764 research outputs found
Amplitudes and Spinor-Helicity in Six Dimensions
The spinor-helicity formalism has become an invaluable tool for understanding
the S-matrix of massless particles in four dimensions. In this paper we
construct a spinor-helicity formalism in six dimensions, and apply it to derive
compact expressions for the three, four and five point tree amplitudes of
Yang-Mills theory. Using the KLT relations, it is a straightforward process to
obtain amplitudes in linearized gravity from these Yang-Mills amplitudes; we
demonstrate this by writing down the gravitational three and four point
amplitudes. Because there is no conserved helicity in six dimensions, these
amplitudes describe the scattering of all possible polarization states (as well
as Kaluza-Klein excitations) in four dimensions upon dimensional reduction. We
also briefly discuss a convenient formulation of the BCFW recursion relations
in higher dimensions.Comment: 26 pages, 2 figures. Minor improvements of the discussio
U-box E3 ubiquitin ligase PUB17 acts in the nucleus to promote specific immune pathways triggered by Phytophthora infestans
Ubiquitination regulates many processes in plants, including immunity. The E3 ubiquitin ligase PUB17 is a positive regulator of programmed cell death (PCD) triggered by resistance proteins CF4/9 in tomato. Its role in immunity to the potato late blight pathogen, Phytophthora infestans, was investigated here. Silencing StPUB17 in potato by RNAi and NbPUB17 in Nicotiana benthamiana by virus-induced gene silencing (VIGS) each enhanced P. infestans leaf colonization. PAMP-triggered immunity (PTI) transcriptional responses activated by flg22, and CF4/Avr4-mediated PCD were attenuated by silencing PUB17. However, silencing PUB17 did not compromise PCD triggered by P. infestans PAMP INF1, or co-expression of R3a/AVR3a, demonstrating that not all PTI- and PCD-associated responses require PUB17. PUB17 localizes to the plant nucleus and especially in the nucleolus. Transient over-expression of a dominant-negative StPUB17V314I,V316I mutant, which retained nucleolar localization, suppressed CF4-mediated cell death and enhanced P. infestans colonization. Exclusion of the StPUB17V314I,V316I mutant from the nucleus abolished its dominant-negative activity, demonstrating that StPUB17 functions in the nucleus. PUB17 is a positive regulator of immunity to late blight that acts in the nucleus to promote specific PTI and PCD pathways
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
Electronic structure, magnetism and superconductivity of MgCNi
The electronic structure of the newly discovered superconducting perovskite
MgCNi is calculated using the LMTO and KKR methods. The states near the
Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while
the electron-phonon coupling constant is estimated to be about 0.7, which
suggests that the material is a conventional type of superconductor where T
is not affected by magnetic interactions. However, the proximity of the Fermi
energy to a large peak in the density of states in conjunction with the
reported non-stoichiometry of the compound, has consequences for the stability
of the results.Comment: 3 pages, 4 figure
Superconductivity in the Correlated Pyrochlore Cd_2Re_2O_7
We report the observation of superconductivity in high-quality
CdReO single crystals with room-temperature pyrochlore structure.
Resistivity and ac susceptibility measurements establish an onset transition
temperature T = 1.47 K with transition width T = 0.25
K. In applied magnetic field, the resistive transition shows a type-II
character, with an approximately linear temperature-dependence of the upper
critical field H. The bulk nature of the superconductivity is confirmed
by the specific heat jump with C = 37.9 mJ/mol-K. Using the
value extracted from normal-state specific heat data, we obtain
C/T = 1.29, close to the weak coupling BCS value. In the
normal state, a negative Hall coefficient below 100 K suggests electron-like
conduction in this material. The resistivity exhibits a quadratic T-dependence
between 2 and 60 K, i.e., +AT, indicative of Fermi-liquid
behavior. The values of the Kadowaki-Woods ratio A/ and the Wilson
ratio are comparable to that for strongly correlated materials.Comment: 4 pages, 5 figure
Structure, stability and stress properties of amorphous and nanostructured carbon films
Structural and mechanical properties of amorphous and nanocomposite carbon
are investigated using tight-binding molecular dynamics and Monte Carlo
simulations. In the case of amorphous carbon, we show that the variation of
sp^3 fraction as a function of density is linear over the whole range of
possible densities, and that the bulk moduli follow closely the power-law
variation suggested by Thorpe. We also review earlier work pertained to the
intrinsic stress state of tetrahedral amorphous carbon. In the case of
nanocomposites, we show that the diamond inclusions are stable only in dense
amorphous tetrahedral matrices. Their hardness is considerably higher than that
of pure amorphous carbon films. Fully relaxed diamond nanocomposites possess
zero average intrinsic stress.Comment: 10 pages, 6 figure
Generalized E(7(7)) coset dynamics and D=11 supergravity
The hidden on-shell E(7(7)) symmetry of maximal supergravity is usually
discussed in a truncation from D=11 to four dimensions. In this article, we
reverse the logic and start from a theory with manifest off-shell E(7(7))
symmetry inspired by West's coset construction. Following de Wit's and
Nicolai's idea that a 4+56 dimensional "exceptional geometry" underlies maximal
supergravity, we construct the corresponding Lagrangian and the supersymmetry
variations for the 56 dimensional subsector. We prove that both the dynamics
and the supersymmetry coincide with D=11 supergravity in a truncation to d=7 in
the expected way.Comment: 42 pages, v2: references adde
Connected Green function approach to ground state symmetry breaking in -theory
Using the cluster expansions for n-point Green functions we derive a closed
set of dynamical equations of motion for connected equal-time Green functions
by neglecting all connected functions higher than order for the
-theory in dimensions. We apply the equations to the
investigation of spontaneous ground state symmetry breaking, i.e. to the
evaluation of the effective potential at temperature . Within our momentum
space discretization we obtain a second order phase transition (in agreement
with the Simon-Griffith theorem) and a critical coupling of
as compared to a first order phase transition and
from the Gaussian effective potential approach.Comment: 25 Revtex pages, 5 figures available via fpt from the directory
ugi-94-11 of [email protected] as one postscript file (there
was a bug in our calculations, all numerical results and figures have changed
significantly), ugi-94-1
Vanishing Effective Mass of the Neutrinoless Double Beta Decay?
We stress that massive neutrinos may be Majorana particles even if the
effective mass of the neutrinoless double beta decay m_ee vanishes. We show
that current neutrino oscillation data do allow m_ee = 0 to hold, if the
Majorana CP-violating phases lie in two specific regions. Strong constraints on
three neutrino masses can then be obtained. We find that the neutrino mass
spectrum performs a normal hierarchy: m_1 < m_2 < m_3. A possible texture of
the neutrino mass matrix is also illustrated under the m_ee = 0 condition.Comment: RevTex 9 pages (2 PS figures included). More discussions and
references added. Results partly changed. To appear in Phys. Rev.
Relation Between Chiral Susceptibility and Solutions of Gap Equation in Nambu--Jona-Lasinio Model
We study the solutions of the gap equation, the thermodynamic potential and
the chiral susceptibility in and beyond the chiral limit at finite chemical
potential in the Nambu--Jona-Lasinio (NJL) model. We give an explicit relation
between the chiral susceptibility and the thermodynamic potential in the NJL
model. We find that the chiral susceptibility is a quantity being able to
represent the furcation of the solutions of the gap equation and the
concavo-convexity of the thermodynamic potential in NJL model. It indicates
that the chiral susceptibility can identify the stable state and the
possibility of the chiral phase transition in NJL model.Comment: 21 pages, 6 figures, misprints are correcte
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