45 research outputs found
Rho-omega mixing in asymmetric nuclear matter via QCD sum rule approach
We evaluate the operator product expansion (OPE) for a mixed correlator of
the isovector and isoscalar vector currents in the background of the nucleon
density with intrinsic isospin asymmetry [i.e. excess of neutrons over protons]
and match it with its imaginary part, given by resonances and continuum, via
the dispersion relation. The leading density-dependent contribution to
mixing is due the scattering term, which turns out to be larger
than any density dependent piece in the OPE. We estimate that the asymmetric
density of induces the amplitude
of mixing, equal in magnitude to the mixing amplitude in vacuum,
with the constructive interference for positive and destructive for negative
values of . We revisit sum rules for vector meson masses at finite
nucleon density to point out the numerical importance of the screening term in
the isoscalar channel, which turns out to be one order of magnitude larger than
any density-dependent condensates over the Borel window. This changes the
conclusions about the density dependence of , indicating
MeV increase at nuclear saturation density.Comment: 8 pages, Revte
Holography, Pade Approximants and Deconstruction
We investigate the relation between holographic calculations in 5D and the
Migdal approach to correlation functions in large N theories. The latter
employs Pade approximation to extrapolate short distance correlation functions
to large distances. We make the Migdal/5D relation more precise by quantifying
the correspondence between Pade approximation and the background and boundary
conditions in 5D. We also establish a connection between the Migdal approach
and the models of deconstructed dimensions.Comment: 28 page
Direct Instantons in QCD Nucleon Sum Rules
We study the role of direct (i.e. small-scale) instantons in QCD correlation
functions for the nucleon. They generate sizeable, nonperturbative corrections
to the conventional operator product expansion, which improve the quality of
both QCD nucleon sum rules and cure the long-standing stability problem, in
particular, of the chirally odd sum-rule.Comment: 10 pages, UMD PP#93-17
Testing factorization in B -> D(*)X decays
In QCD the amplitude for B0 -> D(*)+pi- factorizes in the large Nc limit or
in the large energy limit Q >> Lambda_QCD where Q = {m_b, m_c, m_b-m_c}. Data
also suggests factorization in exclusive processes B-> D* pi+ pi- pi- pi0 and
B-> D* omega pi-, however by themselves neither large Nc nor large Q can
account for this. Noting that the condition for large energy release in B0-> D+
pi- is enforced by the SV limit, m_b, m_c >> m_b-m_c >> Lambda, we propose that
the combined large Nc and SV limits justify factorization in B -> D(*) X. This
combined limit is tested with the inclusive decay spectrum measured by CLEO. We
also give exact large Nc relations among isospin amplitudes for B -> D(*)X and
B -> D(*) D-bar(*)X, which can be used to test factorization through exclusive
or inclusive measurements. Predictions for the modes B-> D(*) pi pi, B-> D(*)K
K-bar and B-> D(*) D-bar(*) K are discussed using available data.Comment: 15 pages, 3 included .eps figures, minor change
Nonfactorization and Color-Suppressed Decays
Using value of the parameter but including a modest
nonfactorized amplitude, we show that it is possible to understand all data,
including polarization, for color-suppressed
decays in all commonly used models of form factors. We show that for decay one can define an effective , which is process-dependent and,
in general, complex; but it is not possible to define an effective for
decay. We also explain why nonfactorized amplitudes do not
play a significant role in color-favored B decays.Comment: 13 pages, Latex, one figure (not included
Bremsstrahlung corrections to the decay
We calculate the O() gluon Bremsstrahlung corrections to the
inclusive decay , involving the full operator basis
-- . Confirming and extending earlier calculations of Ali
and Greub, we give formulas for the total decay width as well as the
perturbative photon spectrum, regarding the former as a necessary part of the
forthcoming complete NLO analysis. We explore in detail the renormalization
scale dependence of our results and find it considerably increased.Comment: 23 pages, LaTeX, uses epsf.sty and rotate.sty. 4 figures (uuencoded
postscript) appended as seperate file. A complete postscript version may be
obtained from URL
ftp://feynman.t30.physik.tu-muenchen.de/pub/preprints/tum-93-95.ps.gz Final
version as to appear in Physical Review D. Some minor errors corrected,
without changes in the numerical results. One reference adde
Long-Range Forces of QCD
We consider the scattering of two color dipoles (e.g., heavy quarkonium
states) at low energy - a QCD analog of Van der Waals interaction. Even though
the couplings of the dipoles to the gluon field can be described in
perturbation theory, which leads to the potential proportional to
(N_c^2-1)/R^{7}, at large distances R the interaction becomes totally
non-perturbative. Low-energy QCD theorems are used to evaluate the leading
long-distance contribution \sim (N_f^2-1)/(11N_c - 2N_f)^2 R^{-5/2} exp(-2 \mu
R) (\mu is the Goldstone boson mass), which is shown to arise from the
correlated two-boson exchange. The sum rule which relates the overall strength
of the interaction to the energy density of QCD vacuum is derived.
Surprisingly, we find that when the size of the dipoles shrinks to zero (the
heavy quark limit in the case of quarkonia), the non-perturbative part of the
interaction vanishes more slowly than the perturbative part as a consequence of
scale anomaly. As an application, we evaluate elastic \pi J/\psi and \pi J/\psi
\to \pi \psi' cross sections.Comment: 16pages, 9 eps figures; discussion extended, 2 new references added,
to appear in Phys.Rev.
Axion-photon Couplings in Invisible Axion Models
We reexamine the axion-photon couplings in various invisible axion models
motivated by the recent proposal of using optical interferometry at the ASST
facility in the SSCL to search for axion. We illustrate that the assignment of
charges for the fermion fields plays an important role in
determining the couplings. Several simple non-minimal invisible axion models
with suppressed and enhanced axion-photon couplings are constructed,
respectively. We also discuss the implications of possible new experiments to
detect solar axions by conversion to -rays in a static magnetic apparatus
tracking the sun.Comment: 14 pages, LaTeX fil
Theta angle versus CP violation in the leptonic sector
Assuming that the axion mechanism of solving the strong CP problem does not
exist and the vanishing of theta at tree level is achieved by some
model-building means, we study the naturalness of having large CP-violating
sources in the leptonic sector. We consider the radiative mechanisms which
transfer a possibly large CP-violating phase in the leptonic sector to the
theta parameter. It is found that large theta cannot be induced in the models
with one Higgs doublet as at least three loops are required in this case. In
the models with two or more Higgs doublets the dominant source of theta is the
phases in the scalar potential, induced by CP violation in leptonic sector.
Thus, in the MSSM framework the imaginary part of the trilinear soft-breaking
parameter A_l generates the corrections to the theta angle already at one loop.
These corrections are large, excluding the possibility of large phases, unless
the universality in the slepton sector is strongly violated.Comment: 5 pages, 2 figure
On Two-Body Decays of A Scalar Glueball
We study two body decays of a scalar glueball. We show that in QCD a spin-0
pure glueball (a state only with gluons) cannot decay into a pair of light
quarks if chiral symmetry holds exactly, i.e., the decay amplitude is chirally
suppressed. However, this chiral suppression does not materialize itself at the
hadron level such as in decays into and , because in
perturbative QCD the glueball couples to two (but not one) light quark pairs
that hadronize to two mesons. Using QCD factorization based on an effective
Lagrangian, we show that the difference of hadronization into and
already leads to a large difference between and , even the decay amplitude is not chirally suppressed. Moreover,
the small ratio of of
measured in experiment does not imply to be a pure glueball. With
our results it is helpful to understand the partonic contents if or is measured reliably.Comment: revised versio