3. HIGH ENERGY PHYSICS DIVISION
 

3.1. THEORETICAL HIGH ENERGY PHYSICS

Masud Chaichian, Kari J. Eskola*, Paul Hoyer* , Kimmo Kainulainen* and Matts Roos

The research activity of the Theoretical Group of 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 and Quark-Gluon Plasma.


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 USA, Japan and with CERN.

* Part of the time

Peer reviewed articles and other publications by this group are listed here, and conference contributions among the conference contributions of the Physics Department.


3.2. EXPERIMENTAL HIGH ENERGY PHYSICS

Jorma Tuominiemi, Tomas Lindén and Sami Lehti

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 1996 the experimental program consisted of the NA52 experiment at the CERN SPS accelerator and on the design and simulation of the CMS experiment for the future Large Hadron Collider at CERN.

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. The analysis of these data was continued in 1996. No clear 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 have been published.


The NA52 detector has been upgraded in 1996. Particularly, a new quartz fiber calorimeter has been added to 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 and parasitically in the ion beams in October and November.


Simulation and design of the Compact Muon Solenoid detector (CMS) was continued in 1996. 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 tau -> e mu. The study is made for the low luminosity running of the LHC with no pile-up effect included. This deccay channel is quite difficult because of the large Z, gamma -> tau tau background. Backgrounds are supperessed by selecting isolated high ptau leptons with zero total charge. The Higgs mass is reconstructed from the momenta of the leptons and the overall missing transverse momentum.