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.