10. THEORETICAL PHYSICS DIVISION (atom.physics.helsinki.fi/~tfo_www)




The research activity of staff of 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 and, secondly, space physics. There is lots of overlap between different orientations, for example, the work of the particle cosmology group is extending in the direction of space physics: 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 and other fields like atomic, molecular, optical and condensed matter physics enter in the graduate study and research program in this way. Collaboration 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 three EU TMR networks.

K. Kajantie




Kari J. Eskola*, Kari Enqvist, Asko Jokinen, K. Kainulainen**, K. Kajantie, Hannu Kurki-Suonio+, Mikko Laine++, John McDonald, Thomas Neuhaus#, Arttu Rajantie ##, Elina Sihvola, Antti Sorri, Kari Rummukainen**

One of the group's main achievements has been a precise and quantitative determination of the phase diagram of electroweak matter, the matter which filled the universe at an early stage of its evolution. Within the standard model one now knows for a given Higgs mass at what temperature the phase transition takes place and what, for example, the associated latent heat is. For a physical Higgs mass there actually will be no transition at all.

More recently, the group has also solved the phase diagram when the matter is in an external hypermagnetic field. This is a conceivable situation in the very early universe and is anyway theoretically an interesting problem. The situation namely is analogous to superconductors in an external magnetic field and there spectacular effects are observed and extensively studied: the superconductor repels the magnetic field but if it forced through the superconductor, it penetrates either through a thick rope or a lattice of thin vortices. Due to the much more involved dynamics of electroweak theory similar but even more spectacular effects may be expected. The group has now numerically solved the main features of the phase diagram of electroweak theory and shown what the response to an external hypermagnetic field is.

Predictions for the electroweak matter can only be studied in cosmology, but similar properties of QCD matter in its two phases, the hadron phase and the quark-gluon plasma phase, can also be studied in the laboratory. A new collider, the Relativistic Heavy Ion Collider RHIC and Brookhaven, permits one to produce QCD matter in collisions of large nuclei at energies ten times larger than ever studied previously and starts producing experimental results in early 2000. The group has performed careful computations on precisely how good generators of quark-gluon plasma these new accelerators will be.

A popular extension of the Standard Model is supersymmetry. The spectrum of supersymmetric theories admits non-topological solitons called B-balls, which the group has studied actively. They form during inflation from the fragmentationof the so called Affleck-Dine condensate and carry a large baryonic charge which they release by decay much after the electroweak phase transition, thus modifying the standard cosmological picture. B-balls could produce also the cold dark matter particles. It was shown that, 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. The group has made a study of the level of sensitivity of Planck to isocurvature perturbations in general. Theoretical Physics Division is involved in the Low Frequency Instrument Consortium of Planck.

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. It was shown that the space of oscillation parameters can be divided into regular and chaotic regions; in the latter small changes in the initial conditions change the sign of the leptonic asymmetry, thus affecting the nucleosynthesis calculation of the light element abundances.

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 Aachen

## Univ. of Sussex

* Univ. of Jyväskylä

** Nordita




Anna Kalliomäki, Jukka Maalampi, Nikolai Romanenko, Ville Sipiläinen, Solveig Skadhauge#, Katri Huitu*, Petteri Keränen**, Juha Peltoniemi***, Martti Raidal+, Kai Puolamäki*, Jouni Laitinen*, and Iiro Vilja++

The question of neutrino masses and mixings, whose existence was confirmed by the Super-Kamiokande experiment on atmospheric neutrino oscillations two years ago, is of fundamental importance and one of the most actively studied subjects in phenomenological particle physics today. Also the flux of solar neutrinos and some laboratory result point towards neutrino oscillations, and the combined data constrains the possible mass and mixing patterns. We have shown that measuring the neutrino fluxes from active galactic nuclei would provide a robust test of various mixing schemes, e.g. the possible existence a fourth, sterile neutrino species. We have also investigated the effect of leptonic CP violation in a long baseline oscillation experiments, showing that in schemes with a sterile neutrino the matter effects in Earth's crust are not important and would not shadow the genuine CP violation signature. A relevant problem, which we have also addressed, is to find an explanation for the lightness of a sterile neutrino, if it exists. Our group has also studied the behaviour of neutrinos in matter using the wave packet formalism.

Apart from neutrino physics, our group has continued its well-established activity in the phenomenology of the beyond-the-Standard-Model physics at linear collider, concentrating on extended gauge models, in particular on the left-right symmetric electroweak model (LRM), and supersymmetry. We have studied the phenomenological signatures of the lepton number violation the LRM predicts. The lepton number violation is generated by the Higgs triplets, consisting of neutral, singly charged and doubly charged scalar particles. We have investigated the production and subsequent decay of these particles at the LHC and linear collider. One particular question under study has been how to make a distinction between singly charged iso-doublet and iso-triplet Higgs particles.

The group maintains close contacts with the University of Turku, University of Oulu and the University of Lund, and it has participated in Joint ECFA / DESY Study: Physics and Detectors for a Linear Collider.

# University of Lund

* Helsinki Institute of Physics

** University of Lisboa

*** University of Oulu


++ University of Turku




Gabriel Amoros*, Anthony Green, Jari Heikkinen, Jonna Koponen, Jouni Niskanen, Petrus Pennanen**, Pekko Piirola, Antonio Polosa and Mikko Sainio***

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 two- and four-quark systems has been continued and is being extended to include more realistic cases containing 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.

However, chiral symmetry is not exact, but is broken due to the quark masses. Related to this is the breaking of charge symmetry in the strong interaction, which can be observed in nucleon-nucleon scattering and is also investigated in the reaction np Æ dp0. Presently an isospin breaking pion rescattering mechanism based on quark mass differences is being implemented in earlier calculations and appears to give an extraordinarily large effect in pion production.

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 in NN Æ NNp and NN Æ 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, which started in April 1998, another extension of European collaboration has evolved.

* also at Lund

** Nordita, Copenhagen

*** also at HIP




Christofer Cronström, Samuli Hemming, Jari Laamanen, Juha Loikkanen, Claus Montonen, Ossi Pasanen, Tommi Raita, Syksy Räsänen and Antti Salmela

At the Division of Theoretical Physics quantum field theory related research is carried out under the supervision of C. Cronström and C. Montonen (on leave, presently at the Helsinki Institute of Physics). The topics studied are principally related to gauge theories, and in particular to quantisation and confinement in non-Abelian gauge theories. The question of confinement in BRST quantised QCD has been the object of study of S. Räsänen. Questions related to quantisation of QCD using a generalised Coulomb gauge condition (quantisation methods andboundary conditions, topological classification of gauge potentials, the role of the holonomy group, the geometry of the generalised Coulomb gauge in the modular space of equivalence classes of gauge potentials) has been the object of study by C. Cronström, S. Hemming, T. Raita and A. Salmela.

The relation between two-dimensional conformal field theory and three-dimensional topological field theory has been studied by J. Loikkanen, and the particle limit of QFT models has been considered by J. Laamanen. O. Pasanen has been engaged in investigating the Anti-DeSitter - Conformal Field Theory equivalence in string theory.

Various aspects of supersymmetric gauge theories have been the object of the research of C. Montonen. In N = 2 super-Yang-Mills theory the general structure of the low-energy effective action has been investigated. The structure of the enigmatic 1/4-BPS states in N = 4 super-Yang-Mills has been under investigation. A review of N = 1 superconformal theories is in preparation. Montonen has also participated in a collaboration investigating the response of different kinds of matter to extremely strong magnetic fields..




Research and education in Space Physics is based on co-operation between the Department of Physics at the University and the Geophysical Research Division of the Finnish Meteorological Institute (FMI/GEO): Education is given at the University whereas most of research staff, including several graduate students, are located at FMI/GEO. In 1999 the University personnel consisted of Hannu Koskinen (professor), of an assistant (Pentti Pulkkinen, 1.1.–31.3., Ville Honkkila, 1.4.–31.12.), and of a graduate school student (Jakke Mäkelä, 1.1.–31.7., Noora Partamies, 1.8.–31.12). The main research line of the University-FMI collaboration is solar-terrestrial physics where the study objects include the physics of the Sun, the solar wind and the terrestrial and planetary magnetospheres.

Hannu Koskinen



Our research programme is built mainly around space plasma observations and their analysis. The observations are conducted from the ground, using magnetometers, radars, and all-sky cameras, and in space, utilizing a variety of spacecraft with plasma instrumentation. In 1999 four graduate students in general and theoretical physics were working with these data. The FMI-University collaboration is presently one of the largest and strongest solar-terrestrial physics groups in Europe. One of the major observational assets is the extensive ground-based instrument network, called MIRACLE, whichs is managed by the FMI. It reaches from Nurmijärvi in the Southern Finland to Svalbard in the North. MIRACLE observations are used world-wide in various studies and they will play a central role in the upcoming Cluster II mission of ESA (to be launched in the summer of 2000).

Our observation-based work is not limited to the terrestrial magnetosphere but also the plasma environment of Mars belongs to the objects under study. Presently we are participating in the development of the ASPERA-C instrument for the Mars Express mission of ESA. ASPERA-C will measure the interaction between the solar wind plasma and the Martian atmosphere. The spacecraft will be launched in 2003 and the observations will begin in 2004. 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 forms the basis for the PhD work of one graduate student in experimental physics.



The work led by Docent Pekka Janhunen at FMI to develop a global 3D magnetospheric MHD simulation code has continued. There are about 10 comparable attempts worldwide, this being the only one in Europe. One of our graduate students participates in the theoretical development of the MHD methods. The simulation work is carried in close collaboration with analysis of space observations.

Another important project in theoretical model development in 1999 was an adaptation of our earlier model of plasma flow around Mars to the terrestrial environment. This model is expected to be a very useful tool in the analysis of energy input to the magnetosphere as well as in the analysis of Cluster II data.



During 1999 the analysis of long-term sunspot data was continued but the focus of the work shifted towards the analysis of coronal mass ejections and their efficiency in driving geomagnetic storms. From year 2000 this work will be continued as a PhD project of one of our new graduate students.



Space weather is a relatively new 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. In fact, our FMI-University collaboration has grown to one of the leading European experts in this field. Soon after our previous ESA contract study on space weather was concluded, we participated in an international team bidding for a new one and the contract started in April 2000. Our work packages will be done at FMI, where Hannu Koskinen will act as the study manager. Our responsibilities include comprehensive investigation of space weather effects on various technological systems both in space and on ground and formulation of event-based scenarios to direct future development of European space weather services.

Our success in these application-oriented projects is based on our broad approach in the fundamental research over the entire chain of events from the surface of the Sun thorugh the magnetosphere and ending in induced currents in the ground-based systems. Also this latter field employs presently one of our graduate students in theoretical physics.



Year 1999 was very productive in terms of Master Theses in space physics. In total 6 theses were produced and five of these students continue as graduate students in the field. The former "Graduate School in Solar-Terrestrial Physics" was extended to the "Graduate School in Astronomy and Space Physics" from the beginning of 1999. Now the GS covers practically all groups in space physics and astronomy in Finland. At present two of our graduate students (one at FMI, one at the Department of Physics) are supported through the graduate school funding.