Searching for the Z ′ boson

Computer graphics of the simulated event, in which the Z & # 39; boson is formed during e + e collisions, in combination with two muons (green line and hits) and decays into invisible particles. In this figure, the Z boson decays into an invisible neutrino and antineutrino, but it can also decay into a dark matter particle and its antiparticle. Credit: KEK / Belle II Collaboration

The Belle II experiment collected physical measurement data for approximately one year. After several years of restoration work, the SuperKEKB electron-positron accelerator and Belle II detector were improved compared to their predecessors, which made it possible to increase the data transfer rate by 40 times.

Scientists from 12 German institutes are involved in the design and operation of the detector, the development of evaluation algorithms and data analysis. Physical Institute. Max Planck made a significant contribution to the development of the highly sensitive internal detector Pixel Vertex Detector.

With Belle II, scientists are looking for traces of new physics that could explain the uneven occurrence of matter, antimatter and the mysterious dark matter, One of the particles not yet discovered that the Belle II detector is looking for is the Z-boson, a variant of the Z-boson that acts as an exchange particle for weak interaction.

As far as we know, about 25% of the Universe consists of dark matter, while visible matter makes up slightly less than 5% of the energy budget. Both forms of matter attract each other through gravity. Dark matter, thus, forms a kind of template for the distribution of visible matter. This can be seen, for example, in the arrangement of galaxies in the universe.

The connection between dark and normal matter

The Z-boson can play an interesting role in the interaction of dark and visible matter (that is, it can be a kind of intermediary between the two forms of matter). The Z-boson can – at least theoretically – result from the collision of electrons (matter) and positrons (antimatter) in SuperKEKB and then decay into invisible particles of dark matter.

Thus, the Z-boson can help scientists understand the behavior of dark matter. Moreover, the discovery of the Z boson may also explain other observations that are not consistent with standard modelfundamental theory of particle physics

Important tip: detecting muon pairs

But how to detect a Z-boson in a Belle II detector? Not directly – that's for sure. Theoretical models and simulations predict that the Z-boson can manifest itself through interactions with muons, heavier relatives of electrons. If scientists find an unusually large number of muon pairs of the opposite charge after collisions of electrons with positrons, as well as unexpected deviations in the conservation of energy and momentum, this will be an important indicator of the Z & # 39; boson.

However, the new Belle II data has not yet given any indication of the Z & # 39; boson. But with new data, scientists can limit the mass and traction of Z ′ boson with previously unattainable accuracy.

More data, more accurate analysis.

“Despite the still small amount of data, we can now carry out measurements that have never been taken before,” said a representative of German groups, Dr. Thomas Kur, from Ludwig Maximilian University of Munich. “This highlights the important role of the Belle II experiment in particle research.”

These initial results came from the analysis of a small amount of data collected during the launch phase of SuperKEKB in 2018. Belle II fully earned March 25, 2019. Since then, during the experiment, data were collected with continuous improvement in the collision rate of electrons and positrons.

If the experiment is finely tuned, it will provide significantly more data than in recently published analyzes. Thus, physicists hope to gain a new understanding of the nature of dark matter and other unanswered questions.

Belle II measures the collisions of the first particle

Additional Information:
I. Adachi et al. Search for the invisible decaying Z-boson in Belle II in final states plus the missing energy e + e− → μ + μ− (e ± μ∓), Physical Review Letters (2020). DOI: 10.1103 / PhysRevLett.124.141801

Provided by
Max Planck Society

Belle II gives the first results: in search of the Z & # 39; boson (2020, April 7)
retrieved April 7, 2020

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