1,536 research outputs found
Perturbative Relations between Gravity and Gauge Theory
We review the relations that have been found between multi-loop scattering
amplitudes in gauge theory and gravity, and their implications for ultraviolet
divergences in supergravity.Comment: LaTex with package axodraw.sty. 10 pages. Presented by L.D. at
Strings 99. Cosmetic changes onl
On the Coupling of Gravitons to Matter
Using relationships between open and closed strings, we present a
construction of tree-level scattering amplitudes for gravitons minimally
coupled to matter in terms of gauge theory partial amplitudes. In particular,
we present examples of amplitudes with gravitons coupled to vectors or to a
single fermion pair. We also present two examples with massive graviton
exchange, as would arise in the presence of large compact dimensions. The gauge
charges are represented by flavors of dynamical scalars or fermions. This also
leads to an unconventional decomposition of color and kinematics in gauge
theories.Comment: RevTex, 4 page
Timescales of carbon turnover in soils with mixed crystalline mineralogies
Organic matter–mineral associations stabilize much of the carbon
(C) stored globally in soils. Metastable short-range-order (SRO) minerals
such as allophane and ferrihydrite provide one mechanism for long-term
stabilization of organic matter in young soil. However, in soils with few SRO
minerals and a predominance of crystalline aluminosilicate or Fe (and
Al) oxyhydroxide, C turnover should
be governed by chemisorption with those minerals. Here, we correlate mineral
composition from soils containing small amounts of SRO minerals with mean
turnover time (TT) of C estimated from radiocarbon (<sup>14</sup>C) in bulk soil,
free light fraction and mineral-associated organic matter. We varied the
mineral amount and composition by sampling ancient soils formed on different
lithologies in arid to subhumid climates in Kruger National Park (KNP), South
Africa. Mineral contents in bulk soils were assessed using chemical
extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our
interest in the role of silicate clay mineralogy, particularly smectite
(2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of
the clay-sized fraction using X-ray diffraction (XRD) and measured <sup>14</sup>C
on the same fraction.
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Density separation demonstrated that mineral associated C accounted for
40–70 % of bulk soil organic C in A and B1 horizons for granite,
nephelinite and arid-zone gabbro soils, and > 80 % in other
soils. Organic matter strongly associated with the isolated clay-sized
fraction represented only 9–47 % of the bulk soil C. The mean TT of C
strongly associated with the clay-sized fraction increased with the amount of
smectite (2 : 1 clays); in samples with > 40 % smectite it
averaged 1020 ± 460 years. The C not strongly associated with
clay-sized minerals, including a combination of low-density C, the C
associated with minerals of sizes between 2 µm and 2 cm (including
Fe oxyhydroxides as coatings), and C removed from clay-sized material by
2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface
horizons. Summed over the bulk soil profile, we found that smectite content
correlated with the mean TT of bulk soil C across varied lithologies. The SRO
mineral content in KNP soils was generally very low, except for the soils
developed on gabbros under more humid climate that also had very high Fe and
C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO
minerals are metastable and sequester C for long timescales. We hypothesize
that in the KNP, SRO minerals represent a transient stage of mineral
evolution and therefore lock up C for a shorter time.
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Overall, we found crystalline Fe-oxyhydroxides (determined as the difference
between Fe in dithionate citrate and oxalate extractions) to be the strongest
predictor for soil C content, while the mean TT of soil C was best predicted
from the amount of smectite, which was also related to more easily measured
bulk properties such as cation exchange capacity or pH. Combined with
previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our
results hold promise for predicting C inventory and persistence based on
intrinsic timescales of specific carbon–mineral
interactions
QCD corrections to associated t anti-t h production at hadron colliders
We briefly present the status of QCD corrections to the inclusive total cross
section for the production of a Higgs boson in association with a top-quark
pair within the Standard Model at hadron colliders.Comment: 3 pages, talk given at 31st International Conference on High Energy
Physics (ICHEP 2002), Amsterdam, The Netherlands, 24-31 Jul 200
Associated Top Quark-Higgs Boson Production at the LHC
We compute the O(alpha_s^3) inclusive cross section for the process pp ->
t-tbar-h in the Standard Model, at sqrt(s)=14 TeV. The next-to-leading order
corrections drastically reduce the renormalization and factorization scale
dependence of the Born cross section and increase the total cross section for
renormalization and factorization scales larger than m_t. These corrections
have important implications for models of new physics involving the top quark.Comment: 5 pages, 4 figures, RevTeX
A technique for loop calculations in non-Abelian gauge theories - with application to five gluon amplitude
A powerful tool for calculations in non-Abelian gauge theories is obtained by
combining the background field gauge, the helicity basis and the color
decomposition methods. It has reproduced the one-loop calculation of the
five-gluon amplitudes in QCD, is applicable to electroweak processes and
extendable to two-loop calculations.Comment: Latex 22 pages, 3 figure
Dual conformal constraints and infrared equations from global residue theorems in N=4 SYM theory
Infrared equations and dual conformal constraints arise as consistency
conditions on loop amplitudes in N=4 super Yang-Mills theory. These conditions
are linear relations between leading singularities, which can be computed in
the Grassmannian formulation of N=4 super Yang-Mills theory proposed recently.
Examples for infrared equations have been shown to be implied by global residue
theorems in the Grassmannian picture. Both dual conformal constraints and
infrared equations are mapped explicitly to global residue theorems for
one-loop next-to-maximally-helicity-violating amplitudes. In addition, the
identity relating the BCFW and its parity-conjugated form of tree-level
amplitudes, is shown to emerge from a particular combination of global residue
theorems.Comment: 21 page
From lightcone actions to maximally supersymmetric amplitudes
In this article actions for N=4 SYM and N=8 supergravity are formulated in
terms of a chiral superfield, which contains only the physical degrees of
freedom of either theory. In these new actions, which originate from the
lightcone superspace, the supergravity cubic vertex is the square of the gauge
theory one (omitting the color structures). Amplitude calculations using the
corresponding Feynman supergraph rules are tedious, but can be simplified by
choosing a preferred superframe. Recursive calculations of all MHV amplitudes
in N=4 SYM and the four-point N=8 supergravity amplitude are shown to agree
with the known results and connections to the BCFW recursion relations are
pointed out. Finally, the new path integrals are discussed in the context of
the double-copy property relating N=4 SYM theory to N=8 supergravity.Comment: 29 pages, 2 figures, v2: title modified, published versio
The Last of the Finite Loop Amplitudes in QCD
We use on-shell recursion relations to determine the one-loop QCD scattering
amplitudes with a massless external quark pair and an arbitrary number (n-2) of
positive-helicity gluons. These amplitudes are the last of the unknown
infrared- and ultraviolet-finite loop amplitudes of QCD. The recursion
relations are similar to ones applied at tree level, but contain new
non-trivial features corresponding to poles present for complex momentum
arguments but absent for real momenta. We present the relations and the compact
solutions to them, valid for all n. We also present compact forms for the
previously-computed one-loop n-gluon amplitudes with a single negative helicity
and the rest positive helicity.Comment: 45 pages, revtex, 7 figures, v2 minor correction
New Relations for Gauge-Theory Amplitudes
We present an identity satisfied by the kinematic factors of diagrams
describing the tree amplitudes of massless gauge theories. This identity is a
kinematic analog of the Jacobi identity for color factors. Using this we find
new relations between color-ordered partial amplitudes. We discuss applications
to multi-loop calculations via the unitarity method. In particular, we
illustrate the relations between different contributions to a two-loop
four-point QCD amplitude. We also use this identity to reorganize gravity tree
amplitudes diagram by diagram, offering new insight into the structure of the
KLT relations between gauge and gravity tree amplitudes. This can be used to
obtain novel relations similar to the KLT ones. We expect this to be helpful in
higher-loop studies of the ultraviolet properties of gravity theories.Comment: 40 pages, 7 figures, RevTex, v2 minor correction
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