Limits on the accuracy of drift chambers and calibration beams

A glance at the letters of intent for experiments at LEP which have been submitted to CERN on 31 January 1982 show that drift chambers of one type or another are still very popular and are considered as being the main candidate for detecting charged particles even eight years from now. The understanding of intrinsic limits on the spatial accuracy of drift chambers, and the development of new methods for calibrating them, are therefore still relevant. The present talk attempts to review the state of the art. Two classes are considered: i) Extremely accurate detectors for vertex reconstruction around the interaction region. ii) Very large detectors of the projection-chamber or imaging-chamber type, where the moderate accuracy of individual measurement (sigma/sub w/ approx. = 200 μm) is compensated by the relatively large number of points on each track. Two questions studied in this talk are: i) What are the fundamental limits on the accuracy and the two-particle resolution in a gaseous drift chamber. How can performances be improved, by the proper choice of gas and running conditions, or by better detection methods. ii) What, in practice, are the limits encountered in large detectors. Is it possible to limit effects of systematics by proper design of the detector. Can X-ray and UV calibration beams be used