Something is amiss in particle physics: two different methods of measuring the lifespan of neutrons yield substantially different results. This mystery has remained unsolved for fifteen years. There has been intense international debate recently regarding a potential explanation: could it be that some neutrons decay into dark matter - invisible particles that have so far been impossible to measure?
Scientists at TU Wien have been pursuing this theory. Large quantities of data from high-precision neutron experiments have been re-analysed and additional experiments have been conducted - but no dark matter has been encountered. On the contrary, 95% of the energy field in which the dark matter could theoretically be concealed could be definitively ruled out. There must be other reasons for the discrepancies in the measurements of neutron lifespans.
The average neutron's lifespan is barely fifteen minutes
The neutrons in an atomic nucleus remain largely stable, but free neutrons which are not bound to other particles decay after a few minutes. This decay creates protons, electrons and neutrinos. “There are essentially two different ways of measuring the average lifespan of free neutrons,” explains Prof. Hartmut Abele from TU Wien Institute of Atomic and Subatomic Physics. “You either try to hold the neutrons in place and count how many of them remain after a while. Or you watch out for decaying material and count the number of decays.”
However, strangely enough, the results of the two methods are different: For one the measurement is 879 seconds, for the other it is 888. The discrepancy can be explained if we assume that there is an additional, previously unknown, type of neutron decay that reduces the number of neutrons, but does not produce any decaying material, which we have previously been looking for. Around one percent of neutrons would have to decay in this somewhat exotic fashion to explain the measurement results.
This hypothesis is appealing as it could provide clues to the mysterious dark matter - previously undetected particles which, according to astronomic measurements, must exist in the universe but have so far remained undetected.
“On account of other observations, such as the fact that neutron stars exist, there is only a certain energy range in which these undetected particles could theoretically exist,” says Erwin Jericha from TU Wien's Institute of Atomic and Subatomic Physics, who also worked on the data analysis. “We have now analysed the remaining energy range to the best of our ability.”
In these experiments, it is crucial to analyse the energy of the electrons which are created in the neutron decay. If some neutrons decay into dark matter, this would have an impact on the energy distribution of the electrons - and this energy distribution can be deduced from the results of previously conducted experiments. In Grenoble, the PERKEO detector system, developed by Hartmut Abele as part of his dissertation, was used to collect data on neutron decay. Also involved in the measurements were T. Soldner, who was responsible for the polarisation analysis, and B. Märkisch from the Technical University of Munich.
The data from these experiments has now been re-analysed by the team at the Institute of Atomic and Subatomic Physics together with research groups from the Technical University of Munich and ILL. Michael Klopf, who worked with Hartmut Abele on his dissertation, developed extensive computer simulations and, together with Heiko Saul, analysis programs that allow the distribution of electron energy to be extracted from existing data. Further experiments were also conducted at TU Wien.
No trace of dark matter
Nevertheless, the data was found to be in excellent agreement with the standard model of particle physics - with no trace of any dark matter. “For 95% of the theoretically possible energy field we can rule out the possibility that neutrons decay into previously undetected particles,” said Hartmut Abele. “This theory therefore now appears to be rather implausible. It is far more likely that the discrepancies between the different methods for measuring the lifespan of neutrons can be attributed to systematic errors previously evaluated.”
And so the search for the mysterious dark matter continues. Nevertheless, it has once again been confirmed that precision measurements with neutrons are perfectly suited to the pursuit of fundamental questions in physics.
Prof. Hartmut Abele
Institute of Atomic and Subatomic Physics
Stadionallee 2, 1020 Wien