Project leader: Anca Tureanu    Personnel


The project “Fundamental particle interactions beyond the Standard Model” has a strong com-munity component, covering the diverse theoretical particle physics research performed at the University of Helsinki. The broad fields of research are matter-antimatter asymmetry and the prob-lem of dark matter. For the year 2023, the focus will be in the three major areas detailed below.

Research plan

Baryon number violation and CP violation: we shall continue the research in neutron oscillation, as prototypical process with baryon number violation. Our plan includes also the investigation of neutron-mirror (hidden) neutron oscillations, which is connected with the dark matter problem. In effect, to be an efficient ingredient in a baryogenesis scenario, neutron oscillations have to involve C- and CP-violations. None of these processes has yet been observed, though there are experimental bounds. New precision experiments are planned for the close future at ESS Lund (HIBEAM/NNBAR), ORNL USA, ILL Grenoble and DUNE.

If neutrons oscillate only into antineutrons, or only into mirror neutrons, CP is automatically conserved. However, if they oscillate into both species, the mixing matrix becomes four-dimensional and CP violation is possible (similar to the Kobayashi–Maskawa case, but with more Dirac CP-violating phases). We shall investigate the theoretical and phenomenological aspects of this novel scenario, which is likely to lead to a successful model of baryogenesis, and contribute to the dark matter content at the same time. Also, we shall consider its potential signatures in the physics of neutron stars, where the oscillations into hidden neutron species can create antimatter cores, with interesting observational consequences.

Dark matter direct detection: Recent years have seen an increased interest in direct detection of light dark matter (mass below 1 GeV). Phonon-based detectors have been developed with sensitivity to O(10) eV recoil energies, but currently these experiments suffer from an unidentified background that prohibits the search for DM. Understanding and overcoming this background is currently a top priority for the global direct detection community. In solid state detectors, the threshold energy for defect creation in nuclear recoils coincides with the recently reached O(10) eV scale. Understanding the role of defects in phonon based detection is crucial for accurate measurements, as the energy loss due to defects not only affects the calibration of the energy scale, but also alters the shape of the observed recoil spectrum.

We will investigate the nuclear recoil induced defect creation process is solid state detectors, utilizing molecular dynamics simulations. We will explore how lattice defects might affect the unidentified low energy excess, and how the uncertainties induced due to energy loss can be mitigated or even utilized for gain in background rejection. One such approach is to consider the directional dependence of the defect creation probability due to the inherent anisotropy of the crystal lattice.

Members of our group are coordinating the membership of HIP in the COSINUS experiment. COSINUS is a direct detection experiment that uses sodium iodine (NaI) crystals as cryogenic scintillating calorimeters. The selected target material (NaI) allows for direct model independent comparison with the DAMA/LIBRA experiment, while the cryogenic calorimetric operation allows for particle identification, improved energy resolution and lower detection threshold. Our contribution to the experiment includes an improved analysis of the interpretation and model independent comparison of the results, with the most notable improvement to being the inclusion of possible energy dependent quenching factors of the NaI crystals in the analysis. Another main contribution will be an improved understanding of the response of the NaI crystal to nuclear recoils, achieved via MD simulations as described above.

Neutrino physics: The theory of neutrino oscillations is still the only experimentally confirmed result of physics beyond the SM. The ongoing and future neutrino oscillations experiments are entering the precision stage, and new results regarding the existence of sterile neutrinos and CP violation in the lepton sector will shape up the theoretical research. One of our focus points is the elaboration of a quantum field theory of oscillating neutrino states, which includes a mechanism of coherent production and detection of those states. We have already made significant progress in this direction, by exploiting analogies with the theory of superconductivity.

The plans for the year 2023 are to formulate and interpret in the language of quantum field theory the standard approach to neutrino oscillations from the perspective of quantum mechanical two- (or n-)level systems. Our preliminary results show that the Gribov-Pontecorvo flavour states are valid only in the ultrarelativistic limit and cannot be extended, for example, to seesaw mechanism with light and heavy neutrinos (type I and III). The latter are an important ingredient in leptogenesis, and through it in alternative mechanisms of baryogenesis, as well as dark matter problem (sterile neutrinos). We shall establish the formulation of flavour neutrino states for the whole spectrum of energies and all possible compositions of massive states, and investigate the phenomenological implications regarding the signals of non-relativistic neutrinos in the KATRIN experiment for the determination of absolute masses and the PTOLEMY experiment, proposed for detecting the cosmic neutrino background.


Our main collaborators during 2023 include Zurab Berezhiani (University of L’Aquila), for the research on neutron oscillations, Merab Gogberashvili (University of Tbilisi), for the research on mirror dark matter and effects on neutron stars and black hole mergers, Gabriela Barenboim (University of Valencia), for the research on neutrino physics. We are planning also a closer co-operation with groups involved in other HIP theory projects, for examples on the research on neutron stars. The project is involved also in the COSINUS collaboration.