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), and Many Body Physics (Jouko Arponen, Juha Honkonen).
An important new element in the activity of the Division has been due to the chair in space physics started this year in co-operation with the General Division. This will further strengthen and widen our already previously active collaboration with the Department of Geophysics of the Finnish Meteorological Institute.
The collaboration with the Helsinki Institute of Physics (HIP) has been fruitful in both research and education. Close contacts are maintained also with theoretical physics groups at CERN, NORDITA and several other institutions and universities in Finland and abroad.
10.2. PARTICLE COSMOLOGY
Jarkko Ahonen, Kari Enqvist, K. Kajantie*, Mika Karjalainen, Hannu Kurki-Suonio**, Mikko Laine*, John McDonald, Janne Peisaý, Arttu Rajantie and Kari Rummukainen¶
The main interest of the particle cosmology group is phase transitions in the early universe. Both analytic and numerical methods have been used. In particular, the electroweak phase transition and associated baryogenesis has received much attention. This is a very topical problem, with an emerging consensus that baryogenesis requires new physics. An attractive possibility is the Minimal Supersymmetric Standard Model (MSSM). At high temperature it has mainly been studied by using the effective theory obtained through dimensional reduction. A comparison between the effective potential approach and dimensional reduction was performed to show that the differences are due to higher-order perturbative corrections. Both methods yield a similar range in the space of parametes in the MSSM where baryogenesis is possible. In addition, dimensional reduction has been applied to the QCD phase transition. Numerical simulations have revealed the existence of large non-pertubative corrections in finite temperature field theory, for instance in the case of Debye screening mass.
Furthermore, parity violation in dimensional reduction has also been clarified, and thermalization of supersymmetric particles has been studied. It was also discovered that metastable solitons, called Q-balls, exist in the MSSM and can play a significant role in baryogenesis. More generally, the high temperature behaviour of the grand unified model SU(5) has been investigated, serving as a pilot study for the phase structure of grand unified models.
The stability of the vacuum, both in the MSSM and in the electroweak theory, has also been investigated. Vacuum decay induced by cosmic rays was shown in realistic models to be less likely than spontaneous decay. Generation of magnetic fields by bubble collisions in a first order electroweak phase transition has been studied in the context of a simplified Abelian model, with some positive results. A study has also been made of the transport coefficients, including electrical conductivity (which is of interest for magnetic field generation), of the plasma of the early universe.
The group maintains close contacts with theoretical physics groups at CERN (Geneva), Nordita (Copenhagen), University of Heidelberg, Bloomington (Indiana, USA), University of Newcastle, University of Stockholm, McGill University, and elsewhere. The members of the group belong to the EU "Finite T phase transitions in particle physics" -network and to 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".
* CERN, Geneva, Switzerland
** Helsinki Institute of Physics
ý Theor. Phys., Dept. of Mathematical Sciences, Univ. Liverpool
¶ Nordita, Copenhagen
10.3. PHENOMENOLOGICAL PARTICLE PHYSICS
Petteri Keränen, Jukka Maalampi, Martti Raidal*, Katri Huitu**, Kai Puolamäki**, Raimo Vuopionperä**, Iiro Vilja*** and Aarre Pietilä***
The research in phenomenological particle physics, carried on in a close collaboration with the Helsinki Institute of Physics (HIP) and University of Turku, has concentrated on extended gauge models and supersymmetry. In supersymmetric models one usually introduces so-called R-parity, which distinguishes supersymmetric particles from the ordinary ones. In the Minimal Supersymmetric Model, there may exist several terms in the Lagrangian that are allowed by gauge symmetry and supersymmetry but which explicitly break the R-parity. We have derived constraints on certain explicit R-parity breaking couplings from µ-e conversions in nuclei and found them more stringent than bounds obtained in other processes. In supersymmetric left-right model the R-parity is automatically a symmetry of the Lagrangian as it is related to one of the generators (B-L) of the gauge transformations. However, according to our previous discovery, in a physically acceptable model the R-parity has to be spontaneously broken via non-vanishing VEVs of neutrino fields. As a result, ordinary charged leptons and neutrinos will mix with supersymmetric chargino and neutralino states, respectively, and lepton number L is violated. We have investigated phenomenological consequences of such mixings in high-energy lepton collisions by studying the single production of neutralinos and charginos.
We have investigated the possibility for a new test of exotic neutrino-neutrino forces via interaction of very energetic neutrinos from active galactic nuclei (AGN) with cosmic background neutrinos. It was found that in a certain parameter range one can indeed obtain more stringent constraints than achievable by the other methods previously suggested.
The group maintains close contacts with University of Bergen, University of Valencia and University of Silesia, Katowice. The group has belonged to the Nordic network Fundamental Constituents of Matter, and has participated in Joint ECFA / DESY Study: Physics and Detectors for a Linear Collider, whose final report Conceptual Design Report of a 500 GeV e+e Linear Collider with Integrated X-ray Laser Facility appeared this year.
** Helsinki Institute of Physics
*** University of Turku
10.4. PHYSICS OF HADRONS
Anthony Green, Jouni Niskanen, Petrus Pennanen*, Mikko Sainio and Mikael Vestama
Hadronic physics can be defined as quantum chromodynamics in the confinement range. A versatile set of tools to address these problems is employed in our group.
Lattice methods provide, in principle, the solution to QCD problems. However, high accuracy can be reached only in special cases. Of particular interest in our group is the four-quark system. A model has been developed which reproduces in a general geometry the energies of a static four-quark system calculated in lattice gauge theory. The ground state and excited state flux distributions around two and four quarks have been measured and related to the above model.
Another approach for the low-energy hadronic phenomena is to make systematic use of the symmetry properties of QCD, namely of the chiral symmetry. This method is called chiral perturbation theory (ChPT). The main field of interest has been the study of the two-pion interaction where one can hope to reach a few percent precision in two-loop calculations. This will, with the new generation data, lead to a more definite understanding of the QCD vacuum properties.
In the meson-nucleon sector work has continued both on pion-nucleon and on eta-nucleon studies. Standard phenomenological tools have been used to extract information of the pion-nucleon amplitude at the Cheng-Dashen point and on the eta-nucleon scattering length.
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 normally assumed charge independence. Our calculation indicates that in high precision experiments one should apply an energy dependent correction to this assumption. This can have important consequences also for recent controversies about the pion-nucleon coupling constant, where np scattering data are part of the experimental input. The quasifree pn -> pppi, reaction with polarized proton beam has been calculated for a collaboration with the TRIUMF team, which measured its cross cection and analyzing power for S-wave final state protons. This reaction is closely related to controversial puzzles in pp -> pp pi0, at threshold.
The group maintains close contact with a number of institutions. The main partners have been the Universities of Bern, Liverpool and Mainz and research institutes IUCF (Indiana), LNF (Frascati), PSI (Villigen, Switzerland), Soltan Institute for Nuclear Studies (Warsaw) and TRIUMF (Vancouver). A major step in the field in 1997 has been the approval of the EU/TMR Network EURODAPHNE where University of Helsinki together with the University of Lund form the Nordic node.
* Helsinki Institute of Physics
10.5. THEORETICAL ATOMIC, MOLECULAR AND OPTICAL PHYSICS
Kalle-Antti Suominen and Asta Paloviita*
The Atomic, Molecular and Optical Physics group has concentrated on studies of a) cold collisions between laser-cooled atoms, b) quantum control of molecular wave packets, and c) output coupling of magnetically trapped Bose-Einstein condensates.
Together with our international collaborators we have developed a new semiclassical approach to collisional loss of atoms from a magneto-optical trap.
We have studied the effects of the envelope area and the duration of ultrashort pulses on molecular excitation. Also, in collaboration with the laser physics project of the Helsinki Institute of Physics (HIP), we have studied the description of the molecular excitation as a decay process. This work was summarised in the thesis of A. Paloviita (May 1997). A new method (APLIP) for adiabatic transfer between electronic molecular states was also developed.
We have also studied the release of magnetically trapped Bose-Einstein condensates with oscillating magnetic radiofrequency fields (output coupling). This is the first step in developing atomic lasers, i.e., beams of coherent matter waves. An analytic description for the output coupling for any rf-pulse for fast interactions was also discovered. This result contains as a special case the model that the MIT group of Wolfgang Ketterle used to describe quantitatively the first atom laser experiment ever performed. We have also studied how a Bose-Einstein condensate reacts to oscillating magnetic fields which are not fast enough for output coupling, but instead shake the condensate by moving the centre of the magnetic trap. Our results show that this leads to spatial dichotomy of the trapped condensate.
The group took part in organising the Helsinki Minisymposium of Bose-Einstein Condensation (March 25-27, 1997) at the University of Helsinki (K.-A. Suominen, with S. Stenholm). A pleasant surprise was the 1997 Nobel prize in physics, which was awarded to the field of laser cooling. There are strong research traditions in Helsinki in the theory of laser cooling, including the recent work by our group. Important changes took place when S. Stenholm accepted a professorship in quantum optics at Kungliga Tekniska Högskolan, Stockholm, in August 1997, and K.-A. Suominen replaced him as the leader of the HIP laser physics project. This has united the laser physics activities at the Division and at HIP.
* NOKIA R&D, Tampere
10.6. SPACE PHYSICS
The space physics research group consists, in addition to Hannu Koskinen at the university, of a dozen scientists and graduate students working at the Geophysical Research Division of the Finnish Meteorological Institute (FMI/GEO). The research is conducted in space plasma physics and solar-terrestrial physics. The investigations include both observational research with own ground-based and space-borne instruments and theoretical studies applying modern modelling and simulation tools. The space physics research at FMI/GEO produced in 1997 some 30 refereed publications, of which the six involving personnel from the university are listed in the Annual Report.
SPACE PLASMA OBSERVATIONS AND THEIR ANALYSIS
The second half of the year was important for this research because a long-term goal to modernize the key observational elements to study the ionospheric electrodynamics, magnetometers, coherent scatter radar, and all-sky cameras was carried to a stage where all these elements were operational simultaneously. In magnetospheric physics the past investments in equipping two Russian Interball satellites with plasma instruments are carrying fruits with unique observational data and the data analysis has started. The first peer-reviewed paper on these results appeared in 1997  and another publication reporting on solar-wind plasma entry into the magnetosphere with previously unknown features was of submitted to Geophys. Res. Lett. . Also older data from previous spacecraft are actively investigated, e.g., the study by Mäkelä et al.  on the properties of plasma cavities above the auroral region based on measurements by the Swedish Freja satellite 5 years ago.
Other members of the FMI/GEO group are active on studying observations obtained by various other spacecraft by ESA, NASA and Japan. An especially important activity concentrates around the group's first ESA instrument onboard the SOHO spacecraft which has now been operation for two years producing new information on the large-scale structure of the solar-wind.
1. I. Sandahl et al., Ann. Geophysicae 15 (1997) 542-552
2. I. Sandahl et al., Geophys. Res. Lett. in press
3. J. Mäkelä et al., J. Geophys. Res., in press
Dr. P. Janhunen at FMI/GEO has developed a top-tier 3D MHD code for large-scale studies of magnetospheric plasma. This model was applied in an investigation of how electric current that is magnetic field-aligned above the ionosphere is closed in the magnetosphere . The problem of mapping ionospheric electric fields into the magnetosphere was studied by Toivanen et al. . H. Koskinen is also one of the core-group members of the International Space Science Institute study on source and loss processes of magnetospheric plasma. The goal of the project is to produce a comprehensive reference book on this wide topic to be published in 1999. The first stage of the project resulted in a number of review papers in Space Sci. Rev. .
4. P. Janhunen and H. Koskinen, Geophys. Res. Lett. 24 (1997) 1419-1422
5. Toivanen et al., J. Geophys. Res. in press
6. H. Koskinen, Space Sci. Rev. 80 (1997) 133-152
FMI/GEO is leading an ESA Contract to study plasma and energetic electron environment and effects on spacecraft with Hannu Koskinen as a study manager. This study is a part of world-wide enterprise to develop the research results of solar-terrestrial physics toward useful products to predict and avoid harmful consequences of the space environment. This activity is often called Space Weather and the FMI/GEO is one of the leading groups in Europe in this field.
The Department of Physics and FMI/GEO are involved in the Graduate School in Solar-Terrestrial Physics in co-operation with the Universities of Oulu and Turku. The new professorship in space physics strengthens this educational line. During the fall term two introductory courses in plasma physics and its space applica-ions were given to advanced undergraduate students. These are planned to be given regularly whereas the spring term courses will be of variable content and directed to more advanced students.