52,811 research outputs found
Small-Recoil Approximation
In this review we discuss a technique to compute and to sum a class of
Feynman diagrams, and some of its applications. These are diagrams containing
one or more energetic particles that suffer very little recoil in their
interactions. When recoil is completely neglected, a decomposition formula can
be proven. This formula is a generalization of the well-known eikonal formula,
to non-abelian interactions. It expresses the amplitude as a sum of products of
irreducible amplitudes, with each irreducible amplitude being the amplitude to
emit one, or several mutually interacting, quasi-particles. For abelian
interaction a quasi-particle is nothing but the original boson, so this
decomposition formula reduces to the eikonal formula. In non-abelian situations
each quasi-particle can be made up of many bosons, though always with a total
quantum number identical to that of a single boson. This decomposition enables
certain amplitudes of all orders to be summed up into an exponential form, and
it allows subleading contributions of a certain kind, which is difficult to
reach in the usual way, to be computed. For bosonic emissions from a heavy
source with many constituents, a quasi-particle amplitude turns out to be an
amplitude in which all bosons are emitted from the same constituent. For
high-energy parton-parton scattering in the near-forward direction, the
quasi-particle turns out to be the Reggeon, and this formalism shows clearly
why gluons reggeize but photons do not. The ablility to compute subleading
terms in this formalism allows the BFKL-Pomeron amplitude to be extrapolated to
asymptotic energies, in a unitary way preserving the Froissart bound. We also
consider recoil corrections for abelian interactions in order to accommodate
the Landau-Pomeranchuk-Migdal effect.Comment: 21 pages with 4 figure
Generalized Background-Field Method
The graphical method discussed previously can be used to create new gauges
not reachable by the path-integral formalism. By this means a new gauge is
designed for more efficient two-loop QCD calculations. It is related to but
simpler than the ordinary background-field gauge, in that even the triple-gluon
vertices for internal lines contain only four terms, not the usual six. This
reduction simplifies the calculation inspite of the necessity to include other
vertices for compensation. Like the ordinary background-field gauge, this
generalized background-field gauge also preserves gauge invariance of the
external particles. As a check of the result and an illustration for the
reduction in labour, an explicit calculation of the two-loop QCD
-function is carried out in this new gauge. It results in a saving of
45% of computation compared to the ordinary background-field gauge.Comment: 17 pages, Latex, 18 figures in Postscrip
Superalgebra and Conservative Quantities in N=1 Self-dual Supergravity
The N=1 self-dual supergravity has SL(2,C) and the left-handed and right
-handed local supersymmetries. These symmetries result in SU(2) charges as the
angular-momentum and the supercharges. The model possesses also the invariance
under the general translation transforms and this invariance leads to the
energy-momentum. All the definitions are generally covariant . As the SU(2)
charges and the energy-momentum we obtained previously constituting the
3-Poincare algebra in the Ashtekar's complex gravity, the SU(2) charges, the
supercharges and the energy-momentum here also restore the super-Poincare
algebra, and this serves to support the reasonableness of their
interpretations.Comment: 18 pages, Latex, no figure
Magnetoelectric properties of magnetite thin films
Resistivity, DC Hall effect and transverse magnetoresistance measurements were made on polycrystalline thin films of magnetite (Fe3O4) from 104K to room temperature. The Verwey transition is observed at TV=123K, about 4K higher than reported for bulk magnetite. The ordinary and extraordinary Hall coefficients are negative over the entire temperature range, consistent with negatively charged carriers. The extraordinary Hall coefficient exhibits a rho 1/3 dependence on the resistivity above TV and a rho 2/3 dependence below TV. The magnetoresistance is negative at all temperatures and for all magnetic field strengths. The planar Hall effect signal was below the sensitivity of the present experiment
Pressure-induced Spin-Peierls to Incommensurate Charge-Density-Wave Transition in the Ground State of TiOCl
The ground state of the spin-Peierls system TiOCl was probed using
synchrotron x-ray diffraction on a single-crystal sample at T = 6 K. We tracked
the evolution of the structural superlattice peaks associated with the
dimerized ground state as a function of pressure. The dimerization along the b
axis is rapidly suppressed in the vicinity of a first-order structural phase
transition at Pc = 13.1(1) GPa. The high-pressure phase is characterized by an
incommensurate charge density wave perpendicular to the original spin chain
direction. These results show that the electronic ground state undergoes a
fundamental change in symmetry, indicating a significant change in the
principal interactions.Comment: 5 pages, 4 figure
Irreducible MultiQutrit Correlations in Greenberger-Horne-Zeilinger Type States
Following the idea of the continuity approach in [D. L. Zhou, Phys. Rev.
Lett. 101, 180505 (2008)], we obtain the degrees of irreducible multi-party
correlations in two families of -qutrit Greenberger-Horne-Zeilinger type
states. For the pure states in one of the families, the irreducible 2-party,
-party and -party () correlations are nonzero, which is
different from the -qubit case. We also derive the correlation distributions
in the -qutrit maximal slice state, which can be uniquely determined by its
-qutrit reduced density matrices among pure states. It is proved that
there is no irreducible -qutrit correlation in the maximal slice state. This
enlightens us to give a discussion about how to characterize the pure states
with irreducible -party correlation in arbitrarily high-dimensional systems
by the way of the continuity approach.Comment: 5p, no fi
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