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, Noncommutative Geometry, Supersymmetry, Gauge and String Theories, BRS-Symmetry and Confinement, and Neutrino Physics.
Quantum field theory on noncommutative space-time has been intensively investigated by the group and a precise formulation, compatible with the general axioms of QFT, have been given. Clearcut results based on this approach have been obtained. 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 (and become even more severe for some types of commutation relations for the coordinates), while the theory on a noncommutative cylinder is ultraviolet-finite. Thus, ultraviolet behaviour of a field theory on noncommutative spaces is sensitive to the topology of the 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 space-time to a compact one (such as to a de Sitter space), noncommutativity alone does not help in removing the ultraviolet divergences.
Recently, it has been shown by N. Seiberg and E. Witten that the noncommutative geometry naturally appears in string theory. Thus, the question about interrelation of these two fundamental approaches to the description of small-scale physics, becomes actual. As is well known, one of the principal objects of the string theory is the Virasoro algebra. The group has shown that there is a direct link between the model on a noncommutative cylinder and the deformed Virasoro algebra suggested earlier by members of the group, which describes the symmetry of the noncommutative field theory. This indicates that there are various relations between noncommutative geometry, string theory and/or various deformed structures appearing in physics.
Another area of research activity of the group is Supersymmetric Gauge Theories. Such theories with the 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.
In several works of the members of the group a specific formulation for colour confinement has been presented in which the phenomenon can be attributed to an act of the massless quanta of the non-Abelian colour gauge fields. The proof has been given based on the BRS-symmetry of the theory and its asymptotic freedom.
In works carried out by the group, it has been argued that the familiar gauge hierarchy between the fundamental Planck scale and the electroweak scale, can be naturally explained in higher-dimensional theories with extra dimensions of relatively large radii, much bigger than the Planck length. In particular, it has been shown that it is possible that the electroweak Higgs mass at high energies is of the order of Planck mass, but radiative corrections drive it to an infrared stable fixed-point at the electroweak scale at low energies, thus inducing a large hierarchy without any fine tuning of the parameters.
The Theoretical High Energy Physics Group maintains close research and scientific contacts with the Helsinki Institute of Physics (HIP), with several theoretical high energy groups in Europe and in other Nordic countries, as well as with several research centres in U.S.A., Japan and with CERN.
3.2. EXPERIMENTAL HIGH ENERGY PHYSICS
The experimental high energy physics activity concentrates on the analysis of the Large Electron Positron Collider (LEP) data and on preparing for the experiments at the Large Hadron Collider (LHC) at CERN. With its extensive experience in hands-on development and construction of detector systems at LEP, the group of the Physics Department is well prepared for the next step: Design of a sub-detector for forward physics at LHC.
PHYSICS ANALYSIS AT LEP
Contributions of the Finnish group in the LEP-DELPHI experiment at CERN are based on the systematic ground work in the detector technologies which are considered most relevant for the physics discovery potential. These include both semiconductor and gas amplification based precision detectors. In 1999 the DELPHI group has contributed to a number of physics studies of the CERN LEP-program. As a result, several group members now coordinate important areas of physics and data analysis of the large international collaboration.
The Finnish group has introduced a new way of analysing hard scattering final states. The method enables one to fully match the QCD view of the strong interactions with the experimental analysis. With the new analysis approach the colour dipoles between the final state partons can be utilized to study both the subtle QCD effects and to carry out more efficient searches for the new particles such as the Higgs boson.
The group has distinguished itself in developing new analysis methods for flavour tagging and reconstructing colour flow structures of multiparton final states. After a detailed study of the bbg final state, preliminary results of a QCD analysis of heavy quark fragmentation have been obtained and reported in the HEP-99 Conference in Tampere. A study of the Vub element of the CKM matrix has been finalized and represents the worlds most accurate measurement of this quantity. The analysis technique pioneered by the group is going to be used by the newly started experiments at the B-factories. The group has also applied its novel event reconstruction method in Higgs searches and a new mass limit of 75 GeV/c2 has been established for the charged Higgs boson.
With LEP-2 entering its last year of operation in 2000, the group is looking forward to the future pp- and e+e- experiments at LHC and at the linear collider. The studies which are being carried out by the group, directly benefit from the expertise gained in detector design and operation and in the data and physics analysis at LEP.
DETECTOR DEVELOPMENT FOR LHC
In August 1999 the group was invited to coordinate a feasibility study on the possibility of expanding the acceptance of an LHC experiment to the very forward region. This coverage is becoming highly important due to the recent new physics observations at DESY, Germany, and at Tevatron in the U.S. A novel technique, fully integrated with the beam lattice, has been introduced by the group and will be an essential part of the proposal prepared for the LHC community in the year 2000.
A detector upgrade of an LHC experiment is proposed in order to enable studies of forward physics at the LHC along with luminosity and total cross section measurements. The new detector system consists of several inelastic detector stations inside the experimental cavern and a number of Micro Stations in more distant locations in the machine tunnel to provide measurement of scattered protons. Sensors of two different technologies will be utilized in the forward detector: silicon and gaseous (GEM-based) pixel detectors. As these new detectors have to meet the severe performance requirements of the LHC, a dedicated R&D project is carried out.
The detector R&D activities of the group were supported by the Academy of Finland grant and recognized by the European Commission, which invited the group leader (R. Orava) to chair a EC Concerted Action on novel detector techniques in X-ray imaging.
In addition to the above Finnish contributions to some of the leading experiments in high energy physics, the Department continued, in 1999, its previous engagement in the NA52 and CMS experiments at CERN (Docent J. Tuominiemi, T. Lindén). In NA52, searching for a form of strange matter ("strangelets"), no believable candidate events were found. However, antiprotons, antideuterons and another anti 3He nucleus have been observed in the experiments. The Department is involved in the CMS experiment through several members of its personnel who are, at present, on leave of absence from the Department and are employed by the Helsinki Institute of Physics (HIP). This participation ranges from the fast simulation of Higgs production (J. Tuominiemi, R. Kinnunen, S. Lehti) to the optical links of the CMS muon detectors (E. Pietarinen).