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.