375 research outputs found
Noether's second theorem for BRST symmetries
We present Noether's second theorem for graded Lagrangian systems of even and
odd variables on an arbitrary body manifold X in a general case of BRST
symmetries depending on derivatives of dynamic variables and ghosts of any
finite order. As a preliminary step, Noether's second theorem for Lagrangian
systems on fiber bundles over X possessing gauge symmetries depending on
derivatives of dynamic variables and parameters of arbitrary order is proved.Comment: 31 pages, to be published in J. Math. Phy
Noether's second theorem in a general setting. Reducible gauge theories
We prove Noether's direct and inverse second theorems for Lagrangian systems
on fiber bundles in the case of gauge symmetries depending on derivatives of
dynamic variables of an arbitrary order. The appropriate notions of reducible
gauge symmetries and Noether's identities are formulated, and their equivalence
by means of certain intertwining operator is proved.Comment: 20 pages, to be published in J. Phys. A (2005
Covariant Hamiltonian field theory
We study the relationship between the equations of first order Lagrangian
field theory on fiber bundles and the covariant Hamilton equations on the
finite-dimensional polysymplectic phase space of covariant Hamiltonian field
theory. The main peculiarity of these Hamilton equations lies in the fact that,
for degenerate systems, they contain additional gauge fixing conditions. We
develop the BRST extension of the covariant Hamiltonian formalism,
characterized by a Lie superalgebra of BRST and anti-BRST symmetries.Comment: 35 pages, Late
A setup for integral measurements of multiple scattering angular distributions by 10- to 100-keV electrons
A dedicated setup has been developed to study the angular distributions of electrons traversing thin films employing the low-energy beam available from the gun of the SĂŁo Paulo Microtron. In this first stage, only integral measurements are possible using the fraction of the electron beam current collected in a Faraday cup and in a ring surrounding the entrance of the former. The overall normalisation is provided by measuring the beam current collected by the scattering chamber as well, thus covering the full solid angle. In this work, important construction specifications are presented. The experience gained by operating this system is also discussed regarding its critical aspects like: avoiding cross talk between the cup and the ring both at the physical and electronic levels, measurements of small charges deposited on large objects (the chamber in particular), and conditioning of the various surfaces involved. Finally, some selected results are compared to the well-known theory by Goudsmit and Saunderson with good agreement
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
Linear to Circular Polarisation Conversion using Birefringent Properties of Aligned Crystals for Multi-GeV Photons
We present the first experimental results on the use of a thick aligned Si
crystal acting as a quarter wave plate to induce a degree of circular
polarisation in a high energy linearly polarised photon beam. The linearly
polarised photon beam is produced from coherent bremsstrahlung radiation by 178
GeV unpolarised electrons incident on an aligned Si crystal, acting as a
radiator. The linear polarisation of the photon beam is characterised by
measuring the asymmetry in electron-positron pair production in a Ge crystal,
for different crystal orientations. The Ge crystal therefore acts as an
analyser. The birefringence phenomenon, which converts the linear polarisation
to circular polarisation, is observed by letting the linearly polarised photons
beam pass through a thick Si quarter wave plate crystal, and then measuring the
asymmetry in electron-positron pair production again for a selection of
relative angles between the crystallographic planes of the radiator, analyser
and quarter wave plate. The systematics of the difference between the measured
asymmetries with and without the quarter wave plate are predicted by theory to
reveal an evolution in the Stokes parameters from which the appearance of a
circularly polarised component in the photon beam can be demonstrated. The
measured magnitude of the circularly polarised component was consistent with
the theoretical predictions, and therefore is in indication of the existence of
the birefringence effect.Comment: 12 pages, 12 figures, 1 table, REVTeX4 two column, Version for
publicatio
Results on the Coherent Interaction of High Energy Electrons and Photons in Oriented Single Crystals
The CERN-NA-59 experiment examined a wide range of electromagnetic processes
for multi-GeV electrons and photons interacting with oriented single crystals.
The various types of crystals and their orientations were used for producing
photon beams and for converting and measuring their polarisation.
The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm
thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the
String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised
photon beams.
A new crystal polarimetry technique was established for measuring the linear
polarisation of the photon beam. The polarimeter is based on the dependence of
the Coherent Pair Production (CPP) cross section in oriented single crystals on
the direction of the photon polarisation with respect to the crystal plane.
Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set
of synthetic Diamond crystals were used as analyzers of the linear
polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the
photon beam into circular polarisation, was observed. This was achieved by
letting the linearly polarised photon beam pass through a 10 cm thick Silicon
single crystal that acted as a "quarter wave plate" (QWP) as suggested by N.
Cabibbo et al.Comment: Presented at International workshop "Relativistic Channeling and
Related Coherent Phenomena", Frascati (Rome) 23-26 March 200
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