539 research outputs found
Scattering of twisted particles: extension to wave packets and orbital helicity
High-energy photons and other particles carrying non-zero orbital angular
momentum (OAM) emerge as a new tool in high-energy physics. Recently, it was
suggested to generate high-energy photons with non-zero OAM (twisted photons)
by the Compton backscattering of laser twisted photons on relativistic electron
beams. Twisted electrons in the intermediate energy range have also been
demostrated experimentally; twisted protons and other particles can in
principle be created in a similar way. Collisions of energetic twisted states
can offer a new look at particle properties and interactions. A theoretical
description of twisted particle scattering developed previously treated them as
pure Bessel states and ran into difficulty when describing the OAM of the final
twisted particle at non-zero scattering angles. Here we develop further this
formalism by incorporating two additional important features. First, we treat
the initial OAM state as a wave packet of a finite transverse size rather than
a pure Bessel state. This realistic assumption allows us to resolve the
existing controversy between two theoretical analyses for non-forward
scattering. Second, we describe the final twisted particle in terms of the
orbital helicity --- the OAM projection on its average direction of propagation
rather than on the fixed reaction axis. Using this formalism, we determine to
what extent the twisted state is transferred from the initial to final OAM
particle in a generic scattering kinematics. As a particular application, we
prove that in the Compton backscattering the orbital helicity of the final
photon stays close to the OAM projection of the initial photon.Comment: 18 pages, 4 figures; v2: expanded introduction and section 4.2 on
final orbital helicit
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
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
Double lepton pair production with electron capture in relativistic heavy--ion collisions
We present a theoretical study of a double lepton pair production in
ultra--relativistic collision between two bare ions. Special emphasis is placed
to processes in which creation of (at least one) pair is accompanied
by the capture of an electron into a bound ionic state. To evaluate the
probability and cross section of these processes we employ two approaches based
on (i) the first--order perturbation theory and multipole expansion of Dirac
wavefunctions, and (ii) the equivalent photon approximation. With the help of
such approaches, detailed calculations are made for the creation of two
bound--free pairs as well as of bound--free and free--free
pairs in collisions of bare lead ions Pb. The results of
the calculations indicate that observation of the double lepton processes may
become feasible at the LHC facility.Comment: 12 pages, 1 figur
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