Searches for Dark Matter and Dark Forces at Belle II

The aim of this proposal is to put under experimental test different dark sector and dark matter models using data coming from the asymmetric energy collisions of electrons and positrons collected by the Belle II experiment at the Super-KEKB accelerator facility in Tsukuba (Japan). The ambition is to discover dark matter, dark forces or both within the four years research line of this standalone project. Searches for low mass dark matter will be performed through the study of processes in which final states are invisible with large missing energy. Since invisible decays of the Y(1S) can be enhanced by the existence of low mass dark matter, this channel will be studied with unprecedented precision in di- pion transitions from Y(3S). The dark Z` , hypothetical mediator of a dark force, will be searched first in electron positron collisions resulting in the production of muons plus a Z` boson, followed by the decay of Z` to muons. This search for visible Z` decays will set the basis to the understanding of systematic effects at Belle II that will be needed when performing for the first time the search of invisible decays of the Z` boson in electron-positron collisions resulting in the production of muons plus a Z` boson, followed by the decay of Z` to invisible final states, which is the natural follow-up of this standalone project.

The FWF P31361 project, conducted between 2018 and 2023, aimed to explore dark sector particle production using data from the Belle II experiment. This endeavour was part of the ongoing quest in physics to understand dark matter, a significant yet elusive component of the universe. The project aimed to search for these particles through electron-positron collisions. Despite facing substantial challenges, such as the Covid-19 pandemic and the underperformance of the SuperKEKB collider, which limited the amount of data available for analysis, the project team achieved significant milestones. We focused on the search for "dark sector" particles, hypothetical particles that could explain dark matter. One of the major achievements of the project was the search for invisible decays of a Z' boson produced in association with muons. This pioneering search at the Belle II experiment marked the first research paper from Belle II, published in the Physical Review Letters journal in 2020. This particular search also explored the potential violation of lepton flavoor by the Z' boson. Building on this momentum, the team then searched for a low-mass dark Higgs boson produced alongside a dark photon undergoing muonic decays. This analysis, another first for a B factory, led to the exclusion of a large parameter space for the existence of these particles. The results were published in Phys. Rev. Lett. in 2023. Additionally, the project investigated the muonic decay of a Z' boson. While the results of this study are still under review within the Collaboration, the research has matched the sensitivity of previous experiments conducted by Belle and Babar, but with only a fraction of the data. During the final part of the project, the possibility of linking this type of fundamental physics to gravitational wave studies was explored. Beyond the scientific achievements, the project had a profound impact on the career development of its members. The principal investigator was awarded an ERC starting grant in 2020, significantly expanding their research scope in Austria. The PhD students hired for the project received FWF Schroedinger grants, enabling them to gain international experience before returning to the group. Moreover, some of the master's students who contributed to the project were later hired as PhD students for the ERC project. The project also led to significant collaborations with international groups, enhancing the research's global impact. Team members had opportunities for professional growth, including participation in conferences and contributions to the Belle II experiment, even though the pandemic restricted travel and in-person interactions. In summary, the FWF P31361 project not only contributed to the understanding of dark matter through innovative searches at the Belle II experiment but also fostered professional development and international collaboration, thereby enhancing the field of particle physics.