T2K Experiment: A Step Towards Solving the Mystery of the Missing Antimatter


RWTH contributes to international large-scale project investigating the matter-antimatter asymmetry.



Stefan Roth



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A team of more than 500 physicists from 60 universities and research institutions succeeded in proving a difference between matter and antimatter in the T2K experiment in Japan. The measurement results are being published in the current issue of the scientific journal Nature. The journal’s front page headline reads: "The mirror crack'd," alluding to the detected violation of the assumed mirror image symmetry between matter and antimatter. The T2K experiment also involves a team from RWTH Aachen University headed by Professor Stefan Roth from the Physics Institute III B.

According to theory, the Universe should consist of equal parts of matter and antimatter, both created in the Big Bang about 14 billion years ago. However, there is not much antimatter to be found in the Universe. Scientists have now studied a physical process in which the otherwise prevailing symmetry between matter and antimatter favors one of the two sides and is thus able to explain the absence of antimatter. The neutrinos used in this process are the most common elementary particles in the Universe, but they are still the least understood. This is due to the fact that these particles are very hard to detect, as they may traverse the earth without even leaving a trace. It is only very rarely that a neutrino interacts with matter.

Neutrinos and antineutrinos, which are generated in the J-PARC research center in Tokai at the Japanese east coast in the form of a highly intense neutrino beam, are sent on a 300 km journey to the 50,000 ton Super Kamokande neutrino detector, which is located 1,000 meters underground in the Kamioka region. This explains the name of the experiment, “Tokai-to-Kamioka,” T2K for short.

During their journey, the neutrinos show an interesting behavior, as some of them change into another type, or "flavor," of neutrino. Neutrinos have been observed to show this morphing behavior more than their antimatter counterparts, an effect which constitutes a violation of the fundamental symmetry between matter and antimatter.

After analysis of the data from nine years of operation, the violation of the symmetry between matter and antimatter could be proven at a 95 percent confidence level. Further measurements are planned for the coming years to confirm this result.

The RWTH research team has been involved in the T2K collaboration since 2007. The Aachen physicists participated in the planning and construction of the T2K experiment and have contributed to the now published results through their research activities. The team receives funding from DFG, the German Research Foundation.

The article Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations, published in "nature", is now available.