The research activity of the Theoretical Physics Division comprises of a number of fields. The main thrust is in Particle Cosmology, Phenomenological Particle Physics, Physics of Hadrons and in Atomic, Molecular and Optical Physics. The activity in these fields is described below. There is also research in Biophysics (Raimo Keskinen), Mathematical Physics (Christofer Cronström, Claus Montonen), Astrophysics (Pentti Pulkkinen) and Many Body Physics (Jouko Arponen, Juha Honkonen).
There is considerable interaction with the Research Institutes for Theoretical Physics (TFT) and High Energy Physics (SEFT), now known as the Helsinki Institute of Physics (HIP), and close contacts are maintained with the Department of Geophysics of the Finnis h Meteorological Institute, as well as with theoretical physics groups at CERN, NORDITA and several other institutions and universities in Finland and abroad.
The personnel of the Division had a significant role in organizing "The XVII International Conference on Neutrino Physics and Astrophysics" with more than 300 participants in Helsinki on 13-19 June.
The main source of the outside funding of the research carried on at the Division has been the Academy of Finland, whose support is gratefully acknowledged.
Jarkko Ahonen, Kari Enqvist, Keijo Kajantie*, Mika Karjalainen, Petteri Keränen, Hannu Kurki-Suonio, Mikko Laine**, Jukka Maalampi, John McDonald+, Janne Peisa**, Arttu Rajantie, Kari Rummukainen ***, Hannes Uibo ++
During the year 1996 an important discovery has been made by the group. The electroweak phase transition, which takes place at temperatures of the order of 100 GeV, has been studied in an effective 3-d field theory approach both by analytic and numerical methods. This is a very topical problem in particle cosmology. It has been found that for large Higgs masses there actually is no first order transition, but only a continuous cross-over, in the Minimal Standard Model. This has important implications for particle cosmology since it implies that for the generation of the observed matter-antimatter asymmetry one needs some completely new physics. The minimal supersymmetric extension of the Standard Model has been studied in this co ntext and it has been shown that it forms a possible alternative.
A further non-equilibrium issue investigated is the dynamical evolution of the phase transition bubbles, their appearance by nucleation and the subsequent collisions of the shock fronts. The role of primordial magnetic fields has been studied both in the context of neutrino and axion physics and magnetohydrodynamics. We have calculated the electrical conductivity in the early universe. The growth of primordial fields has been simulated by a generalized shell model, which reveals a cascade of magnetic energy from small to large length scales. Big Bang nucleosynthesis, and its constraints on neutrino properties and abundances of Dirac neutrinos, is also one of the main research topics.
The group maintains close contacts with theoretical physics groups at CERN (Geneva), Niels Bohr Institute (Copenhagen), Universities of Bielefeld, Heidelberg, Bloomington (Indiana, USA), Liverpool, Newcastle, Stockholm and Turku. The members of the group belong in various networks, such as the EU Theoretical Astroparticle Network and the Nordic Network "The Standard Model in Extreme Environments". The group serves as the host for the EU Training and Mobility programme "A Critical Investigation of Electroweak Baryogenesis Models"
* on leave of absence at CERN, Geneva, Switzerland
** Inst. fûr Theor. Physik, Heidelberg, Germany
+ from October 1996
** Theor. Phys., Dept. of Mathematical Sciences, Univ. Liverpool
*** Univ. Bielefeld, Fakultät fûr Physik, Bielefeld, Germany
++ until September 1996
Jukka Maalampi, Martti Raidal*
The research in phenomenological particle physics has concentrated on extended gauge models, in particular on the so-called left-right symmetric electroweak gauge model (LRM). The LRM gives answers to some open questions of the Standard Model of electroweak interactions, such as the origin of parity violation and the lightness of neutrinos. It is an effective low-energy theory of the SO(10) grand unified model, whose ability to shed light to the fermion mass hierarchy problem has been much studied recently.
We have investigated the possible probes of the LRM in high-energy electron-positron, electron-electron, electron-photo n and photon-photon collision experiments by searching signatures of the new interactions and new particles present in the theory. We have studied the production of the doubly charged dilepton scalar, a novel prediction of the model, via WW fusion in proton-proton collisions at the Large Hadron Collider, to be built at CERN, and the violation of lepton number, another non-standard prediction of the LRM, at the planned linear collider. We have also analyzed so-called R-parity breaking effects in the supersymmetric version of the LRM.
The work has been done in a close collaboration with the phenomenology group at SEFT (HIP) (Katri Huitu, Kai Puolamäki and Raimo Vuopionperä) and with University of Turku (Aarre Pietilä, Jukka Sirkka). The group maintains also close contacts with Universi ty of Bergen, University of Valencia, PSI (Villigen) and University of Silesia, Katovice. The group belongs to the Nordic network "Fundamental Constituents of Matter", and it has actively participated and contributed to the "Joint ECFA / DESY Study: Physics and Detectors for a Linear Collider" related to the plan of building a linear collider at DESY, Hamburg.
* now in Valencia
Jouni Niskanen, Mikko Sainio, Petrus Pennanen, Mikael Vestama
Hadron physics at low and intermediate energies is QCD in the nonperturbative region, a very tough problem for exact theoretical approaches. One possibility to simplify the situation is to use the symmetries of QCD in effective field theories, such as chiral perturbation theory (ChPT). In 1996 the research activities in this field include chiral perturbation calculations of the pion-pion interaction to two-loop order. Other work includes development of technical tools for two-loop integrals more generally in ChPT. Hadron level calculations include also pion production in two-nucleon collisions, eta-meson interaction with the few-nucleon system, charge symmetry and charge independence breaking in the NN interaction and pion production, and photoreactions in few-nucleon systems. Further, at a more microscopic level the few-quark problem has been investigated using lattice gauge techniques.
The two-loop amplitude of the elastic pion-pion scattering has been evaluated analytically in chiral perturbation theory . Such a precision in a theoretical prediction has not been achieved in low-energy hadron physics before. With the forthcoming pion-pion data one may eventually obtain experimental information on the size of the quark-antiquark condensate in QCD in this manner.
In intermediate energy neutron scattering experiments the beam intensity is commonly normalized by the pion production reaction np -> d pi0, which is related to the well known pp -> d pi+ by assuming charge independence. Our result  indicates that in high precision experiments one should apply an energy dependent correction to this assumption. This can have important consequences for recent controversies about the pion-nucleon coupling constant, where np scattering data are part of the experimental input.
Close contacts with theoretical and experimental groups at the universities of Bern, British Columbia (Vancouver), Indiana (Bloomington), Mainz, Vienna and Warsaw as well as the major laboratories at LNF (Frascati), IUCF (Bloomington), Jülich, Nordita (Copenhagen), PSI (Villigen) and TRIUMF (Vancouver) have been essential for the work of the Hadron Physics group.
1. J. Bijnens, G. Colangelo, G. Ecker, J. Gasser and M.E. Sainio, Phys.
Lett. B374 (1996) 210-216
2. J.A. Niskanen and M. Vestama, preprint HU-TFT-96-38, accepted to Physics Letters B
Kalle-Antti Suominen, Asta Paloviita*
The Theoretical Atomic, Molecular and Optical Physics group has continued its previous research on three fields. The methods for treating cold atomic collisions have been developed and used successfully in describing some recent experimental data; the recent work was summarised in a review article for Journal of Physics B. The high light of the cold collision work was a study of light interactions with ultracold gases, i.e., Bose-Einstein condensates; together with our collaborators abroad we have shown that with suitable arrangements light can be used to probe condensates without considerable loss of atoms, and even to study the scattering length, which describes completely the low temperature encounters in alkali gases. Furthermore, we have studied the molecular excitation processes induced by ultrashort pulses, and examined the r ole of decoherence in quantum computers, especially in registers and during quantum Fourier transforms.
The group collaborates closely with several research groups: the quantum optics group at the Helsinki Institute of Physics, the laser physics group at Imperial College, London, the laser cooling and quantum information groups at Clarendon Laboratory, University of Oxford, the laser cooling and molecular theory groups at Atomic Physics Division at National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA and atomic and molecular physics group at Departments of Chemistry and Physics, Ben-Gurion University, Beer Sheva, Israel. The main funding source for the group has been the Academy of Finland.
* NOKIA R&D, Tampere
The Graduate School in Solar-Terrestrial Physics which started in 1995 as co-operation between University of Oulu, University of Helsinki, University of Turku, Finnish Meteorological Insitute, and Sodankylä Geophysical Observatory, gives higher education in space physics. The program of the graduate school is focused on space plasma physics, especially in ionospheric and magnetospheric research. In Helsinki the practical training is given by the Geophysical Research Division of the Finnish Meteorological Institute where two graduate students of the Department of Physics of the Unversity of Helsinki are supported through this program. During the spring term a lecture series in space plasma physics was given at the University of Helsinki. In March 1996 the school had a joint seminar in The Finnish Meteorological Institute and in June a two-week course in data analysis methods was held at the Sodankylä Geophysical Observatory.
*Finnish Meteorological Institute, Geophysical Research