In search of the Z’ boson


The Belle II experiment started almost exactly a year ago. The renowned journal Physical Review Letters is now publishing the first results of the detector. The work deals with a new particle in connection with dark matter, which, according to current knowledge, makes up about 25 percent of the universe.

The Belle II experiment has been taking data for physical measurements for about a year. Both the SuperKEKB electron-positron accelerator and the Belle II detector had been refurbished over several years in order to achieve a 40 times higher data rate.

Scientists at twelve institutes in Germany are significantly involved in the construction and operation of the detector, the development of evaluation algorithms and the analysis of the data. The Max Planck Society's semiconductor laboratory made a key contribution to the new development of the highly sensitive innermost detector, the pixel vertex detector.

With Belle II, scientists are looking for traces of new physics that can be used, for example, to explain the unequal occurrence of matter and antimatter or the mysterious dark matter. One of the previously undiscovered particles that the Belle II detector is looking for is the Z'-boson - a variant of the already detected Z-boson. The latter acts as an exchange particle for the weak interaction.

As far as we know, about 25 percent of the universe is made up of dark matter, whereas visible matter makes up just under 5 percent of the energy budget. Both forms of matter attract each other via gravity. Dark matter forms a kind of template for the distribution of visible matter, which is shown, for example, in the arrangement of galaxies in the universe.

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

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