Theoretical High Energy Physics
M a s u d C h a i c h i a n

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.

Experimental High Energy Physics
J o r m a T u o m i n i e m i

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 Large Electron Positron Collider, LEP, and of contributions to the design and simulation of the Compact Muon Solenoid detector, CMS, and ATLAS experiments for the future Large Hadron Collider at CERN. All these experiments are frontline research projects realized via extensive international collaboration.

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 providing information on the complex collision process.

Until now some 1012 lead ions with an energy of 158 GeV/nucleon have been shot onto 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 10­7-10­9 (model dependent) for their production probability. Results on the production of strangelets, antiprotons, antideuterons and anti 3He-nuclei have been published. The experiment will continue taking new data in the autumn 1998.

Simulation and design of the CMS was continued in 1997. The High Energy Physics Division has contributed to the simulation and assessment 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 Large Hadron Collider, LHC, was studied in the decay channel h, A0, H -> tau tau -> e µ. The study is designed for the low luminosity running of the LHC with no pile-up effect included. Backgrounds from other processes are suppressed by selecting isolated high transverse momentum tau leptons with zero total charge. The Higgs mass is reconstructed from the momenta of the leptons and the overall missing transverse momentum. Results have been published in a CMS report.

The ATLAS experiment is the other large general-purpose experiment planned to take data at the LHC at CERN. The High Energy Physics Division has been coordinating 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.

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

The High Energy Physics Division, in collaboration with the Helsinki Institute of Physics, HIP, is actively involved in the DELPHI experiment at 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.