1993 Annual Report

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The energy scale, needed for research of basic building blocks of matter and forces acting between them, can be reached only at huge accelerators. Complicated detector systems using state-of-the-art technology record collisions of high energy particles. Resources, both human and financial, required by this field of science, exceed the capacities even of the worlds' most developed countries. Research through international collaboration in a few specialized laboratories around the world is therefore the only way today to pursue this activity. Building the Large Hadron Collider (LHC) at CERN will present the ultimate evolvement: the unification of resources of the entire world for a unique machine to test the frontiers of human knowledge.

Slovenian particle physicists have followed the path of their colleagues and already twenty years ago they have joined their first international project - the OMICRON collaboration at the European Laboratory for Particle Physics (CERN) in Geneva. The group, gathered around Prof. Gabrijel Kernel, grew gradually and in 1992 the Experimental Particle Physics Department was established.

Today scientists of the Experimental Particle Physics Department are members of three collaborations:

Last year the ARGUS spectrometer stopped data taking after ten years of fruitful operation. The ARGUS collaboration, a group of 86 physicists from Canada, Germany, Russia, Sweden, Slovenia and USA, is considered as one of the most successful in particle physics. In ten years more than hundred papers were published with the number of citations exceeding two thousand. In 1993, the main contribution of the Slovenian group was in the analysis of photon-photon reactions. These reactions provide a very useful tool since the probability to produce a meson in such a process depends on the charge of the quark and the antiquark as well as on the binding force. Since the results of our recent measurement of indicate a possible existence of a four-quark state, the study was focused on production of other vector meson pairs. By analyzing the partial wave composition in the reaction a spin-parity dominance was established. The cross-sections for and were measured. In addition, the partial wave analysis of was performed and the cross-section of the reaction was measured.

The CPLEAR collaboration upgraded their data-acquisition system which resulted in an improvement of data-recording speed up to 1.2 MBy/s. Further optimizations of the trigger were made. In eleven weeks beam time around 13000 cartridges containing more than 1 billion events were recorded. In the analysis of CP violation in the channel an improved accuracy on the CP violation parameters was obtained, but the error still exceeds that of the world's average. In the three-pion decay the upper limit on the CP violation parameter was improved by an order of magnitude and for the first time the CP-allowed decay was observed. The semileptonic decay analysis yielded new upper limits on the validity of time-reversal and CPT invariance as well as on the validity of the rule. Slovenian physicists were involved in the three pion decay analysis and particle identification using the electromagnetic calorimeter. They also started to look at a possible observation of CP violation in the decay channel.

In 1993, researchers from the Jozef Stefan Institute in the DELPHI collaboration were taking part in the very forward tracking upgrade as well as in the analysis of data taken by the spectrometer operating at the large electron positron collider at energies around the resonance. In 1995 an installation of a multilayered silicon strip tracking detector in the forward region of the spectrometer is foreseen. In a cooperation with the Ceramics Department and the Centre for Natural Sciences and Technology of the Jozef Stefan Institute our researchers have started production and testing of hybrids for detector readout and verifying assembled detectors using a laser setup. Data analysis was concentrated on B-meson decays produced in decays of the boson. Using reconstructed momenta of decay products (K and mesons) and particle identification featuring a Ring Imaging Cerenkov Detector our physicists were able to reconstruct and mesons and obtain their mass difference (Fig. 1). The performed analysis increased the number of reconstructed mesons which will enable a measurement of their mixing in the future. The reconstructed meson together with an identified lepton is a clear significance of the meson decay. At the moment such a measurement is the only way to determine the lifetime of and the resulting lifetime is among the first measurements in the world.

The laboratory for research and development of particle detectors is an essential part of the Experimental Particle Physics Department. On one hand, it is intended to support Slovenian participation in international particle physics projects and on the other hand it provides for application of high energy physics measuring methods to other fields of science and technology.

Development of a fast ring imaging Cerenkov detector (RICH) is in progress. This RICH detector is to be used as a part of a proposal to study B mesons at the HERA-B facility in Hamburg. Most of the studies to date have been concentrated on CsI photocathodes, their quantum efficiency and stability in a multiwire chamber environment. Cerenkov rings produced by cosmic particles have been measured with a test module and custom-made readout electronics (Fig. 3). A larger prototype detector is currently under construction and will be tested in the near future with test beams at DESY, Hamburg.

Multiwire chambers (MWC's), with their good position resolution and large surfaces, offer a possibility for improving the position resolution beyond that achieved with commercial Positron Emission Tomographs based on scintillators. A resolution of 3 mm FWHM has been obtained with a small prototype and the construction of a larger system, enabling 3-dimensional imaging of the human brain is in progress. With their experience and knowledge obtained in this research, our physicists are contributing to the repair and upgrade of a scintillator based, commercial PET apparatus at the University Clinical Centre.

In the field of environmental physics, a detector for measuring low activities has been developed and its performance tested. Using a multiwire chamber in coincidence with a silica-aerogel Cerenkov, levels as low as 1 Bq may be determined in a few hours of measurement. This is a considerable improvement over conventional methods that require laborious chemical separation of and subsequent counting of the total activity.

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Tue Mar 1 14:12:41 MET 1994