3. HIGH ENERGY PHYSICS DIVISION ________________________________________________________________________________________________

3.1. THEORETICAL HIGH ENERGY PHYSICS

The research activity of the Theoretical Group of the High Energy Physics Division covers several topics of current interest in theoretical physics and in the theory of elementary particle physics. These topics include Anyons and Fractional Statistics, Cosmology, Quantum Field Theory, Quantum Chromodynamics, Quantum Groups, Meson Spectroscopy, Supersymmetry and Weak Interactions, Ultra-Relativistic Heavy Ion Collisions, Quark-Gluon Plasma, Neutrino Physics and Astrophysics.

The group maintains close research and scientific contacts with several theoretical high energy groups in Europe and in other Nordic countries, as well as with several research centres in the USA, Japan and with CERN.

Peer reviewed articles and other publications published by this group are listed in the Appendix of the Annual Report and conference contributions here .

Masud Chaichian, Kimmo Kainulainen and Juha Peltoniemi

3.2. EXPERIMENTAL HIGH ENERGY PHYSICS

The experimental research of the High Energy Physics Division is based on experiments done with the big particle accelerators at CERN, the European Laboratory for Particle Physics. In 1997 the experimental program consisted of the NA52 experiment at the CERN SPS accelerator, the DELPHI experiment at the LEP collider and of contributions to the design and simulation of the CMS and ATLAS experiments for the future Large Hadron Collider at CERN. All these experiments are frontline research projects realized via extensive international collaboration.

Jorma Tuominiemi

NA52 EXPERIMENT

Jorma Tuominiemi* and Tomas Lindén

The NA52 experiment searches for a new form of strange matter, the strangelet particles. This form of matter could have been formed in the early universe and in neutron stars and it is a possible candidate for dark matter in the universe. It could also be produced in collisions of heavy ions of high enough energy. In NA52 a fully ionized lead ion beam at the CERN SPS accelerator is shot at different lead targets. The energy density and strangeness concentration in these collisions is such that strangelets could be formed, if they exist. Their production would provide a signal for the creation of a quark gluon plasma as well.

The second topic of the NA52 experiment is the study of antinuclei production. Also here the experiment has unique capabilities in the world. In the lead ion collisions a large number of particles are produced. Their properties give information on the complex collision process. Antinuclei production is particularly interesting from this point of view.

The NA52 experiment is a collaborative effort with the University of Bern and the Research Institutes of Annecy and Strasbourg. The High Energy Physics Division is mainly responsible for the development of the on-line and off-line software for the analysis of the NA52 experiment.

Until now some 1012 lead ions with an energy of 158 GeV/nucleon have been shot on targets to produce new particles. The momentum, energy and time of flight of these particles were recorded to determine their mass. No completely unambiguous candidates for strangelets have yet been found, which in the mass range of 5-50 GeV/c2 sets an upper limit of 10­7­10­9 (model dependent) for their production probability. Results on the production of antiprotons, antideuterons and anti 3He-nuclei were published in 1996. The analysis of baryon and antibaryon as well as charged K meson production from the 1994 and 1995 data was completed and submitted for publication.

The NA52 detector was upgraded in 1996. Particularly, a new quartz fiber calorimeter was installed in the beam line immediately after the target station. This calorimeter will bring information on the centrality of the collision events as well as on the energy flow. The new equipment was tested in a proton run in September 1996 and parasitically in the ion beams in October and November 1996.

Analysis of pion production and nuclear fragments data from the runs in 1994, 1995 and 1996 were continued. The main task of NA52 in 1997 was to prepare and carry out the planned ion run in October-November. Due to a most unfortunate fire in one of the SPS RF power supplies on the 13th of May the SPS and LEP accelerators were stopped for more than two months. As a consequence of this the complete lead ion run for 1997 was cancelled. The NA52 members now eagerly wait for the SPS lead ion run scheduled for the fall of 1998.

CMS

Jorma Tuominiemi* and Sami Lehti

Simulation and design of the Compact Muon Solenoid detector (CMS) was continued in 1997. The High Energy Physics Division has contributed to the simulation and assesment of the physics discovery potential of the CMS design, particularly in the search of the Higgs bosons and supersymmetric scalar top quarks.

Search for the neutral MSSM Higgs bosons h, A0, H with the proposed CMS-detector at the LHC is studied in the decay channel h, A0, H -> tau t au -> eµ. The study is made for the low luminosity running of the LHC with no pile-up effect included. This decay channel is quite difficult because of the large Z, gamma -> tau tau background. Backgrounds are supperessed by selecting isolated high pt leptons with zero total charge. The Higgs mass is reconstructed from the momenta of the leptons and the overall missing transverse momentum.

* now at Helsinki Institute of Physics

ATLAS

Paula Eerola

The ATLAS experiment is a large general-purpose experiment planned to study proton-proton collisions at the Large Hadron Collider, LHC, at CERN. The ATLAS experiment is being constructed by 1700 collaborators in 144 institutes in 33 countries around the world. The experiment will start operation in 2005.

The High Energy Physics Division has had a coordinating role in the B-physics working group of ATLAS. B-physics at LHC is aimed at unravelling the origin of CP-violation, violation of space reflection and particle-antiparticle symmetries, through rare decays involving B-mesons. CP-violation in B-decays is an intensive field of research - besides the LHC, which in terms of B hadron production rates is the ultimate B-factory, dedicated B-meson production facilities are being constructed at SLAC and at KEK.

Activities in 1997 have concentrated in improving the understanding of the B-physics capabilities of ATLAS by more and more realistic simulations of the detector response. In 1997, ATLAS submitted several Technical Design Reports, and in particular in the Inner Detector TDR, simulations of some selected B-meson final states were presented as the most complex reconstruction tasks the detector can accomplish.

DELPHI

Paula Eerola and Kenneth Österberg

The High Energy Physics Division, in collaboration with the Helsinki Institute of Physics, HIP, is actively involved in the DELPHI experiment at Large Electron Positron Collider, LEP, at CERN. The Finnish group in DELPHI has been contributing to several analyses, in particular rare B-decays and decays of tau-leptons. The Finnish group has initiated a new method of reconstructing jets, which potentially could disentangle quarks and gluons and thus open up a completely new view on partons.