Experimental research in the field of Particle physics is nowadays pursued at a few international centres. Hundreds of physicists gather around their huge spectrometers at accelerators capable of obtaining beams of high energy particles. Experiments exploring the secrets of nature at high energies and correspondingly small scales are conducted by collaborations, thus adding their human and financial resources. Researchers of the Experimental Particle Physics Department take part in the research of four collaborations: CPLEAR and DELPHI in the European Laboratory for Particle Physics CERN in Geneva and ARGUS and HERA-B at DESY in Hamburg.
In the scope of an upgrade of the DELPHI detector to match the higher energies of the LEP-200 collider, members of the Department take part in the upgrade of the vertex detector in the forward region. Characteristics of microstrip silicon detectors (fig. 1) were measured in a test beam. In cooperation with the Solid State and Ceramic departments, hybrid circuits for the readout electronics have been designed and manufactured. The assembly of double-sided modules has started with the aim of being ready for insertion into the spectrometer at the end of 1995.
Concerning the analysis of DELPHI data, members of the Department performed an on-line calibration of the RICH detector using decays. Measurements of the lifetime were continued, using events with a and a lepton (90 % of such events originate from a semileptonic decay) and events containing a and a lepton (50 % are decays). An evaluation of the decay width of the into a pair of quarks has started, using a neural network to identify b-quarks on the basis of kinematical properties of jets. Such measurements in combination with others allow an indirect determination of the top quark mass with almost the same precision as from the direct measurement.
The CPLEAR detector at the low-energy antiproton ring (LEAR) at CERN continued data-taking. In 80 days of running more than 1 billion events were recorded. In parallel, analysis of data taken in previous years yielded results on CP violation parameters with better precision than the current world average. Time reversal violation was demonstrated for the first time on a two standard deviation level. Slovenian researchers performed an analysis of three-pion decays improving the upper limit for CP-violation in this decay and for the first time determining the branching ratio for the decay (fig. 2). They have finalized a method for electron-pion separation with the electromagnetic calorimeter and continued the analysis of the decay .
Despite the fact that the ARGUS detector stopped data-taking two-years ago the analysis of data is still continuing. Recently, three precise investigations of the lepton decays have been reported. The decay lepton spectra were parametrized by four Michel parameters, closely related to the nature of the weak interaction. The complete set, measured for the first time, was found in good agreement with the Standard model. Polarization effects in the decay were also measured and the mixing parameter redetermined using partial reconstruction or by tagging the flavour of the meson by a charged kaon. These results are of great importance for future experiments, planned to reveal the violation in the system of b-mesons. The focus of activities of the Slovenian members remained on the analysis of hadron formation via two-photon interactions. The results, published in two articles, concern a variety of fields: from light meson spectroscopy and description of hadronic interactions by mesonic degrees of freedom, to the determination of the strong coupling constant, extracted from hadronic corrections to the two-photon formation of .
The design study of HERA-B, an experiment to measure the CP symmetry violation in the neutral B meson system, was continued. B mesons will be among the reaction products of high energy protons () with copper nuclei. The target will be set-up from a system of thin wires placed in the beam halo of the proton beam of the HERA collider at DESY in Hamburg. The trigger system will be tuned to recognize the well defined final state . The study was concluded by a proposal for the HERA-B experiment. Following the positive decision of the DESY directorate on the future of the proposed experiment, a technical design report was completed and published.
The research and development of one of the HERA-B spectrometer components, the RICH (Ring Imaging Cerenkov counter), was carried on. A prototype was designed and assembled in the test beam T24 of the DESY accelerator system. The measurements have shown (fig. 3) that such an apparatus allows for a precise measurement of charged particle's velocity by measuring its Cerenkov angle. To measure the ageing properties of photon detectors in the hostile environment as expected in the HERA-B experiment, a set-up was constructed that will allow for testing the detector with cosmic rays, radioactive sources and UV light.
Related to the development of a MWPC based PET tomograph, a NaI scintillator based system was repaired and improved. The whole readout chain and reconstruction algorithms were tested by imaging a guinea pig injected with a solution (fig. 4).
In September 1994 the Department organized the Third Biennial Conference on Low-Energy Antiproton Physics (LEAP'94) in Bled, attended by more than 100 scientists from all over the world. Results of the second generation of experiments at CERN's low-energy antiproton ring presented the major part of the contributions. Among the highlights were the CPLEAR results on CP and T violation, indications for a glueball from Crystal Barrel and the improved comparison of antiproton and proton masses by detecting radiofrequency signals from the cyclotron motion of a single antiproton in a Penning trap of the PS-196 collaboration.
Figure 1: Silicon detector module for the Very Forward Tracker at DELPHI. Readout electronics (VLSI chips) is bonded to a fan-in, produced by photolithography. Each module has 256 readout channels.
Figure 2: Cerenkov rings accumulated in the prototype photon detectors, a TMAE-methane chamber (above) and a MWPC with a CsI photocathode (below).
Figure 3: Slices through the 3D distribution of F-18 activity in the test animal (guinea pig) measured with a scintillation PET apparatus. Four consecutive slices displayed in the left frame show the time evlution of the concentration of activity in the bladder of the animal.