2014 Fiscal Year Annual Research Report
ダークマター探索，二重ベータ崩壊及びエキゾチック原子核への精密原子核計算
| Project/Area Number |
14F04323
|
|---|
| Research Institution | The University of Tokyo |
|---|
Principal Investigator |
大塚 孝治 東京大学, 理学(系)研究科(研究院), 教授 (20201379)
|
|---|
| Co-Investigator(Kenkyū-buntansha) |
MENENDEZ SANCHEZ Javier 東京大学, 理学(系)研究科(研究院), 外国人特別研究員
|
|---|
| Project Period (FY) |
2014-04-25 – 2017-03-31
|
| Keywords | Double-beta decay / Dark matter scattering / Nuclear structure / Spectroscopy / Shell model / Chiral EFT / Three-nucleon forces / Meson-exchange currents |
|---|
| Outline of Annual Research Achievements |
Nuclear matrix elements for the neutrinoless double-beta decay of 48Ca have been calculated for the fist time in the shell model approach in a valence space comprised by two-major harmonic oscillator shells, whereas previous calculations were restricted to one major shell. As a result, the two-major-shell nuclear matrix element is about 50% larger than in the one-major-shell calculation. This has immediate implications for experimental searches, as larger matrix elements reduce the experimental lifetime of the process. A research article presenting these results is in preparation.
Nuclear structure calculations of the quadrupole electric and magnetic moments of the ground states of odd-mass calcium isotopes have been performed. Nuclear forces predicted by chiral effective field theory, including state-of-the-art two- and three-nucleon forces, were used. Theoretical predictions have been compared to very recent measurements performed at ISOLDE (CERN, Switzerland), and theory and experiment come in very good agreement. These results, including the theory-experiment comparison, have been presented at an international conference held at TRIUMF, Vancouver (Canada), and have been submitted for publication to the journal Physical Review C.
In addition, a review article on the impact of three-nucleon forces on neutron-rich systems, including nuclear structure calculations in the medium-mass oxygen and calcium isotopes, has been prepared and submitted for publication to the journal Annual Review of Nuclear and Particle Science.
|
| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Double-beta decay investigations have been performed to identify which parts of the nuclear force are more relevant for double-beta decay. Systematic calculations on the neutrinoless double-beta decay of calcium, titanium and chromium isotopes have been performed. Even though only 48Ca is relevant for experimental searches, a systematic exploration is useful to identify the most relevant correlations for double-beta decay. We found that nuclear matrix elements are particularly sensitive to the proton-neutron pairing part of the nuclear interaction, and these correlations have to be treated carefully in all double-beta decay calculations. The results will be prepared for publication as a research article.
Nuclear matrix elements for dark matter particles off nuclei have previously been calculated assuming spin-independent and spin-dependent interactions. However, six new couplings of dark matter particles with nucleons have been recently proposed in the literature, and these are being implemented in large-scale shell model calculations.
Finally, nuclear structure calculations for medium-mass nuclei in the "sd" shell are being performed with different initial Hamiltonians using two- and three-body forces. This will be the first study of these nuclei including three-nucleon forces with neutrons and protons in the valence space, and will also quantify the uncertainties in nuclear structure calculations due to the nuclear force. These results will also be prepared for publication in a research article.
|
| Strategy for Future Research Activity |
We plan to include two-body (meson-exchange) currents to neutrinoless double-beta decay nuclear matrix elements. Complete expressions for currents have been very recently published, and involve new momentum-transfer dependence, important for neutrinoless double-beta decay large momentum transfers (200MeV). In addition, a comparison of decay calculations with the same Hamiltonian is very valuable to constrain the nuclear matrix elements. We plan to compare shell model with quasiparticle random phase approximation (QRPA), the two methods most widely used in double-beta decay calculations. Finally, calculation of nuclear matrix elements beyond the closure approximation is also planned, which is expected to modify present results in about 10% in one major-shell, but has not been explored in two major-shells.
The completion of the calculation of dark matter-nucleus interactions in all channels considered in the literature will allow for a complete study of this process, including coherent effects due to nuclear structure, and interference between different channels. Since this is very relevant for experimental analyses, collaboration for experimental groups involved in direct dark matter detection will reinforce and give visibility to our findings.
Furthermore, nuclear structure calculations beyond the "sd" shell including two- and three-nucleon forces are planned. This will significantly extend the scope of medium-mass nuclei calculations based on chiral forces. In addition, spectroscopic information for these nuclei, such as electromagnetic transitions, will be explored.
|
Research Products
(2 results)
