Link between dark and normal matter

The Z'-boson could play an interesting role in the interaction of dark and normal, visible matter, i.e. it could be a kind of mediator between the two forms of matter. The Z 'can - at least theoretically - result from the collision of electrons (matter) and positrons (antimatter) in the SuperKEKB and then disintegrate into invisible dark matter particles.

Thus, the Z'-boson can help to understand the behavior of dark matter - and not only that: the discovery of the Z 'could also explain other observations that are not in line with the standard model, the fundamental theory of particle physics .

Important indication: detection of muon pairs

But how can the Z'-boson be found in the Belle II detector? Not directly, that's for sure. Theoretical models and simulation calculations predict that the Z 'could reveal itself through interactions with muons, heavier relatives of the electrons: If, after the electron / positron collisions, scientists have an unusually high number of muon pairs with opposite charges as well as unexpected ones Discovering deviations in energy and momentum conservation would be an important indicator for the Z '.

However, the new Belle II data did not yet show any signs of the Z'-boson. However, with the new data, the scientists can limit the mass and coupling strengths of the Z'-boson with an unprecedented level of accuracy.

More data, more precise analyzes

"Despite the still small amount of data, we can now take measurements that have never existed before," said the spokesman for the German groups, Prof. Thomas Kuhr from LMU Munich. "This underlines the important role of the Belle II experiment in the research of elementary particles."

These first results come from analyzing a small amount of data that SuperKEKB started up in 2018. Belle II started full operation on March 25, 2019. Since then, the experiment has been collecting data, while the collision rate of electrons and positrons has been continuously improved.

If the experiment is set up perfectly, it will provide a multiple of the data that has gone into the currently published analyzes. The physicists hope to gain new insights into the nature of dark matter and other unanswered questions.


The German working groups in the Belle II experiment are funded by the following institutions and programs:

Federal Ministry of Education and Research: Framework Program for Research on Universe and Matter (ErUM)
German Research Foundation (DFG) as part of the excellence strategy of the federal and state governments:
"ORIGINS": EXC-2094 - 390783311
"Quantum Universe": EXC-2121 - 390833306
European Research Council
European Union’s Horizon 2020 - grant agreement No 822070
Helmholtz Association
Max Planck Society



Search for an invisibly decaying Z 'boson at Belle II in e + e– ® m + m– (e + - m– +) + missing energy final states

The Belle II Collaboration

Physical review letters; Volume 124, 14; April 10, 2020

DOI: 10.1103 / PhysRevLett.124.141801

Go to Editor View