5,866 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
A new possibility to monitor collisions of relativistic heavy ions at LHC and RHIC
We consider the radiation of particles of one bunch in the collective field
of the oncoming bunch, called coherent bremsstrahlung (CBS). The main
characteristics of CBS for LHC (in the Pb-Pb mode) and for RHIC are calculated.
At LHC about photons per second are expected for
photon energies eV. It seems that CBS
can be a potential tool for optimizing collisions and for measuring beam
parameters. The bunch length can be found from the critical energy of the CBS
spectrum; the transverse bunch size is related to the photon rate. A specific
dependence of photon rate on the impact parameter between the beams allows for
a fast control over the beam displacement.Comment: 9 pages + 4 figures, latex with poscript figures uuencode
Charge asymmetry of pions in the process
The study of the charge asymmetry of produced particles allows to investigate
the interference of different production mechanisms and to determine new
features of the corresponding amplitudes. In the process the two-pion system is produced via two mechanisms: two-photon
(C-even state) and bremsstrahlung (C-odd state) production. We study the charge
asymmetry of pions in a differential in the pion momenta cross section
originating from an interference between these two mechanisms. At low effective
mass of dipions this asymmetry is directly related to the s- and p-phases of
elastic scattering. At higher energies it can give new information
about the meson family, meson, etc. The asymmetry is
expressed via the pion form factor and helicity amplitudes for
the subprocess as where we have calculated analytically the coefficients
for the region giving the main contribution to the effect. Several
distributions of pions are presented performing a numerical analysis in a model
with point-like pions. In the region near the dipion threshold the asymmetry is
of the order of 1%. We show that with suitable cuts the signal to background
ratio can be increased up to about 10%.Comment: 16 pages, 10 figures, LaTeX, style files for EPJC include
Photoemission study of the spin-density wave state in thin films of Cr
Angle-resolved photoemission (PE) was used to characterize the spin-density
wave (SDW) state in thin films of Cr grown on W(110). The PE data were analysed
using results of local spin density approximation layer-Korringa-Kohn-Rostoker
calculations. It is shown that the incommensurate SDW can be monitored and
important parameters of SDW-related interactions, such as coupling strength and
energy of collective magnetic excitations, can be determined from the
dispersion of the renormalized electronic bands close to the Fermi energy. The
developed approach can readily be applied to other SDW systems including
magnetic multilayer structures.Comment: 4 figure
Real-time dynamics in Quantum Impurity Systems: A Time-dependent Numerical Renormalization Group Approach
We develop a general approach to the nonequilibrium dynamics of quantum
impurity systems for arbitrary coupling strength. The numerical renormalization
group is used to generate a complete basis set necessary for the correct
description of the time evolution. We benchmark our method with the exact
analytical solution for the resonant-level model. As a first application, we
investigate the equilibration of a quantum dot subject to a sudden change of
the gate voltage and external magnetic field. Two distinct relaxation times are
identified for the spin and charge dynamics.Comment: 5 pages, 5 figure
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