The research activity at the Theoretical Physics Division is firstly particle physics interpreted in a wide sense, containing particle cosmology, phenomenological particle physics, physics of hadrons and mathematical physics. Further, there is a research group in atomic, molecular and optical physics. A particularly significant recent addition is the research group in space physics, which extends beyond the theory division also to the general division. Also the work of the particle cosmology group is extending in this direction: a participation in the Planck satellite program aiming at the measurement of the cosmic background radiation inhomogeneities has been formalised and K. Enqvist has been appointed a co-investigator.
The division is small, effectively only seven positions have been filled. Thus it is clear that collaboration with other units is essential. This has been very close with HIP (the Helsinki Institute of Physics), CERN (Geneva), Nordita (Copenhagen) and several other institutions in Finland and abroad. The division is associated with two EU TMR networks.
One of the group's main projects has been a precise and quantitative study of electroweak matter, the matter which filled the universe at an early stage of its evolution. Within the standard model this has now been brought to a conclusion: the precise nature of the endpoint of the electroweak phase transition has been definitely establised. The transition belongs to the universality class of the three-dimensional Ising model and the value of the Higgs mass corresponding to the endpoint is about 72 GeV. As the mass of the physical Higgs particle is experimentally larger than this, there actually is no phase transition in the simplest standard electroweak model corresponding to physical reality: something different is required.
A popular extension of the Standard Model is supersymmetry. The spectrum of supersymmetric theories admits non-topological solitons called B-balls. They carry a large baryonic charge and can decay much after the electroweak phase transition, thus modifying the standard cosmological picture. Their formation from the so-called Affleck-Dine condensate, and their decay, has been studied, and it has been found that B-balls could produce also to the cold dark matter particles. During inflation, fluctuations of the Affleck-Dine condensate give rise to isocurvature perturbations that could be observable in the power spectrum of the microwave background as measured in the future satellite experiments, such as Planck Surveyor Mission. Theoretical Physics Division is involved in the Low Frequency Instrument Consortium of Planck.
A new direction in the study of the electroweak transition has been finding out how it behaves in an external magnetic field. Cosmological magnetic fields of a relevant magnitude have conceivably existed in the very early universe. As a first result the phase diagram in the magnetic field has been determined, and the inhomogeneities of the magnetic field in a "type I" phase haven been observed.
Another new direction is the study of topological structures, vortices, in field theories. These have so far not been observed but are nevertheless a very suggestive proposal and form theoretically an unexplored territory if full field theory dynamics is included.
Oscillations between active and sterile species can affect the abundances of light elements in a significant way. As a consequence of the recent observation of the neutrino mass, this issue has gained in topicality. It has been realized that a small leptonic asymmetry, which in the Standard Model is equal to the baryon asymmetry, can be enhanced by oscillations. The net effect is, however, very sensitive to the fluctuations in the leptonic asymmetry.
The problems studied are complicated and can often only be solved by numerical means. Thus the computing facilities, especially the Cray T3E, of the Center of Scientific Computing in Finland have been essential.
*** Univ. of Liverpool
+ Univ. of Sussex
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 investigated the production of single sneutrinos, supersymmetric partners of neutrinos, as a probe of these R-parity violating couplings.
We have investigated the production and decay of doubly charged bosons predicted by the left-right symmetric electroweak model. These particles are members of scalar triplets which play a central role in the model taking care of the breaking of the left-right symmetry and giving rise to the seesaw mechanism of neutrino masses. The work is a collaboration with the particle physics group in the University of Lund and aims at a simulation of the production and decay processes at high-energy colliders.
Another field of study has been neutrino astrophysics. Ultra high energy neutrinos supposed to be produced in active galactic nuclei (AGN) and gamma ray bursts (GRB) provide a new test bench for the properties of neutrinos. We have investigated the possibility to probe the magnetic moments of neutrinos by studying the effects the large magnetic fields in the vicinity of AGN and other sources of high-energy cosmic on the neutrino flux. We have also studied the possibility to test neutrino stability by using the AGN and GRB neutrinos. The high energy and long flight distance of these neutrinos allows one to extend such studies to a new region in parameter space.
The group maintains close contacts with DESY, the University of Turku
and the University of Lund, and it has participated in Joint ECFA / DESY Study: Physics and Detectors for a Linear Collider.
** Helsinki Institute of Physics
+ University of Turku
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.
The lattice work on four-quark systems has been continued and is being extended to include the more realistic case of two static and two light quarks in SU(3) and with dynamical fermions. The development of a nuclear physics inspired model to understand the lattice energies has also continued.
Another approach for 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 work in this field has focused on the development of techniques to calculate two-loop integrals needed in the pion-pion and pion pair production amplitudes.
In the meson-nucleon sector, work has continued on the extraction of the subthreshold expansion parameters from pion-nucleon data and their application in the Goldberger-Miyazawa-Oehme sum rule. A multi-channel K-matrix model has been developed for pion and photon induced pion- and eta-production.
Recently there has been considerable experimental interest in meson production to three-body final states as NN --> NNp and NN --> NNh. Research in these reactions continues. For example, it has been found that a popular approximation often employed to take into account final state interactions in theoretical interpretations can lead to physically incorrect results and conclusions. Interaction with an experimental group in TRIUMF has continued to be fruitful.
The group maintains close contacts with a number of institutions. The main partners have been the Universities of Bern, Liverpool, Lund and Mainz, and the research institutes IUCF (Indiana), LNF (Frascati), PSI (Villigen, Switzerland), Soltan Institute for Nuclear Research (Warsaw) and TRIUMF (Vancouver). With the EU/TMR network EURODAPHNE starting in April 1998 another extension of European collaboration has evolved.
* Nordita, Copenhagen
We have continued our well-established research on time-dependent quantum systems, cold atomic collisions, quantum information, and wave packet dynamics with Bose-Einstein condensates. Some of this work was performed in collaboration with theoreticians and experimentalists at the University of Copenhagen and at NIST, USA. During 1998 the AMO physics activities were officially transferred to the Helsinki Institute of Physics (HIP). In fact the AMO research has been conducted completely at HIP already since 1997. Further information can be found in the HIP 1998 Annual Report.
* Helsinki Institute of Physics
At the Division of Theoretical Physics Christofer Cronström and Claus Montonen (on leave, and at present at the Helsinki Institute of Physics) do quantum field theory related research, in particular research on non-Abelian gauge theory and on supersymmetric field and string theory.
The research of C. Cronström aims at developing a quantum theory for a Yang-Mills field coupled to a quark field in which the proper dynamical degrees of freedom have been identified. The ultimate purpose of this is to understand colour confinement in QCD, i.e. the confinement of quarks and gluons, which is one of the great unsolved problems in particle physics.
In recent publications of C. Cronström, a novel way of establishing a canonical Hamiltonian formulation of pure Yang-Mills theory has been presented, in which Gauss' law is identically satisfied in principle. This has necessitated the introduction of a new gauge condition, which is a natural generalization of the Coulomb gauge condition in electrodynamics.
On a semiclassical level, the research programme above is related to the analysis of certain systems of elliptic partial differential equations and to an analysis of topological questions related to the choice of the generalized Coulomb gauge as a unique gauge condition.
The research of C. Montonen has been concerned with supersymmetric gauge
theories, with (SQCD) and without (SYM) matter fields, and with or without
extended supersymmetry. Parts of the low energy effective action for N
= 2 SYM have been explicitly derived in order to ascertain that Seiberg-Witten
duality is not broken by unforeseen quantum effects. The various phases
of N = 1 SQCD have been studied and the effect on them of a soft breaking
of the supersymmetry has been investigated.
SPACE PLASMA OBSERVATIONS AND THEIR ANALYSIS
Space plasma observations and their analysis form the backbone of our research programme in space physics. These are conducted from the ground, using magnetometers, radars, and all-sky cameras, and in space, utilizing a variety of spacecraft with plasma instrumentation. In 1998 three graduate students of the Department of Physics were working with these data. Analysis of plasma observations from the Swedish Freja satellite constituted the licentiate thesis of Jakke Mäkelä. Also the doctoral dissertation by Petri Toivanen made extensive use of satellite observations. In February 1998 we organized an international workshop on the results from the Russian Interball satellites in Finland and the members of our group presented their results in all major conferences and topical meetings.
Our observation-based work is not limited to the terrestrial magnetosphere but also the plasma environment of Mars belongs to the objects under study. In 1998 we were among those who were selected to provide instrumentation for the Mars Express mission of ESA. The team of the instrument, called ASPERA-C, is led by the Swedish Institute of Space Physics with Hannu Koskinen and Esa Kallio (FMI) as Finnish co-investigators. Our aim is to study the interaction between the solar wind and the neutral atmosphere of the planet. Our responsibility is to deliver the data processing unit for the instrument and to develop analysis tools based on our expertise from previous observations near Mars. The programming and testing of the unit will form the basis for the PhD work of one graduate student in experimental physics.
Petri Toivanen defended in August 1998 his PhD thesis titled "Large-scale electromagnetic fields and particle drifts in time-dependent Earth's magnetosphere". It contained both data analysis and original theoretical work how to handle the electrodynamical coupling between the ionosphere and magnetosphere during a temporal evolution of the magnetospheric magnetic field.
The work led by Pekka Janhunen at FMI to develop a global 3D magnetospheric MHD simulation code continued. There are about 10 comparable attempts world-wide, this being the only one in Europe.
Pentti Pulkkinen defended also in August 1998 his PhD thesis titled "Solar differential rotation and its generators: Computational and Statistical Studies". It consistent both on simulation studies of solar MHD and statistical analysis of the sunspot data over 143 years.
Space weather is a relatively new major world-wide effort in applied space physics. It deals with problems on technological systems and humans in space and on ground caused by disturbances of mostly solar origin in the plasma and energetic particle environment. While our observational and theoretical work in space physics is directed at fundamental research the space weather applications are carefully taken into account when new activities are planned. As one of our major efforts in the field of space weather was the ESA contract study on space weather where FMI/GEO was the prime contractor with two subcontractors from Sweden. Hannu Koskinen acted as the study manager of the project. In 1998 the technical note of the central FMI work package was published as an FMI Report (1998:4): H. Koskinen and T. Pulkkinen, State of the art of space weather modelling and proposed ESA strategy. More information of the project can be found through its public WWW-server: http://www.geo.fmi.fi/spee/
The first two years of organized space physics education at the Department of Physics have shown that there is demand on both introductory and more advanced courses in space physics. There is now a standard one-semester course with the title "Introduction to Plasma Physics and its Space Applications" (in Finnish) that has been lectured during the fall terms of 1997 and 1998. In spring 1998 the special course "Advanced Space Physics" was lectured. The graduate students were registered to the "Graduate School in Solar-Terrestrial Physics" which from the beginning of 1999 was extended to the "Graduate School in Astronomy and Space Physics". At present two of the graduate students (one at FMI, one at the Department of Physics) are paid by the graduate school funding.