Annual Report 1995

As throughout the world, the activities of the Experimental Particle Physics Department are tied to experiments at large international centres for particle physics. Scientists of the Department pursue their research as members of four collabrations CPLEAR and DELPHI at the European Laboratory for Particle Physics (CERN) in Geneva and ARGUS and HERA-B at Germany's centre DESY in Hamburg.

Head: As. Prof. Dr. Marko Mikuz

For the CPLEAR collabration, 1995 meant the beginning of the harvest after more than five years of collecting data on neutral kaon decays. Four articles were published on precise measurements of CP violation parameters and tests of CPT. At the same time, the introduction of a new tracking chamber in the early stage of the trigger enabled us to double the amount of useful data in the last year of data-collecting. Complementary to benefits in event reconstruction, the improved apparatus is also less sensitive to systematic errors in the trigger domain. The Slovenian role in the project, in addition to general duties in data collection and off-line data production, mostly involved analysis of three pion and semileptonic decays. Members of the Department continued collaboration at the DELPHI spectrometer operating at the Large Electron Positron Collider at CERN. They took part in data collection as well as in computer analysis of recorded data. Measurements were conducted from March to November. In this period 4 x 105 hadronic decays of Z 0 vector bosons were recorded on tapes.

The analysis followed the line of previous work. The main area of activity was hadronic systems containing a b quark. Measurements include the determination of B 0s meson lifetime and of the partial decay width of the Z 0 boson into a pair of b quarks. In the latter measurement, the researchers took advantage of the artificial neural network technique.

Figure 1: Time dependence of two-pion decays of K0 mesons and their anti-particles as measured by the CPLEAR spectrometer. The difference of decay curves of a particle and anti-particle represents a manifest sign of CP violation.

One of the promising decay channels for the evaluation of B0s meson lifetime is the one giving rise to a F meson and a lepton in the final state. This channel has not yet been used in a similar analysis. Around 50% of the reconstructed events in such a sample come from semileptonic decays of the B0s meson. Accurate identification of final state particles in this decay channel is essential, which can be achieved with the Ring Imaging Cherenkov detector of the DELPHI spectrometer. Members of our Department are continuously involved in the calibration of this detector, enabling a clean separation between charged kaons and pions in the reactions under study.

In the second half of 1995, an energy increase in collisions in LEP to 140 GeV opened up a new frontier of research in high energy physics. Given the good quality of data recorded by DELPHI and the other three experiments at LEP during this period, one can anticipate confirmation or disproof of certain theoretical models (Supersymmetric Minimal Standard Model) in a short time.

Construction of the mini-strip silicon detector for an upgrade of the forward region of the DELPHI spectrometer has been continued. 90% of the required double-sided modules were tested and mounted on the support structure by the end of the year. During tests, the modules showed stable behaviour and high efficiency (up to 99%) essential for the reconstruction of charged-particle tracks in the forward region - particularly important for the physics at the increased energies of the LEP 200 collider.

Figure 2: Reconstructed tracks in the DELPHI spectrometer resulting from an electron-positron collision at an energy of 130 GeV

The analysis of data, collected by the ARGUS detector in 1982 - 1992, is slowly coming to an end. In 1995 we developed a new improved method for the analysis of t-lepton decays. Using that method, it was possible to diminish errors on Michel parameters, used to describe the decays of the t lepton. The measured values agree well with the predictions of the Standard Model, the present theory of fundamental particles and their interactions. Neutrino-less t decays to Goldstone axions a, which are beyond the framework of the Standard model, were also sought, but none were found. On the other hand, two clean candidates for rare B-meson decays were seen. In the Standard Model, such decays are described by the so-called penguin graphs. Results of the analysis of two-photon interactions, performed by the Slovenian members of the ARGUS collaboration are described in two diploma and two doctoral theses; the publications are in print. These results are important in the scope of light meson spectroscopy and for the theory of strong interactions in its non-perturbative regime.

In 1995 we started, together with physicists from eight countries, construction of HERA-B, an experiment to measure CP symmetry violation in the neutral B meson system. B mesons will be among the reaction products of high energy protons (p = 800 GeV/c) 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 recognise a well-defined final state B to J/Psi, K0 s to l+ l- p+ p-. By the end of 1995, we assembled the following components in the experimental area: the magnet of the spectrometer (Fig. 3) together with the compensation coil for the electron beam of the HERA collider, muon absorber and the vacuum-tight housing of the silicon micro-strip vertex detector. Part of the tracking system is already under construction, and will be used in test measurements starting in the summer of 1996. The aim of the first round of measurements is to test overall system performance, and to study the background. The whole spectrometer is expected to start taking measurements by spring 1998.

Research and development of one of the HERA-B spectrometer's components, the RICH (Ring Imaging Cherenkov counter), was continued. Measurements were continued with the prototype constructed and first tested in 1994. The scope of tests were ageing and high rate performance of photon detectors in conditions expected in the hostile environment of the HERA-B experiment. The ageing studies were also pursued in the home laboratory, with particular emphasis on performance of the CsI based photon detector. Measurements have shown that the original version of the MWPC becomes unstable at high rates. A modification, involving an additional plane of cathode wires, was shown to eliminate this problem. To concentrate the efforts connected to the development of a CsI based photon counter, we have joined the RD-26 collaboration, an international research and development group based at CERN.

In connection with the development of an apparatus for positron emission tomography, in the year 1995, a system for computer controlled detector movement was manufactured. Test measurements with point sources have shown a position resolution of 2 cm FWHM. Measurements of a standard phantom allow a comparison with the SPECT system, which is used at the Clinic for Nuclear Medicine at the University Clinical Centre. Two multiwire proportional detectors have also been constructed, which should lead to improved position resolution.

Figure 3: HERA-B magnet in the experimental hall

In 1995, a decision was taken which is bound to determine the path of the Department for the next 15 years. At the LHC, two big experiments, ATLAS and CMS, were approved by CERN. As our intention was to participate already in the R & D phase which is to be finished by end of 1997, intensive talks with representatives of both collaborations were held in 1995. The decision reached by the end of the year was to join the ATLAS collaboration. In the initial phase, involvement in the development of position-sensitive semiconductor detectors is foreseen, a technology which, in addition to representing a breakthrough in high-energy physics, has promising applications in such fields as medical diagnostics.

In July 1995 the ICFA'95 Instrumentation School was organised by the Department. In a fortnight, more than 130 participants from 29 countries across four continents benefited from lectures from 28 renowned experts in the field. In the afternoons, they had the opportunity to try out their newly acquired knowledge in seven laboratory exercises, set up especially for the school.

Figure 4: Participants of the ICFA'95 school.