18,230 research outputs found
Quark electric dipole moment induced by magnetic field
We show numerically that quarks develop an electric dipole moment in the
direction of a sufficiently intense magnetic field due to local fluctuations of
topological charge. This anomalous CP-odd effect is a spin analogue of the
Chiral Magnetic Effect in QCD.Comment: 6 pages, 4 figures, RevTex 4.0; revision: comments added, published
versio
Model study of generalized parton distributions with helicity flip
Generalized parton distributions with helicity flip are studied in the quark
sector, within a simple version of the MIT bag model, assuming an SU(6) wave
function for the proton target. In the framework under scrutiny it turns out
that only the generalized transversity distribution, H_T^q, is non vanishing.
For this quantity, the forward limit is properly recovered and numerical
results are found to underestimate recent lattice data for its first moment.
Positivity bounds recently proposed are fulfilled by the obtained distribution.
The relevance of the analysis for the planning of measurements of the quark
generalized transversity is addressed.Comment: 15 pages, 4 figures; shortened version accepted for publication in
Phys. Rev.
Exotic hybrid mesons in hard electroproduction
We estimate the sizeable cross section for deep exclusive electroproduction
of an exotic hybrid meson in the Bjorken regime. The production
amplitude scales like the one for usual meson electroproduction, i.e. as
. This is due to the non-vanishing leading twist distribution amplitude
for the hybrid meson, which may be normalized thanks to its relation to the
energy momentum tensor and to the QCD sum rules technique. The hard amplitude
is considered up to next-to-leading order in and we explore the
consequences of fixing the renormalization scale ambiguity through the BLM
procedure. We study the particular case where the hybrid meson decays through a
meson pair. We discuss the generalized distribution
amplitude and then calculate the production amplitude for this process. We
propose a forward-backward asymmetry in the production of and
mesons as a signal for the hybrid meson production. We briefly comment on
hybrid electroproduction at very high energy, in the diffractive limit where a
QCD Odderon exchange mechanism should dominate. The conclusion of our study is
that hard electroproduction is a promissing way to study exotic hybrid mesons,
in particular at JLAB, HERA (HERMES) or CERN (Compass)
Background Configurations, Confinement and Deconfinement on a Lattice with BPS Monopole Boundary Conditions
Finite temperature SU(2) lattice gauge theory is investigated in a 3D cubic
box with fixed boundary conditions provided by a discretized, static BPS
monopole solution with varying core scale . Using heating and cooling
techniques we establish that for discrete -values stable classical
solutions either of self-dual or of pure magnetic type exist inside the box.
Having switched on quantum fluctuations we compute the Polyakov line and other
local operators. For different and at varying temperatures near the
deconfinement transition we study the influence of the boundary condition on
the vacuum inside the box. In contrast to the pure magnetic background field
case, for the self-dual one we observe confinement even for temperatures quite
far above the critical one.Comment: to appear in EPJ
Interfering Doorway States and Giant Resonances. II: Transition Strengths
The mixing of the doorway components of a giant resonance (GR) due to the
interaction via common decay channels influences significantly the distribution
of the multipole strength and the energy spectrum of the decay products of the
GR. The concept of the partial widths of a GR becomes ambiguous when the mixing
is strong. In this case, the partial widths determined in terms of the - and
-matrices must be distinguished. The photoemission turns out to be most
sensitive to the overlapping of the doorway states. At high excitation
energies, the interference between the doorway states leads to a restructuring
towards lower energies and apparent quenching of the dipole strength.Comment: 17 pages, LaTeX, 5 figures as JPEG, to appear in PRC (July 1997
Infrared behavior and Gribov ambiguity in SU(2) lattice gauge theory
For SU(2) lattice gauge theory we study numerically the infrared behavior of
the Landau gauge ghost and gluon propagators with the special accent on the
Gribov copy dependence. Applying a very efficient gauge fixing procedure and
generating up to 80 gauge copies we find that the Gribov copy effect for both
propagators is essential in the infrared. In particular, our best copy dressing
function of the ghost propagator approaches a plateau in the infrared, while
for the random first copy it still grows. Our best copy zero-momentum gluon
propagator shows a tendency to decrease with growing lattice size which
excludes singular solutions. Our results look compatible with the so-called
decoupling solution with a non-singular gluon propagator. However, we do not
yet consider the Gribov copy problem to be finally resolved.Comment: 9 pages, 9 figure
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