In January 2018, a team of 9 bachelor students, led by researcher Shin’ichiro Ando and postdoc Bradley Kavanagh, investigated radiative neutrino decay and its impact on the cosmic microwave background — relic radiation from the hot Big Bang, almost fourteen billion years ago. The team set precise constraints on the neutrino lifetime, which was found to be orders of magnitude longer than the age of the universe. Shin’ichiro Ando, who supervised the group, said: “Theoretical physics and astrophysics might have a relatively short span in publishing papers compared with other fields, but still, supervising bachelor students for only for four weeks to get an outcome worth publishing was an extraordinary challenge. The students were indeed responsible for a major part of the paper. They were extremely motivated and worked completely professionally as a group. I am proud of their achievement.”

Neutrino lifetime

Neutrinos, nearly massless elementary particles, might secretly interact with photons – particles of light – through their magnetic moments. Such interactions induce decays of a heavier neutrino into a lighter neutrino and a photon. If the relic neutrinos that are left from the Big Bang, and that currently fill the entire universe, decay, the emitted photons will have microwave frequency. Since the cosmic microwave background is known through observations to look like the radiation of a nearly perfect black body, even a tiny distortion in its emitted spectrum of wavelengths through the neutrino decay is not allowed. The team used the data from the COBE-FIRAS instrument to place precise constraints on the neutrino decay lifetime, and also made projections for future cosmic microwave background projects such as PIXIE. The obtained constraints are very precise and stringent, as competitive as or even stronger than the state-of-the-art laboratory constraints of the neutrino magnetic moment.

Great experience

The work was carried out in the context of the 4-week long ‘ITFA Workshop’ course in January 2018, which is part of the joint bachelor programme in Physics and Astronomy of the University of Amsterdam and the Vrije Universiteit Amsterdam. Dylan van Arneman, one of the participating students, said: “I really enjoyed working on this project. Not only did I learn the basics of writing a paper, but it also helped me understand the structure of scientific papers, which allowed me to be able to read and interpret papers better. It was really nice to see how the three sub-groups, who were in principle doing things completely independently of each other, managed to share their results and findings with the other groups every week to, in the end, make a nice flowing final project.”

Bradley Kavanagh, one of the supervisors, added: “Supervising the workshop was a great experience. The students all seemed engaged and motivated by the idea of doing a real research project, with a well-defined aim and the possibility of writing a paper at the end. At times, supervising the workshop was challenging. We couldn't necessarily plan everything in advance. We had to constantly adapt and adjust the lectures and classes, for example, if the students were progressing more quickly than expected or if they needed more time and support to solve a particular problem. In the end, we were very impressed by the high quality of research the students produced!”

Reference

The outcome of the project has been summarized as a paper, which was published in Physical Review D on 2 July.

Precision constraints on radiative neutrino decay with CMB spectral distortion, Jelle L. Aalberts, Shin’ichiro Ando, Wouter M. Borg, Edwin Broeils, Jennypher Broeils, Stephen Broeils, Bradley J. Kavanagh, Gijs Leguijt, Marnix Reemst, Dylan R. van Arneman, and Hoang Vu, Phys. Rev. D 98 (2018) 023001.