We have published our first results in a paper selected as an Editors’ Suggestion in Physical Review Letters The paper reports the first search for a new type of elementary particle that may act as a “portal” between ordinary matter and dark matter, which is understood to make up some 85% of the matter in the universe.
Cosmological observations in recent years provide strong evidence that only 15% of the mass of the matter of the universe is known to us, while the remaining 85% is composed of some still undetected and mysterious particles known as dark matter. A great deal of effort in the international particle physics community, including the Belle II experiment, is now focused on finding evidence of dark matter particles.
A Z’ boson is one of the proposed theoretical candidates that might connect dark matter with the ordinary world. If it exists, it could be produced in electron-positron collisions at SuperKEKB and subsequently decay to invisible dark matter particles. Discovery of a Z’ boson might solve important open issues in particle physics related to the behaviour of dark matter and resolve some anomalies observed in other experiments that cannot be explained by the reigning theory of particle physics (the Standard Model).
Theoretical models and detailed simulations predict that Belle II could detect a clear signal that Z’ particles are being produced in electron-positron collisions by searching for an excess of events containing a pair of two oppositely charged muons (heavy cousins of electrons). So far, the data show no such signal. Further searches in a much larger data set yet to be collected by Belle II will either reveal a feebly interacting Z’ boson—or rule it out.
This first physics result was obtained by analysing a small data sample collected during the commissioning of SuperKEKB in 2018.
Since 2019, SuperKEKB and Belle II have been collecting data in full operation mode, while steadily improving the performance of these sophisticated machines*1. Eventually, the experiment will acquire 180,000 times more data than used in the first published analysis. With a data sample of this unprecedented size, Belle II will perform many studies related to dark matter, searches for new particles, and precision measurements that will help elucidate the fundamental laws of Nature.