8,311 research outputs found
New method for calculating helicity amplitudes of jet-like QED processes for high-energy colliders II. Processes with lepton pair production
As continuation of our previous paper we further develop our new method for
calculating helicity amplitudes of jet-like QED processes described by tree
diagrams, applying it to lepton pair production. This method consists in
replacing spinor structures for real and weakly virtual intermediate leptons by
simple transition vertices. New vertices are introduced for the pair production
case, and previous bremsstrahlung vertices are generalized to include virtual
photons inside the considered jet. We present a diagrammatic approach that
allows to write down in an efficient way the leading helicity amplitudes, at
tree level. The obtained compact amplitudes are particularly suitable for
numerical calculations in jet-like kinematics. Several examples with up to four
particles in a jet are discussed in detail.Comment: 11 pages, 11 figures, Latex, Springer-Latex macros include
New method for calculating helicity amplitudes of jet--like QED processes for high--energy colliders I. Bremsstrahlung processes
Inelastic QED processes, the cross sections of which do not drop with
increasing energy, play an important role at high-energy colliders. Such
reactions have the form of two-jet processes with the exchange of a virtual
photon in the t-channel. We consider them in the region of small scattering
angles , which yields the dominant contribution to
their total cross sections. A new effective method is presented and applied to
QED processes with emission of real photons to calculate the helicity
amplitudes of these processes. Its basic idea is similar to the well-known
equivalent-lepton method. Compact analytical expressions for those amplitudes
up to are derived omitting only terms of the order of , and higher order. The helicity amplitudes are presented
in a compact form in which large compensating terms are already cancelled. Some
common properties for all jet-like processes are found and we discuss their
origin.Comment: 17 pages, LATEX (svjour style files included
Matter near to the Endpoint of the Electroweak Phase Transition
Wave functions and the screening mass spectrum in the 3D SU(2)-Higgs model
near to the phase transition line below the endpoint and in the crossover
region are calculated. In the crossover region the changing spectrum versus
temperature is examined showing the aftermath of the phase transition at lower
Higgs mass. Large sets of operators with various extensions are used allowing
to identify wave functions in position space.Comment: 3 pages, 8 figures, LaTex+espcrc2.sty; LATTICE98(electroweak
Quantum phase transitions, frustration, and the Fermi surface in the Kondo lattice model
The quantum phase transition from a spin-Peierls phase with a small Fermi
surface to a paramagnetic Luttinger-liquid phase with a large Fermi surface is
studied in the framework of a one-dimensional Kondo-Heisenberg model that
consists of an electron gas away from half filling, coupled to a spin-1/2 chain
by Kondo interactions. The Kondo spins are further coupled to each other with
isotropic nearest-neighbor and next-nearest-neighbor antiferromagnetic
Heisenberg interactions which are tuned to the Majumdar-Ghosh point. Focusing
on three-eighths filling and using the density-matrix renormalization-group
(DMRG) method, we show that the zero-temperature transition between the phases
with small and large Fermi momenta appears continuous, and involves a new
intermediate phase where the Fermi surface is not well defined. The
intermediate phase is spin gapped and has Kondo-spin correlations that show
incommensurate modulations. Our results appear incompatible with the local
picture for the quantum phase transition in heavy fermion compounds, which
predicts an abrupt change in the size of the Fermi momentum.Comment: 9 pages, 8 figure
Projecting the Kondo Effect: Theory of the Quantum Mirage
A microscopic theory is developed for the projection (quantum mirage) of the
Kondo resonance from one focus of an elliptic quantum corral to the other
focus. The quantum mirage is shown to be independent of the size and the shape
of the ellipse, and experiences \lambda_F/4 oscillations (\lambda_F is the
surface-band Fermi wavelength) with an increasing semimajor axis length. We
predict an oscillatory behavior of the mirage as a function of a weak magnetic
field applied perpendicular to the sample.Comment: 4 pages 2 figures include
Two-point functions of quenched lattice QCD in Numerical Stochastic Perturbation Theory
We summarize the higher-loop perturbative computation of the ghost and gluon
propagators in SU(3) Lattice Gauge Theory. Our final aim is to compare with
results from lattice simulations in order to expose the genuinely
non-perturbative content of the latter. By means of Numerical Stochastic
Perturbation Theory we compute the ghost and gluon propagators in Landau gauge
up to three and four loops. We present results in the infinite volume and limits, based on a general fitting strategy.Comment: 3 pages, 5 figures, talk at conference QCHS-IX, Madrid 201
Two-point functions of quenched lattice QCD in Numerical Stochastic Perturbation Theory. (I) The ghost propagator in Landau gauge
This is the first of a series of two papers on the perturbative computation
of the ghost and gluon propagators in SU(3) Lattice Gauge Theory. Our final aim
is to eventually compare with results from lattice simulations in order to
enlight the genuinely non-perturbative content of the latter. By means of
Numerical Stochastic Perturbation Theory we compute the ghost propagator in
Landau gauge up to three loops. We present results in the infinite volume and
limits, based on a general strategy that we discuss in detail.Comment: 27 pages, 11 figure
The lattice ghost propagator in Landau gauge up to three loops using Numerical Stochastic Perturbation Theory
We complete our high-accuracy studies of the lattice ghost propagator in
Landau gauge in Numerical Stochastic Perturbation Theory up to three loops. We
present a systematic strategy which allows to extract with sufficient precision
the non-logarithmic parts of logarithmically divergent quantities as a function
of the propagator momentum squared in the infinite-volume and limits.
We find accurate coincidence with the one-loop result for the ghost self-energy
known from standard Lattice Perturbation Theory and improve our previous
estimate for the two-loop constant contribution to the ghost self-energy in
Landau gauge. Our results for the perturbative ghost propagator are compared
with Monte Carlo measurements of the ghost propagator performed by the Berlin
Humboldt university group which has used the exponential relation between
potentials and gauge links.Comment: 8 pages, 6 figures, XXVII International Symposium on Lattice Field
Theory - LAT2009, Beijin
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