The search for the neutrinoless double beta decay (0νββ) is one of the key research topics in neutrino physics with major impact on particle physics and cosmology. It probes the neutrino nature (Majorana vs. Dirac), the neutrino mass scale, and possibly CP-parity violating effects. An observation of 0νββ would also shed light on the origin of the matter dominance in today's universe. In order to derive an effective Majorana neutrino mass from the measured 0νββ half lives, the corresponding nuclear matrix elements (NME) need to be known with sufficient accuracy. Currently, the theoretical calculations of the NMEs differ by a factor of 2-3, depending on the nuclear model framework applied. In the recent report of APPEC (Astroparticle Physics European Consortium), the neutrinoless double beta decay committee therefore explicitly recommends dedicated theoretical and experimental efforts to achieve a more accurate determination of the NMEs.
The OMC4DBD proposal aims to carry out key experimental measurements on several 0νββ daughter isotopes to provide critical input for benchmarking nuclear matrix element calculations. The groups from JINR and PSI, together with members from other institutes in Europe, propose to measure the Ordinary Muon Capture (OMC) process on several isotopes using the muon beams available at the Paul Scherrer Institute (PSI), Villigen. The OMC4DBD proposal has been accepted by the PSI User Committee in January 2020 (PSI R-20-01.1), and beam time has been granted in October 2020 with the perspective of a multi-year program to follow. This proposal extends the previous program of the OMC measurements, initiated and led by the JINR partners of this proposal.
Unfortunately, the beam measurements planned for this year were rescheduled to the next year due to pandemic. Therefore, our team will focus on the analysis of data obtained during the measurements of 2019.
In the years 2021-2022, to be covered by this proposal, we plan to measure the OMC on Ba-136, Se-76 and Mo-96, which are daughter nuclei of corresponding ββ-isotopes Xe-136, Ge-76, and Zr-96. A set of world leading 0νββ-experiments such as nEXO, KamLAND2-Zen, NEXT, DARWIN, and PandaX-III will use Xe-136 isotope as a ββ-source, while Ge-76 will be measured in the LEGEND - next-generation 0νββ-project. Currently, the Zr-96 enrichment method is finalized (Russian companies together with JINR) and up to 0.5 kg of Zr-96 will be available in 2021 (for the first time in the world), which will be measured in the Demonstrator of the SuperNEMO 0νββ-project. Planned OMC measurements in the frame of this project will provide vital experimental data to improve the theoretical knowledge of the ββ-processes.
The PSI and JINR groups will combine their expertise and instrumentation to achieve this goal. All preparation of the measurements, mounting and testing, the data taking and analysis tools will be carried out in close collaboration by the two groups. Analysis workshops are planned to be carried out at PSI in preparation of the publications. The project expenses also include costs of travel and stays at the PSI Guest House during experimental shifts.
Joint Institute for Nuclear Research:
- Dr. Yuri Shitov
- Dr. Daniya Zinatulina
- Dr. Mark Schirchenko
Paul Scherrer Institut:
- Dr. Andreas Knecht