15,300 research outputs found
Chiral 3-exchange NN-potentials: Results for dominant next-to-leading order contributions
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams with one insertion from the
second order chiral effective pion-nucleon Lagrangian proportional to the
low-energy constants . The resulting isoscalar central potential
vanishes identically. In most cases these -exchange potentials are larger
than the ones generated by the diagrams involving only leading order vertices
due to the large values of (which mainly represent virtual
-excitation). A similar feature has been observed for the chiral
-exchange. We also give suitable (double-integral) representations for
the spin-spin and tensor potentials generated by the leading-order diagrams
proportional to involving four nucleon propagators. In these cases the
Cutkosky rule cannot be used to calculate the spectral-functions in the
infinite nucleon mass limit since the corresponding mass-spectra start with a
non-vanishing value at the -threshold. Altogether, one finds that chiral
-exchange leads to small corrections in the region fm where
- and chiral -exchange alone provide a very good strong NN-force as
shown in a recent analysis of the low-energy pp-scattering data-base.Comment: 11 pages, 7 figures, to be published in The Physical Review
A Lattice Study of the Gluon Propagator in Momentum Space
We consider pure glue QCD at beta=5.7, beta=6.0 and beta=6.3. We evaluate the
gluon propagator both in time at zero 3-momentum and in momentum space. From
the former quantity we obtain evidence for a dynamically generated effective
mass, which at beta=6.0 and beta=6.3 increases with the time separation of the
sources, in agreement with earlier results. The momentum space propagator G(k)
provides further evidence for mass generation. In particular, at beta=6.0, for
k less than 1 GeV, the propagator G(k) can be fit to a continuum formula
proposed by Gribov and others, which contains a mass scale b, presumably
related to the hadronization mass scale. For higher momenta Gribov's model no
longer provides a good fit, as G(k) tends rather to follow an inverse power
law. The results at beta=6.3 are consistent with those at beta=6.0, but only
the high momentum region is accessible on this lattice. We find b in the range
of three to four hundred MeV and the exponent of the inverse power law about
2.7. On the other hand, at beta=5.7 (where we can only study momenta up to 1
GeV) G(k) is best fit to a simple massive boson propagator with mass m. We
argue that such a discrepancy may be related to a lack of scaling for low
momenta at beta=5.7. {}From our results, the study of correlation functions in
momentum space looks promising, especially because the data points in Fourier
space turn out to be much less correlated than in real space.Comment: 19 pages + 12 uuencoded PostScript picture
Large normally hyperbolic cylinders in a priori stable Hamiltonian systems
We prove the existence of normally hyperbolic invariant cylinders in nearly
integrable hamiltonian systems
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
Lattice Calculation of Heavy-Light Decay Constants with Two Flavors of Dynamical Quarks
We present results for , , , and their ratios in
the presence of two flavors of light sea quarks (). We use Wilson light
valence quarks and Wilson and static heavy valence quarks; the sea quarks are
simulated with staggered fermions. Additional quenched simulations with
nonperturbatively improved clover fermions allow us to improve our control of
the continuum extrapolation. For our central values the masses of the sea
quarks are not extrapolated to the physical , masses; that is, the
central values are "partially quenched." A calculation using "fat-link clover"
valence fermions is also discussed but is not included in our final results. We
find, for example,
MeV, , MeV, and , where in each case the first error is
statistical and the remaining three are systematic: the error within the
partially quenched approximation, the error due to the missing strange
sea quark and to partial quenching, and an estimate of the effects of chiral
logarithms at small quark mass. The last error, though quite significant in
decay constant ratios, appears to be smaller than has been recently suggested
by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other
lattice computations to date, the lattice quark masses are not very light
and chiral log effects may not be fully under control.Comment: Revised version includes an attempt to estimate the effects of chiral
logarithms at small quark mass; central values are unchanged but one more
systematic error has been added. Sections III E and V D are completely new;
some changes for clarity have also been made elsewhere. 82 pages; 32 figure
Vector form factor in K_l3 semileptonic decay with two flavors of dynamical domain-wall quarks
We calculate the vector form factor in K \to \pi l \nu semileptonic decays at
zero momentum transfer f_+(0) from numerical simulations of two-flavor QCD on
the lattice. Our simulations are carried out on 16^3 \times 32 at a lattice
spacing of a \simeq 0.12 fm using a combination of the DBW2 gauge and the
domain-wall quark actions, which possesses excellent chiral symmetry even at
finite lattice spacings. The size of fifth dimension is set to L_s=12, which
leads to a residual quark mass of a few MeV. Through a set of double ratios of
correlation functions, the form factor calculated on the lattice is accurately
interpolated to zero momentum transfer, and then is extrapolated to the
physical quark mass. We obtain f_+(0)=0.968(9)(6), where the first error is
statistical and the second is the systematic error due to the chiral
extrapolation. Previous estimates based on a phenomenological model and chiral
perturbation theory are consistent with our result. Combining with an average
of the decay rate from recent experiments, our estimate of f_+(0) leads to the
Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V_{us}|=0.2245(27), which is
consistent with CKM unitarity. These estimates of f_+(0) and |V_{us}| are
subject to systematic uncertainties due to the finite lattice spacing and
quenching of strange quarks, though nice consistency in f_+(0) with previous
lattice calculations suggests that these errors are not large.Comment: 23 pages, 11 figures, 7 tables, RevTeX4; v3: one table added, results
and conclusions unchanged, final version to appear in Phys.Rev.
Light Hadron Spectrum in Quenched Lattice QCD with Staggered Quarks
Without chiral extrapolation, we achieved a realistic nucleon to (\rho)-meson
mass ratio of (m_N/m_\rho = 1.23 \pm 0.04 ({\rm statistical}) \pm 0.02 ({\rm
systematic})) in our quenched lattice QCD numerical calculation with staggered
quarks. The systematic error is mostly from finite-volume effect and the
finite-spacing effect is negligible. The flavor symmetry breaking in the pion
and (\rho) meson is no longer visible. The lattice cutoff is set at 3.63 (\pm)
0.06 GeV, the spatial lattice volume is (2.59 (\pm) 0.05 fm)(^3), and bare
quarks mass as low as 4.5 MeV are used. Possible quenched chiral effects in
hadron mass are discussed.Comment: 5 pages and 5 figures, use revtex
Extraction efficiency of drifting electrons in a two-phase xenon time projection chamber
We present a measurement of the extraction efficiency of quasi-free electrons
from the liquid into the gas phase in a two-phase xenon time-projection
chamber. The measurements span a range of electric fields from 2.4 to 7.1 kV/cm
in the liquid xenon, corresponding to 4.5 to 13.1 kV/cm in the gaseous xenon.
Extraction efficiency continues to increase at the highest extraction fields,
implying that additional charge signal may be attained in two-phase xenon
detectors through careful high-voltage engineering of the gate-anode region
Two Photon Contribution to Polarization in
Short distance physics involving virtual top and charm quarks contributes to
(and ) polarization in the decay . Measurement of the parity violating asymmetry , where and are the rates
to produce right and left-handed , may provide valuable information on
the unitarity triangle. The parity violating asymmetry also gets a contribution
from Feynman diagrams with two photon intermediate states. We estimate this two
photon contribution to the asymmetry and discuss briefly the two photon
contribution to time reversal odd asymmetries that involve both the and
polarizations.Comment: (19 pages, 5 figures available on request. Uses phyzzx),
CALT-68-1798, UCSD/PTH 92-2
Bosonization of One-Dimensional Exclusons and Characterization of Luttinger Liquids
We achieve a bosonization of one-dimensional ideal gas of exclusion
statistics at low temperatures, resulting in a new variant of
conformal field theory with compactified radius . These
ideal excluson gases exactly reproduce the low- critical properties of
Luttinger liquids, so they can be used to characterize the fixed points of the
latter. Generalized ideal gases with mutual statistics and non-ideal gases with
Luttinger-type interactions have also similar behavior, controlled by an
effective statistics varying in a fixed-point line.Comment: 13 pages, revte
- …