311 research outputs found
Nucleon-Nucleon Scattering under Spin-Isospin Reversal in Large-N_c QCD
The spin-flavor structure of certain nucleon-nucleon scattering observables
derived from the large N_c limit of QCD in the kinematical regime where
time-dependent mean-field theory is valid is discussed. In previous work, this
regime was taken to be where the external momentum was of order N_c which
precluded the study of differential cross sections in elastic scattering. Here
it is shown that the regime extends down to order N_c^{1/2} which includes the
higher end of the elastic regime. The prediction is that in the large N_c
limit, observables describable via mean-field theory are unchanged when the
spin and isospin of either nucleon are both flipped. This prediction is tested
for proton-proton and neutron-proton elastic scattering data and found to fail
badly. We argue that this failure can be traced to a lack of a clear separation
of scales between momentum of order N_c^{1/2} and N_c^1 when N_c is as small as
three. The situation is compounded by an anomalously low particle production
threshold due to approximate chiral symmetry.Comment: 5 pages, 1 figur
Saxion Emission from SN1987A
We study the possibility of emission of the saxion, a superpartner of the
axion, from SN1987A. The fact that the observed neutrino pulse from SN1987A is
in excellent agreement with the current theory of supernovae places a strong
bound on the energy loss into any non-standard model channel, therefore
enabling bounds to be placed on the decay constant, f_a, of a light saxion. The
low-energy coupling of the saxion, which couples at high energies to the QCD
gauge field strength, is expected to be enhanced from QCD scaling, making it
interesting to investigate if the saxion could place stronger bounds on f_a
than the axion itself. Moreover, since the properties of the saxion are
determined by f_a, a constraint on this parameter can be translated into a
constraint on the supersymmetry breaking scale. We find that the bound on f_a
from saxion emission is comparable with the one derived from axion emission due
to a cancellation of leading-order terms in the soft-radiation expansion.Comment: 18 pages, 2 figures; minor changes, typos corrected, version to
appear in JHE
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North American Influence on Tropospheric Ozone and the Effects of Recent Emission Reductions: Constraints from ICARTT Observations
We use observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign over eastern North America in summer 2004, interpreted with a global 3âD model of tropospheric chemistry (GEOSâChem), to improve and update estimates of North American influence on global tropospheric ozone and the effect of recent U.S. anthropogenic reductions on surface ozone pollution. We find that the 50% decrease in U.S. stationary NOx sources since 1999 has decreased mean U.S. boundary layer ozone concentrations by 6â8 ppbv in the southeast and 4â6 ppbv in the Midwest. The observed dO3/dCO molar enhancement ratio in the U.S. boundary layer during ICARTT was 0.46 mol molâ1, larger than the range of 0.3â0.4 from studies in the early 1990s, possibly reflecting the decrease in the NOx/CO emission ratio as well as an increase in the ozone production efficiency per unit NOx. North American NOx emissions during summer 2004 as constrained by the ICARTT observations (0.72 Tg N fossil fuel, 0.11 Tg N biomass burning, 0.28 Tg N lightning for 1 July to 15 August) enhanced the hemispheric tropospheric ozone burden by 12.4%, with comparable contributions from fossil fuel and lightning (5â6%), but only 1% from biomass burning emissions despite 2004 being a record fire year over Alaska and western Canada.Earth and Planetary Science
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Biogenic Versus Anthropogenic Sources of CO in the United States
Aircraft observations of carbon monoxide (CO) from the ICARTT campaign over the eastern United States in summer 2004 (July 1âAugust 15), interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem), show that the national anthropogenic emission inventory from the U.S. Environmental Protection Agency (93 Tg CO yâ1) is too high by 60% in summer. Our best estimate of the CO anthropogenic source for the ICARTT period is 6.4 Tg CO, including 4.6 Tg from direct emission and 1.8 Tg CO from oxidation of anthropogenic volatile organic compounds (VOCs). The biogenic CO source for the same period from the oxidation of isoprene and other biogenic VOCs is 8.3 Tg CO, and is independently constrained by ICARTT observations of formaldehyde (HCHO). Anthropogenic emissions of CO in the U.S. have decreased to the point that they are now lower than the biogenic source in summer.Earth and Planetary SciencesEngineering and Applied Science
The staggered domain wall fermion method
A different lattice fermion method is introduced. Staggered domain wall
fermions are defined in 2n+1 dimensions and describe 2^n flavors of light
lattice fermions with exact U(1) x U(1) chiral symmetry in 2n dimensions. As
the size of the extra dimension becomes large, 2^n chiral flavors with the same
chiral charge are expected to be localized on each boundary and the full
SU(2^n) x SU(2^n) flavor chiral symmetry is expected to be recovered. SDWF give
a different perspective into the inherent flavor mixing of lattice fermions and
by design present an advantage for numerical simulations of lattice QCD
thermodynamics. The chiral and topological index properties of the SDWF Dirac
operator are investigated. And, there is a surprise ending...Comment: revtex4, 7 figures, minor revisions, 2 references adde
On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system
In this paper we explore the consequences of the recent determination of the
mass m=(8.7 +/- 0.8)M_Sun of Cygnus X-1, obtained from the Quasi-Periodic
Oscillation (QPO)-photon index correlation scaling, on the orbital and physical
properties of the binary system HDE 226868/Cygnus X-1. By using such a result
and the latest spectroscopic optical data of the HDE 226868 supergiant star we
get M=(24 +/- 5)M_Sun for its mass. It turns out that deviations from the third
Kepler law significant at more than 1-sigma level would occur if the
inclination i of the system's orbital plane to the plane of the sky falls
outside the range 41-56 deg: such deviations cannot be due to the first
post-Newtonian (1PN) correction to the orbital period because of its smallness;
interpreted in the framework of the Newtonian theory of gravitation as due to
the stellar quadrupole mass moment Q, they are unphysical because Q would take
unreasonably large values. By conservatively assuming that the third Kepler law
is an adequate model for the orbital period we obtain i=(48 +/- 7) deg which
yields for the relative semimajor axis a=(42 +/- 9)R_Sun. Our estimate for the
Roche's lobe of HDE 226868 is r_M = (21 +/- 6)R_Sun.Comment: Latex2e, 7 pages, 1 table, 4 figures. To appear in ApSS (Astrophysics
and Space Science
Quasi-particle Lifetimes in a d_{x^2-y^2} Superconductor
We consider the lifetime of quasi-particles in a d-wave superconductor due to
scattering from antiferromagnetic spin-fluctuations, and explicitly separate
the contribution from Umklapp processes which determines the electrical
conductivity. Results for the temperature dependence of the total scattering
rate and the Umklapp scattering rate are compared with relaxation rates
obtained from thermal and microwave conductivity measurements, respectively.Comment: 14 pages, 4 figure
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New Constraints on Terrestrial and Oceanic Sources of Atmospheric Methanol
We use a global 3-D chemical transport model (GEOS-Chem) to interpret new aircraft, surface, and oceanic observations of methanol in terms of the constraints that they place on the atmospheric methanol budget. Recent measurements of methanol concentrations in the ocean mixed layer (OML) imply that in situ biological production must be the main methanol source in the OML, dominating over uptake from the atmosphere. It follows that oceanic emission and uptake must be viewed as independent terms in the atmospheric methanol budget. We deduce that the marine biosphere is a large primary source (85 Tg aâ1) of methanol to the atmosphere and is also a large sink (101 Tg aâ1), comparable in magnitude to atmospheric oxidation by OH (88 Tg aâ1). The resulting atmospheric lifetime of methanol in the model is 4.7 days. Aircraft measurements in the North American boundary layer imply that terrestrial plants are a much weaker source than presently thought, likely reflecting an overestimate of broadleaf tree emissions, and this is also generally consistent with surface measurements. We deduce a terrestrial plant source of 80 Tg aâ1, comparable in magnitude to the ocean source. The aircraft measurements show a strong correlation with CO (R2=0.51â0.61) over North America during summer. We reproduce this correlation and slope in the model with the reduced plant source, which also confirms that the anthropogenic source of methanol must be small. Our reduced plant source also provides a better simulation of methanol observations over tropical South America.Earth and Planetary SciencesEngineering and Applied Science
Numerical Study of Gluon Propagator and Confinement Scenario in Minimal Coulomb Gauge
We present numerical results in SU(2) lattice gauge theory for the
space-space and time-time components of the gluon propagator at equal time in
the minimal Coulomb gauge. It is found that the equal-time would-be physical
3-dimensionally transverse gluon propagator vanishes at
when extrapolated to infinite lattice volume, whereas the
instantaneous color-Coulomb potential is strongly enhanced at
. This has a natural interpretation in a confinement scenario in
which the would-be physical gluons leave the physical spectrum while the
long-range Coulomb force confines color. Gribov's formula provides an excellent fit to our data
for the 3-dimensionally transverse equal-time gluon propagator
for relevant values of .Comment: 23 pages, 12 figures, TeX file. Minor modifications, incorporating
referee's suggestion
Nucleon Axial Form Factor from Lattice QCD
Results for the isovector axial form factors of the proton from a lattice QCD
calculation are presented for both point-split and local currents. They are
obtained on a quenched lattice at with Wilson
fermions for a range of quark masses from strange to charm. We determine the
finite lattice renormalization for both the local and point-split currents of
heavy quarks. Results extrapolated to the chiral limit show that the
dependence of the axial form factor agrees reasonably well with experiment. The
axial coupling constant calculated for the local and the point-split
currents is about 6\% and 12\% smaller than the experimental value
respectively.Comment: 8 pages, 5 figures (included in part 2), UK/93-0
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