Project leader: Anca Tureanu Personnel
Background
The major challenges of modern particle physics lie beyond the Standard Model. At the heart of the theoretical and experimental research are the matter-antimatter asymmetry in the Universe, the problem of dark matter and the neutrino properties and interactions. Our local research group in Helsinki has essential expertise in the proposed research topics and a vast and active international collaboration network.
The purpose of this project is to study several important facets of the above topics, from the phenomenological and quantum field theoretical perspective, while exploring the synergies of dedicated experiments at the high energy and high intensity frontier.
Research plan
CPT violation: CPT symmetry is one of the cornerstones of particle physics model building, as it is a consequence of local relativistic quantum field theory. The discovery of CPT violation would have far-reaching implications for high-energy physics and cosmology, especially for the mechanism of baryogenesis. Numerous precision experiments have been setting bounds on potential CPT violation, including ALICE and LHCb. Future experiments are planned at FAIR (low-energy antiproton spectroscopy), DUNE and HyperKamiokande (neutrino oscillations). Recently, we have proposed a reciprocal of CPT theorem, in which we showed that a CPT-violating interaction does not necessarily lead to mass differences between particles and antiparticles. However, all the CPT violation tests are done precisely by looking at those mass differences. We plan to investigate the phenomenological consequences of Lorentz-invariant CPT violation in nonlocal theory and propose experimental tests for CPT violation and its scale.
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.
We shall continue our research on oscillations between neutrons, antineutrons and mirror neutrons, which can exhibit CP violation (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 plan to study the phenomena that hinder the sensitivity for rare event signals and their persistence even at cryogenic temperatures, for semiconductor detectors when the detection threshold is pushed to lower energies.
The membership of HIP in the COSINUS experiment is coordinated by members of our project. 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. COSINUS measures the recoil energy directly via the phonon channel in addition to the scintillation light, removing the ambiguity in the energy scale and allowing for particle identification (nuclear vs electron recoil). The uncertainty on the quenching factors and their possible dependence on the recoil energy and crystal properties is one of the main difficulties in interpreting and comparing the DAMA signal with other experiments. 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. We plan a systematic scan of the dark matter effective theory parameter space to analyze the compatibility of DAMA with other direct detection experiments under relaxed assumptions of the DM-nucleon interactions and DM velocity distributions.
Theory of neutrinos: 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 consistent theory of neutrino oscillations and matter conversion, which includes a mechanism of coherent production and detection of flavour neutrinos. In our recent theoretical proposal flavour neutrinos are treated as waves of massless particles propagating in a “refractive quantum vacuum”. The difference in strength between weak interactions and mass-generating interactions is argued to allow for the production and detection of flavour neutrinos in weak interactions as massless particles. They experience the mass-generating interactions as coherent forward scattering in the Brout–Englert–Higgs vacuum, which induces macroscopically multi-refringent effects. The flavour neutrino wave is then found to have a universal effective refractive mass in vacuum and a unique group velocity for a given energy. Massless neutrinos with energies below the scale of the mass parameters behave as evanescent, non-propagating waves. We plan to investigate further the consequences of this theoretical framework by i) developing the theory of propagation in matter of slowly varying density; ii) elucidating the relativistic invariance and unitarity of the neutrino oscillations and iii) the effect of evanescent neutrinos on structure formation and how this may change the cosmological bounds on the neutrino mass parameters.
Collaboration: Our group has an active international collaboration network with scientists from: RIKEN Wako, Simons Center for Geometry and Physics, Leibniz Institute for Astrophysics Potsdam, Norwegian University of Science and Technology Trondheim, National Institute for Chemical Physics and Biophysics Tallinn, Texas A&M University, University of Lund, University of Göttingen, University of Coimbra, INFN Catania, University of Amsterdam.