Prototype for an Undulator-based Source for Polarised Positrons: International Polarised Positron Collaboration: Project E-166

The full exploitation of the physics potential of a future Linear Collider requires the development of polarised positron beams. A very promising scheme for the technical realisation is the use of helical undulators, generating circular polarised photons of several MeV which are then converted in a thin target to longitudinally polarised positrons. The experiment E-166 tests this scheme. It uses the low-emittance 50-GeV electron beam at the Final Focus Test Beam (FFTB) at SLAC, passing through a 1 meter-long helical undulator. The flux and polarisation of the undulator photons as well as the properties of the positrons will be measured and will be compared with simulations.Comment: 3 pages, 5 postscript figures, epj style, talk given at the EPS conference, Aachen, Germany, July 200

A Future Linear Collider with polarised beams: Searches for New Physics

There exists a world--wide consensus for a future $e^+e^-$ Linear Collider in the energy range between $\sqrt{s}=500--1000$ GeV as the next large facility in HEP. The Linear Collider has a large physics potential for the discovery of new physics beyond the Standard Model and for precision studies of the Standard Model itself. It is well suited to complement and extend the physics program of the LHC. The use of polarised beams at a Linear Collider will be one of the powerful tools. In this paper some highlights of searches for physics beyond the Standard Model at a future Linear Collider with polarised $e^-$ and $e^+$ beams are summarised.Comment: 15 pages, 10 eps figures, Invited plenary talk given at 15th International Spin Physics Symposium, September 2002, BNL, Brookhaven, US

Physics aspects of polarized e+ at the linear collider

Polarized e- and e+ at the International Linear Collider play an important role for discovering physics beyond the Standard Model and for precisely unravelling the structure of the underlying physics. The physics programme at the first energy stage at sqrt{s}=500 GeV benefits strongly from the polarization of both beams. But also at 1 TeV as well as at a possible multi-TeV design of a linear collider, CLIC, the physics output is greatly enriched by beam polarization. An overview is given of the impact of providing polarized e+ at the linear collider in addition to polarized e- for physics studies in top, Higgs, supersymmetry and further models of physics beyond the Standard Model.Comment: 14 pages, 16 eps figures Talk given at the Polarized Positrons Workshop `POSIPOL', CERN, 26-28 April 200

News from polarized e- and e+ at the ILC

The proposed International Linear Collider (ILC) is well-suited for discovering physics beyond the Standard Model and for precisely unravelling the structure of the underlying physics. The physics return of the ILC can be maximized by the use of polarized beams, in particular the simultaneous polarization of the e- and the e+ beam. Ongoing physics studies are accompanied by active R&D on the machine part for generating polarized beams and for measuring the polarization with high precision at the ILC. Some new results on the physics case and on the technical aspects of the polarization of both beams are briefly summarized.Comment: 6 pages, 12 eps figures, style files; Contribution to '2005 International Linear Collider Workshop -Stanford, U.S.A.

Nonlinear lepton-photon interactions in external background fields

Nonlinear phenomena of lepton-photon interactions in external backgrounds with a generalised periodic plane-wave geometry are studied. We discuss nonlinear Compton scattering in head-on lepton-photon collisions extended properly to beyond the soft-photon regime. In addition, our results are applied to stimulated lepton-antilepton pair production in photon collisions with unrestricted energies. Derivations are considered semi-classically based on unperturbed fermionic Volkov representations encoding the full interaction with the background field. Closed expressions for total probabilities considering S-matrix elements have been derived. The general formula is applied to Compton scattering by an electron propagating in an external laser-like background. We obtain additive contributions in the extended unconstrained result which turns out to be stringently required in the highly nonlinear regime. A detailed comparison of contributing harmonics is discussed for various field parameters.Comment: 39 pages, 8 figures, minor changes in introduction, references added, a few misprints foun

Euclidean mirrors: enhanced vacuum decay from reflected instantons

We study the tunneling of virtual matter-antimatter pairs from the quantum vacuum in the presence of a spatially uniform, time-dependent electric background composed of a strong, slow field superimposed with a weak, rapid field. After analytic continuation to Euclidean spacetime, we obtain from the instanton equations two critical points. While one of them is the closing point of the instanton path, the other serves as an Euclidean mirror which reflects and squeezes the instanton. It is this reflection and shrinking which is responsible for an enormous enhancement of the vacuum pair production rate. We discuss how important features of two different mechanisms can be analysed and understood via such a rotation in the complex plane. a) Consistent with previous studies, we first discuss the standard assisted mechanism with a static strong field and certain weak fields with a distinct pole structure in order to show that the reflection takes place exactly at the poles. We also discuss the effect of possible sub-cycle structures. We extend this reflection picture then to weak fields which have no poles present and illustrate the effective reflections with explicit examples. An additional field strength dependence for the rate occurs in such cases. We analytically compute the characteristic threshold for the assisted mechanism given by the critical combined Keldysh parameter. We discuss significant differences between these two types of fields. For various backgrounds, we present the contributing instantons and perform analytical computations for the corresponding rates treating both fields nonperturbatively. b) In addition, we also study the case with a nonstatic strong field which gives rise to the assisted dynamical mechanism. For different strong field profiles we investigate the impact on the critical combined Keldysh parameter. [...]Comment: 54 pages, 23 figures, revised, restructured to improve readability, matches journal versio