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From China to the South Pole: Joining forces to solve the neutrino mass puzzle

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One of the most interesting problems of modern physics is the determination of the neutrino mass order. Physicists from the PRISMA + cluster of excellence at Johannes Gutenberg University of Mainz (JGU) play a leading role in a new study that indicates that the mystery of ordering the neutrino mass can be finally solved in the next few years.This will happen thanks to the joint implementation of two new neutrino experiments, which are under development-modernization of the IceCube experiment at the South Pole and the Jiangmen Underground Neutrino Observatory (JUNO) in China. Soon they will give physicists access to much more sensitive and complementary data on the neutrino mass order.

Neutrinos are chameleons among elementary particles.


Neutrinos are produced by natural sources - for example, in the bowels of the Sun or other astronomical objects - as well as in huge quantities by nuclear power plants. However, they can pass through ordinary matter - such as the human body - almost unhindered, without leaving a trace of their presence. This means that extremely complex methods are required that require the use of massive detectors to observe random rare reactions in which these "ghost particles" are involved.

There are three different types of neutrinos: electronic, muonic, and tau neutrinos. They can vary from one type to another, and scientists call this phenomenon “neutrino oscillations”. The mass of particles can be determined by observing the vibrational patterns. For many years, physicists have tried to establish which of the three types of neutrinos is the lightest and which is the heaviest. Professor Michael Wurm, a physicist from the PRISMA + cluster of excellence, who plays an important role in conducting the JUNO experiment in China, explains: “We believe that the answer to this question will make a significant contribution and will allow us to collect long-term data on the violation of the symmetry of matter and antimatter in neutrino sector. Then, using this data, we hope to find out once and for all why matter and antimatter have not completely destroyed each other after the Big Bang. ”

Global collaboration is profitable


Both large-scale experiments use very different and complementary methods in order to solve the puzzle of ordering the neutrino mass. “The obvious approach is to combine the expected results of both experiments,” says Professor Sebastian Bezer of the PRISMA + cluster, also a neutrino researcher and one of the main participants in the IceCube experiment.

No sooner said than done. In the journal Physical Review D, researchers from IceCube and the JUNO collaboration published a joint analysis of their experiments. For this, the authors modeled the predicted experimental data depending on the measurement time for each experiment. The results vary depending on whether the neutrino masses are in the normal or reverse (inverted) order. Physicists then conducted a statistical test in which they applied a combined analysis to the simulated results of both experiments. This revealed the degree of sensitivity with which both experiments in combination could predict the correct order or, rather, eliminate the wrong order. Since the observed oscillation patterns in JUNO and IceCube depend on the actual ordering of the neutrino mass in a way specific to each experiment,the combined test has a distinguishing ability significantly higher than the individual experimental results. Thus, this combination will finally eliminate the incorrect ordering of the neutrino mass during the measurement period from three to seven years.

“In this case, the whole is really larger than the sum of its parts,” concludes Sebastian Bezer. “Here we have clear evidence of the effectiveness of the complementary experimental approach when it comes to solving the remaining neutrino puzzles.” “No experiment can achieve this on its own, whether it is an update to IceCube, JUNO or any other of the currently launched,” adds Michael Wurm. “Moreover, it just shows what physicists can achieve by working together.”

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