In search of the Z' boson

Belle II detector in Japan delivers first results

Impression of a collision in the Belle II detector. © KEK, Belle II. Zachary Duer, Tanner Upthegrove, Leo Piilonen, George Glasson, W. Jesse Barber, Samantha Spytek, Christopher Dobson (Virginia Tech Institute for Creativity, Arts and Technology, Virginia Tech Department of Physics, Virginia Tech School of Education)

Almost exactly one year ago, the Belle II detector went into operation at the Japanese research centre KEK. Now the international collaboration of scientists is publishing the first results obtained with the help of the detector. The publication in the scientific journal "Physical Review Letters" circles the properties of a new particle related to dark matter. According to current knowledge, dark matter is more than five times more common in the universe than the matter we are familiar with.

The Belle II detector has been taking physics data for about a year now. Both the particle accelerator SuperKEKB and the Belle II detector had been upgraded over several years of rebuilding work to achieve a 40 times higher data rate.

Scientists at twelve institutes in Germany play a major role in the construction and operation of the detector. DESY plays a leading role, especially in the integration and commissioning of the highly sensitive innermost detector, the Pixel Vertex Detector. In addition, the German groups are involved in the development of evaluation algorithms and the data analysis.

With Belle II, scientists are looking for traces of new physics that can be used, for example, to explain the unequal relationship between 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 photon (particle of light), which, unlike the photon, has mass.

As far as we know, about 25 percent of the universe consists of dark matter, whereas visible, known matter makes up only just under five percent of the universe. Both forms of matter attract each other via gravity. Dark matter is thus also located in the visible matter galaxies in the universe and, according to common theories, is also responsible for how visible matter is distributed in the universe.

Link between dark matter and normal matter

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

Thus, the Z' boson could help to understand the behaviour of dark matter - and not only that: the discovery of the Z' could also explain other results of some precision measurements that are not consistent with the Standard Model, the fundamental theory of particle physics.

Important indication: detection of muon pairs

But how can the Z' boson be detected in the Belle II detector? Not the direct route, that's for sure. Theoretical models that explain the precision measurements I mentioned,

predict that the Z' could reveal itself through its interaction with muons, the heavier relatives of the electrons. If scientists were to discover an unusually high number of muon pairs with opposite charges and unexpected deviations in energy and momentum conservation after the electron-positron collisions, this would be an important indication of the Z'.

However, the new Belle II data has not yet provided any indication of the Z' boson. The new data can be used to limit the mass and coupling strengths of the Z' boson with a previously unattainable level of accuracy. The Belle II team will have to evaluate many more collision data before the Z' becomes visible - or it is ruled out as an explanation for the mysterious precision measurements.

More data, more precise analyses

"Despite the still small amount of data, we are now able to make measurements that have never been done before," says Thomas Kuhr, spokesman for the German groups at the Ludwig Maximilian University of Munich. This underlines the important role of the Belle II experiment in the study of elementary particles. "If the Z' boson actually exists, we have already been able to encircle it somewhat with this measurement," adds DESY researcher Ilya Komarov. "With future data, we will be able to draw this circle ever closer."

These initial results are derived from the analysis of a small amount of data collected during the start-up phase of SuperKEKB in 2018. DESY scientists were also significantly involved in this analysis. Belle II went into full operation on 25 March 2019. Since then, the experiment has been collecting data while at the same time steadily increasing the collision rate of electrons and positrons.

Once the experiment is perfectly adjusted, it will provide many times the data that have been used in the currently published analyses. In this way, physicists hope to gain new insights into the nature of dark matter and other unsolved questions.

The German working groups in the Belle II experiment are financially supported by the following institutions and programmes:

> European Research Council
  > Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany´s Excellence Strategy
      > “ORIGINS“: EXC-2094 – 390783311
      > “Quantum Universe”: EXC-2121 – 390833306
> German Federal Ministry of Education and Research (BMBF): BMBF funding of collaborative research “Exploring of the Universe and Matter” (ErUM)
> Helmholtz Society
> European Union’s Horizon 2020 – grant agreement No 822070
> Max Planck Society

Original publication:

Search for an invisibly decaying Z' boson at Belle II in e+e- + - (e+ - - +) + missing energy final states; The Belle II Collaboration; "Physical Review Letters", 2020; DOI: