Peter Schmüser

Evidence for planar events in e+e− annihilation at high energies

Abstract Hadron jets produced in e+e− annihilation between 13 GeV and 31.6 GeV in c.m. at PETRA are analyzed. The transverse momentum of the jets is found to increase strongly with c.m. energy. The broadening of the jets is not uniform in azimuthal angle around the quark direction but tends to yield planar events with large and growing transverse momenta in the plane and smaller transverse momenta normal to the plane. The simple q q collinear jet picture is ruled out. The observation of planar events shows that there are three basic particles in the final state. Indeed, several events with three well-separated jets of hadrons are observed at the highest energies. This occurs naturally when the outgoing quark radiates a hard noncollinear gluon, i.e., e + e − → q q g with the quarks and the gluons fragmenting into hadrons with limited transverse momenta.

Ultraviolet and Soft X-Ray Free-Electron Lasers

The main goal of the book is to provide a systematic and didactic approach to the physics and technology of free-electron lasers. Numerous figures are used for illustrating the underlying ideas and concepts, and many links to other fields of physics are provided. After an introduction to undulator radiation and the low-gain FEL, the one-dimensional theory of the high-gain FEL is developed in a systematic way. Particular emphasis is put on explaining and justifying the various assumptions and approximations that are needed to obtain the differential equations governing the FEL dynamics. The predictive power of the 1D FEL theory is demonstrated with numerous examples and figures, including exponential gain, saturation and FEL bandwidth. One of the most important features of a high-gain FEL, the formation of microbunches, is studied at length and illustrated with several figures. 3D corrections to the 1D theory are discussed. The process of self amplified spontaneous emission (SASE) is explained mathematically, and many experimental results are shown. The layout of the worlds first SASE FEL in the soft X-ray regime, FLASH, is described in some detail and the technological challenges of X-ray FELs are outlined. Important concepts of accelerator physics are explained while some of the more involved mathematical computations are put into the appendices.

Comparison of e+e− annihilation with QCD and determination of the strong coupling constant

We have analyzed 1113 events of the reaction e+e− → hadrons at CM energies of 12 and 30 GeV in order to make a detailed comparison with QCD. Perturbative effects can be well separated from effects depending on the quark and gluon fragmentation parameters to yield a reliable measurement of the coupling constant αS. At 30 GeV, the result is αS = 0.17 ± 0.02 (statistical) ± 0.03 (systematic). QCD model predictions, using the fragmentation parameters determined along with αS, agree with both gross properties of the final states and with detailed features of the three-jet states.

Achievement of 35 MV/m in the superconducting nine-cell cavities for TESLA

Abstract The Tera Electronvolt Superconducting Linear Accelerator (TESLA) is the only linear electron–positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV / m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor Q0 of 1010. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV / m . Using an improved cavity treatment by electrolytic polishing, it has been possible to raise the gradient to 35– 43 MV / m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently 4 nine-cell cavities have achieved 35 MV / m at Q0⩾5×109, and a fifth cavity could be excited to 39 MV / m . In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler. One cavity has been operated for 1100 h at the TESLA-800 gradient of 35 MV / m and 57 h at 36 MV / m without loss in performance.

Evidence for a spin-1 gluon in three-jet events

High-energy e+e--annihilation events obtained in the TASSO detector at PETRA have been used to determine the spin of the gluon in the reaction e+e- → qqg. We analysed angular correlations between the three jet axes. While vector gluons are consistent with the data (55% confidence limit), scalar gluons are disfavoured by 3.8 standard deviations, corresponding to a confidence level of about 10-4. Our conclusion is free of possible biases due to uncertainties in the fragmentation process or in determining the qqg kinematics from the observed hadrons.

The Tasso Gas and Aerogel Cherenkov Counters

Abstract We describe the gas and erogel threshold Cherenkov counters built for the TASSO experiment at the DESY e + e − storage ring PETRA. The counters are arranged in two diametrically opposed groups as part of the two hadron arms. The gas counters contain Freon 114 and CO 2 respectively; 128 ellipsoidal mirrors focus the Cherenkov light through funnels onto Philips 5″ photomultipliers coated with wavelength shifter. β = 1 charged particles are detected with at least 99.9% efficiency. The aerogel counters are of the diffusing wall type. They have a sensitive area of 12 m 2 , divided into 32 cells and use a total of 192 RCA Quantacon 5″ photomultipliers. The average aerogel refractive index is around 1.025. The efficiency for β = 1 charged particles is 98%. Design considerations, prototype evaluation, construction details, and performance of the final counter system are described.

Anregung von Volumen- und Oberflächenplasmaschwingungen in Al und Mg durch mittelschnelle Elektronen

Dispersion and scattering cross section of the bulk plasma losses of Al and Mg are compared with plasma theory. The low lying Mg loss shows an asymmetric intensity distribution for non-normally incident electrons and an angular dependence of intensity as expected from the theory of surface losses. In very thin Al films, coupling effects between the two opposite surfaces are found, causing some new properties of the surface losses. Especially a thickness dependent dispersion of the surface loss is measured which is quite different from that of the bulk losses and characteristic for surface excitation.

The Large Cylindrical Drift Chamber of Tasso

Abstract We have built and operated a large cylindrical drift chamber for the TASSO experiment at the DESY storage ring, PETRA. The chamber has a length of 3.5 m, a diameter of 2.5 m, and a total of 2340 drift cells. The cells are arranged in 15 concentric layers such that tracks can be reconstructed in three dimensions. A spatial resolution of 220 μm has been achieved for tracks of normal incidence on the drift cells.

Study of the transverse coherence at the TTF free electron laser

Double slits with different separations, crossed slits and circular apertures have been used to study the transverse coherence of the VUV light of the SASE Free Electron Laser at the TESLA Test facility at DESY. The resulting diffraction patterns are converted to visible light by a Ce:YAG crystal and imaged by a high-resolution CCD camera. The visibility of the diffraction patterns indicates a high degree of transverse coherence. Measurements have been taken at various operating modes and wavelengths of the FEL. A numeric FEL simulation code has been used to calculate the wavefronts of the light at the exit of the undulator. By propagating the wavefronts through the optical setup, the diffraction at the double slits is computed with the code GLAD. Good agreement with the measurements is found.

Improved surface treatment of the superconducting TESLA cavities

Abstract The proposed linear electron–positron collider TESLA is based on 1.3 GHz superconducting niobium cavities for particle acceleration. For a centre-of-mass energy of 500 GeV , an accelerating field of 23.4 MV/m is required which is reliably achieved with a niobium surface preparation by chemical etching. An upgrade of the collider to 800 GeV requires an improved cavity preparation technique. In this paper, results are presented on single-cell cavities which demonstrate that fields of up to 40 MV/m are accessible by electrolytic polishing of the inner surface of the cavity.