abstract

We present recent progress on the theoretical precision limits of the LEP luminosity process, as calculated by the Monte Carlo event generator BHLUMI4.04. We include exact results for all two-photon radiative corrections to the process \(e^+ e^- \rightarrow e^+ e^-\) at small angles and LEP energies. These results reduce the precision estimate for the \({\cal O}(\alpha ^2)\) photonic radiative correction from \(0.1\%\) to \(0.027\%\), leading to an overall precision of \(0.061\%\) for the currently published version of BHLUMI4.04. This precision level is important for the final precision Z physics measurements at LEP1. We also present precision estimates for LEP2.

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The results for the production of the four-fermions via neutral electroweak currents are presented. The experimental data coming from the DELPHI experiment at the LEP2 at CMS energy from 130 GeV to 189 GeV are compared to the Monte Carlo predictions.

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We review 1) constraints on low energy supersymmetry from the search for Higgs boson and from precision data, 2) dependence of coupling unification on the superpartner spectrum, 3) naturalness and fine tuning in the minimal and non- minimal scenarios.

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Preliminary results are presented from searches for new particles in \(e^+e^-\) collisions at \(\sqrt s=188.6\) GeV. No evidence for new physics was found, but a significant range of parameters in various models was explored and excluded, in some cases with important theoretical implications.

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The status of the Standard Model is reviewed on the basis of precise calculations for the electroweak observables associated with the \(W\) and \(Z\) bosons together with the recent experimental high precision data. A brief discussion of the status of precision observables in the MSSM is also included.

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The parametrisation of the photon structure function in the low \(Q^2\) region is formulated. It includes the VMD contribution and the QCD improved parton model component suitably extrapolated to the low \(Q^2\) region. The parametrisation describes reasonably well existing experimental data on \(\sigma _{\gamma \gamma }\) for real photons and the low \(Q^2\) data on \(\sigma _{\gamma ^* \gamma }\). Predictions for \(\sigma _{\gamma \gamma }\) and for \(\sigma _{\gamma ^* \gamma }\) for energies which may be accesssible in future linear colliders are also given.

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The contribution of the QCD pomeron to the processes: \(e^+ e^- \to e^+ e^- J/\psi J/\psi \) and \(e^+ e^- \to e^+e^- \) hadrons (with tagged electrons) is discussed. We focus on reactions which occur via photon–photon collisions, with virtual photons coming from the Weizsäcker-Wiliams spectrum of the electrons. We stress the importance of the non-leading corrections to the BFKL equation and take into account dominant non-leading effects which come from the requirement that the virtuality of the exchanged gluons along the gluon ladder is controlled by their transverse momentum squared. The \(\gamma ^*\gamma ^*\) cross-sections are found to increase with increasing \(\gamma ^*\gamma ^*\) CM energy \(W\) as \((W^2)^{\lambda _{\rm P}}\) while the cross-section for \(J/\psi \) production is found to increase as \((W^2)^{2\lambda _{\rm P}}\). The parameter \(\lambda _{\rm P}\) is slowly varying with energy \(W\) and takes the values \(\lambda _{\rm P} \sim 0.23\)–0.35 depending on the process. We also analyze the contribution of the soft pomeron for the total \(\gamma ^* \gamma ^*\) cross-section. We compare results of our calculations to the recent data from LEP.

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The BELLE collaboration (about 330 physicists and students from 50 laboratories representing ten countries) prepares a dedicated CP violation experiment at the KEK B-factory, now under commisioning. The BELLE detector was completed in December 1998 and should start data taking in April 1999.

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Muon storage rings are reviewed in an introductory manner. The physics opportunities are exciting. The technical difficulties are considerable, but can be spread over three step scenario. First a neutrino factory would make definitive experiments on neutrino oscillations; then several generations of precision muon colliders could explore the spectrum of particles associated with electroweak symmetry breaking or supersymmetry; finally high energy lepton collisions could be envisaged.

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Higgs physics at prospective \(e^+e^-\) linear colliders is reviewed in the context of the Standard Model and supersymmetric theories. A brief overview is also given on strong interactions of the electroweak bosons at TeV energies in alternative scenarios.

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The physics potential of discovering and exploring supersymmetry at future \(e^+e^-\) linear colliders is reviewed. Such colliders are planned to start to operate at a center-of-mass energy of 500 GeV to 800 GeV, with a final energy of about 2 TeV expected. They are ideal facilities for the discovery of supersymmetric particles. High precision measurements of their properties and interactions will help to uncover the mechanism of supersymmetry breaking and will allow for tests of grand unification scenarios.

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In this contribution I review the physics of top quarks at a future Linear Collider. Main emphasis is put on the process \(e^+ e^- \to t\bar t\) close to threshold. Different physical observables, their sensitivity to the basic parameters and their theoretical prediction are discussed. Recent higher order calculations are shown to have a considerable impact on a precise determination of the top quark mass. It is pointed out how the use of mass definitions different from the pole mass scheme become important in this respect. Continuum top quark production above threshold is discussed briefly.

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When Higgs boson candidates will be found at future colliders, it becomes imperative to determine their properties, beyond the mass, production cross section and decay rates. Other crucial properties are those related to the behaviour under CP transformations, and the self-couplings. This paper addresses the question of measurability of some of the trilinear couplings of MSSM neutral Higgs bosons at a high-energy \(e^+e^-\) collider, and the possibilities of exploring the Higgs boson CP properties at \(e^+e^-\) and \(e^-e^-\) colliders.

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The masses of the neutral \({\cal CP}\)-even Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM) are predicted on the basis of explicit Feynman-diagrammatic calculations. The results, containing the complete diagrammatic one-loop corrections, the leading two-loop corrections of \({\cal O}(\alpha \alpha _s)\) and further improvements taking into account leading electroweak two-loop and higher-order QCD contributions, are discussed and compared with results obtained by renormalization group calculations. Good agreement is found in the case of vanishing mixing in the scalar top sector, while sizable deviations occur if scalar top mixing is taken into account. By means of a Taylor expansion a compact approximation formula for the mass of the lightest Higgs boson, \(m_h\), is derived. The quality of the approximation in comparison with the full result is analyzed.

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Production and decay of gaugino-like charginos are crucially determined by sneutrino exchange. Therefore we study the pair production of charginos \(e^{+}e^{-}\to \tilde {\chi }^{+}_1\tilde {\chi }^{-}_1\) with polarized beams and the subsequent decay \(\tilde {\chi }^{-}_1\to \tilde {\chi }^0_1 e^{-} \bar {\nu }_e\), including the complete spin correlations between production and decay. The spin correlations have strong influence on the decay angular distribution and on the corresponding forward–backward asymmetry. We show for two representative scenarios for \(\sqrt {s}=270\) GeV and for \(\sqrt {s}=500\) GeV that forward–backward asymmetries for polarized beams are an important tool for constraining the sneutrino mass \(m_{\tilde {\nu }_e}\).

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The recent developments in the soft supersymmetry breaking (SSB) sector of Gauge-Yukawa and Finite Unified Theories permit the derivation of exact renormalization group invariant results also in this sector of the theory. Of particular interest is a RGI sum rule for the soft scalar masses holding to all-orders in perturbation theory. In the case of Finite Unified Theories the sum rule ensures the all-loop finiteness also in their SSB sector and in this way are promoted to completely finite ones. Using the sum rule we investigate the minimal supersymmetric Gauge-Yukawa and two Finite-Gauge-Yukawa SU(5) models. The characteristic features of these models are: a) the old agreement of the top quark mass prediction remains unchanged, b) the lightest Higgs boson is predicted to be around 120 GeV, c) the s-spectrum starts above several hundreds of GeV.

abstract

The CLIC study of high-energy (0.5-5 TeV), high-luminosity (10\(^{34}\)-10\(^{35}\, {\rm cm}^{-2}{\rm s}^{-1}\)) \(e^\pm \) linear collider is presented. Beam acceleration using high-frequency (30 GHz) normal-conducting structures operating at high accelerating fields (100 to 200 MV/m) significantly reduces the length and, in consequence, the cost of the linac. Based on new beam and linac parameters derived from a recently developed set of general scaling laws for linear colliders, the beam stability is shown to be similar to lower frequency designs in spite of the strong wake-field dependency on frequency. A new cost-effective and efficient drive beam generation scheme for RF power production by the so-called Two Beam Acceleration (TBA) method is described. It uses a thermionic gun and a fully-loaded normal-conducting linac operating at low frequency (937 MHz) to generate and accelerate the drive beam bunches, and RF multiplication by funnelling in compressor rings to produce the desired bunch structure. Recent 30 GHz hardware developments and results from the CLIC Test Facility (CTF), assessing the feasibility of the scheme, are described.

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A brief overview is given of the JLC project that is promoted to explore a new frontier of \(e^+e^-\) collider physics, selecting topics of the physics motivation, the detector concept, the progress in Accelerator Test Facility at KEK and the internationalization of the project.

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The status of the layout of the linear collider project, TESLA, which employs superconducting accelerating structures, will be presented.

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Physics opportunities in two photon collisions at a future linear collider are discussed. Several QCD related topics are described in detail.

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The physics programme of a future high energy \(e^+e^-\) linear collider requires very accurate three-dimensional measurements of particle positions close to the interaction point. Silicon Active Pixel Sensors are an attractive technology for a Vertex Tracker if their single point resolution can be improved further than the performances envisaged for the generation of detectors being developed for the LHC. Capacitively coupled Active Pixel Sensors with an analog readout electronics should bring the solution. The design and production of the first prototypes of these novel detectors as well as the electronics concept are presented.

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European preparations for building a detector for the \(e^+e^-\) physics up to 1 TeV center-of-mass energies at the linear collider are described.

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The paper presents original experimental results which were chosen to present an application of the synchrotron radiation for study of engineering of the valence band electronic structure of the semimagnetic semiconductors. The results of the resonant photoemission study (Fano type resonance) of transition metal atoms (\(3p\)–\(3d\) electrons transition) and rare earth atoms (\(4d\)–\(4f\) electrons transition) incorporated into the volume of the II-VI and IV-VI compounds or deposited on clean surface of the CdTe crystal will be presented.