuum Effects in Nonlocal Quark Dynamics
A straightforward calculation reveals the essentially nonlocal character of
the leading heavy interaction arising from nonperturbative gluon
field correlations in the model of a fluctuating QCD vacuum. In light of this
quarkonium spin splitting ratio predictions which have supported the scalar
confinement ansatz are reconsidered as a specific example of possible
consequences for spectroscopy.Comment: Latex, 9 page
Pentaquarks with One Color Sextet Diquark
The masses of pentaquarks are calculated within the framework of
a semirelativistic effective QCD Hamiltonian, using a diquark picture. This
approximation allows a correct treatment of the confinement, assumed here to be
similar to a Y-junction. With only color antitriplet diquarks, the mass of the
pentaquark candidate with positive parity is found around 2.2 GeV. It
is shown that, if a color sextet diquark is present, the lowest
pentaquark is characterized by a much smaller mass with a negative parity. A
mass below 1.7 GeV is computed, if the masses of the color antitriplet and
color sextet diquarks are taken similar
Meson-like Baryons and the Spin-Orbit Puzzle
I describe a special class of meson-like \Lambda_Q excited states and present
evidence supporting the similarity of their spin-independent spectra to those
of mesons. I then examine spin-dependent forces in these baryons, showing that
predicted effects of spin-orbit forces are small for them for the same reason
they are small for the analogous mesons: a fortuitous cancellation between
large spin-orbit forces due to one-gluon-exchange and equally large inverted
spin-orbit forces due to Thomas precession in the confining potential. In
addition to eliminating the baryon spin-orbit puzzle in these states, this
solution provides a new perspective on spin-orbit forces in all baryons.Comment: 24 pages, 4 figure
Ab Initio Calculation of Relativistic Corrections to the Static Interquark potential I: SU(2) Gauge Theory
We test the capability of state-of-the-art lattice techniques for a precise
determination of relativistic corrections to the static interquark potential,
by use of SU(2) gauge theory. Emphasis is put on the short range structure of
the spin dependent potentials, with lattice resolution a ranging from a approx
0.04 fm (at beta=2.74) down to a approx 0.02 fm (at beta=2.96) on volumes of
32^4 and 48^4 lattice sites. We find a new short range Coulomb-like
contribution to the spin-orbit potential V_1'.Comment: 37 pages REVTeX with 20 encapsuled ps figure
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