This year, at the annual Spring Meeting of the German Physical Society (DPG), DESY will be represented with five of 14 plenary talks. The experimental and theoretical particle physics themes are physical phenomena being investigated with the HERA experiments, the design of a detector for a future electron-positron linear collider like TESLA, as well as a theory talk on extra dimensions and the grand unification. This year's DPG Spring Meeting of the particle physics section takes place from 10 to 13 March in Aachen and approx. 400 participants are expected. Below you will find the abstracts of the five mentioned plenary talks, listed in the order of delivery.

The Detector for TESLA: Demands, Concepts and Developments

Markus Schumacher, Bonn

The basic physical questions to be addressed at TESLA, a planned e⁺e^- linear accelerator with high luminosity, require precision measurements of known and new phenomena in the energy regime between 90 and 800 GeV.
The basic design of the detector is determined by the goal of precision measurements, the accelerator induced backgrounds and the time structure of the collisions. The resulting requirements for the detector exceed those for previous e⁺e^- detectors significantly.
The necessary resolutions and granularity of the detector components will be explained using selected examples of physics processes. The favored concept for the detector and the different options, which are developments of existing and new technologies, are discussed. An overview about the research and development activities will be given.

Physics at high Q² and the Search for New Phenomena at HERA

Isabell Melzer-Pellmann, DESY

The experiments H1 and ZEUS at HERA measure positron-proton and electron-proton interactions up to a maximal four momentum transfer of about Q² = 100000 GeV². Measurements of inclusive neutral and charged current cross sections allow tests of the Standard Model in a kinematic region that is not accessible to other experiments. Furthermore, parton distributions are measured over a range of several orders of magnitude. In addition, signatures of events which cannot be explained by the Standard Model are investigated.

The Spin Structure of Nucleons

Antje Bruell, MIT

After 20 years of experimental efforts the question how much the building blocks of the nucleon-the quarks of various flavors and the gluons-contribute to the nucleon's spin still is only poorly understood. A large number of precision measurements of the inclusive polarized scattering process confirmed that the quarks contribute only about 20-30%. New semi-inclusive measurements confirm this interpretation of the inclusive data and additionally allow a determination of the contributions of the various flavors. Of special interest here is the polarization of the strange quarks and a possible difference in the polarizations of the up and down sea quarks.
First measurements of limited precision indicate a positive contribution of the gluons to the nucleon's spin. On this important question a huge improvement is expected from the currently running and/or planned experiments at various accelerators. Conclusions about possible contributions from orbital momenta might emerge from measurements of exclusive reactions: recently several such measurements-though with limited statistical precision and within a small kinematical range-have been reported.

Heavy Quark Production at the ep Collider HERA

Andreas Meyer, Hamburg

The study of heavy quark production in ep collisions at HERA is a key method to investigate the theory of strong interactions (QCD). Various aspects of QCD are probed by the measurements, e.g. perturbative parton dynamics, the structure of the proton and of the photon and also fragmentation. The masses of the charm and beauty quarks provide energy scales at which the strong coupling constant alpha_s is small enough for perturbative calculations to be performed, even at small momentum transfer squared Q², in photo production. New measurements of open charm and beauty production and of charmonium production are presented and compared with theoretical expectations. While the production of charm is reasonably well described there are deviations found in the sector of beauty production between the measurements and the expectations. Similar deviations have been seen in photon-photon and proton anti-proton collisions.