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 Quantum Field Theory, Supersymmetry, String Theory, Quantum Chromodynamics, Noncommutative Geometry, Neutrino Physics, Astrophysics and Cosmology.

In particular, supersymmetric gauge theories with matter fields (SQCD) and without them (SYM), and with or without extended supersymmetry have been investigated in the group. In particular, parts of the low energy effective action of N=2 SYM theory based on the conventional effective field theory method have been explicitly derived by the group in order to ascertain that the Seiberg-Witten duality is not broken by unforeseen effects. This work has presented a quantitative clarification for Seiberg’s non-perturbative arguments.

Recently the nonperturbative aspects of supersymmetric gauge theories, confinement, duality and dynamical supersymmetry breaking, have been clarified by Seiberg and Witten. The real world, however, is not supersymmetric and thus it is important to extend these analyses to non-supersymmetric theories, e.g. to QCD. To this end, the group has extensively investigated supersymmetric QCD with generic soft supersymmetry breaking terms and have revealed the fate of non-perturbative aspects of supersymmetric QCD after supersymmetry breaking. It has clarified the vacuum structure for different flavours of quarks, which includes chiral symmetry breaking and non-breaking phases. Furthermore, strong suggestions have been obtained by the group that Seiberg’s duality may be valid even after supersymmetry breaking.

Quantum groups with inherent noncommutative geometry may provide us with new approaches to a fundamental theory having no divergences and comprising all known interactions. It is a viable alternative to the string and brane models. Promising results on this, as well as on topologically massive Yang-Mills theories and their two-loop finiteness have been obtained in the group. Further, quantum field theories on noncommutative space-times have been intensively investigated by the group and a precise formulation, compatible with the general axioms of QFT, is given. Contrary to the common belief that noncommutativity of space-time would be a key to remove the ultraviolet divergences, the group has shown that ultraviolet divergences persist for field theories on a noncommutative plane, while on a noncommutative cylinder they are absent. Thus, the ultraviolet behaviour of quantum field theory on noncommutative spaces is sensitive to the topology of space-time, namely to its compactness.  General arguments have been presented for the case of higher space-time dimensions. This result brings us to a strong suggestion that unless the ordinary QFT is combined with another interaction (gravity), which would necessarily change the basic structure of the space-time to a compact one (such as to an anti-de Sitter space), the noncommutativity alone does not help in removing the ultraviolet divergences.

The theoretical high energy physics group maintains close research and scientific contacts with the Helsinki Institute of Physics (HIP), and 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.

Masud Chaichian
 
 

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 1998 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 experiment for the future Large Hadron Collider at CERN. All these experiments are frontline research projects realized via extensive international collaboration and in close collaboration with the Helsinki Institute of Physics.

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 may 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 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.

Participation in the DELPHI experiment at LEP at CERN was actively continued. The Finnish group in DELPHI has been contributing to several analyses, in particular to studies of 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.

Until now some 4 x 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. Until 1998 no completely unambiguous candidates for strangelets had been found, which in the mass range of 5-50 GeV/c2 sets an upper limit of 10-7-10-9 (model dependent) for their production probability. Results on the production of strangelets, antiprotons, antideuterons and anti 3He-nuclei have been published. In 1998, data taking of the experiment was completed. The statistics of the data sample was doubled and the analysis of this data is now in progress.

Simulation and design of the CMS was continued in 1998. 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 ->  t t ->  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 contributing to the B-physics simulation 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 1998 concentrated on improving the understanding of the B-physics capabilities of ATLAS by increasingly realistic simulations of the detector response.

Jorma Tuominiemi