A team from the Institute for Corpuscular Physics (IFIC), a joint center of the Spanish National Research Council (CSIC) and the University of Valencia, has published in the prestigious journal Physical Review Letters a study showing that it is possible to investigate the decay of the particle known as the muon by observing how the emitted neutrinos and antineutrinos scatter off atomic nuclei through the CEvNS process.

Muons are subatomic particles heavier than electrons. Over time, a muon “breaks apart” (decays), giving rise to other particles, including neutrinos and antineutrinos. Neutrinos and their antiparticles, antineutrinos, are fundamental subatomic particles with extremely small masses, close to zero, and no electric charge, which makes them invisible to electromagnetic fields.

When a neutrino collides with the nucleus of an atom, it can cause that nucleus to move. This tiny “push” is what is known as CEvNS (Coherent Elastic Neutrino–Nucleus Scattering).

In the 1970s, Daniel Z. Freedman noted that this process — coherent elastic neutrino–nucleus scattering (CEvNS) — had, according to the Standard Model of particle physics, a higher-than-usual probability of taking place.

However, in the 1970s the phenomenon was impossible to observe, since its only signal is the slight recoil of the nucleus after the “collision” with the incoming neutrino or antineutrino. Thanks to experimental advances, the COHERENT collaboration was able to measure this process for the first time in 2017, using neutrinos produced at Oak Ridge National Laboratory (USA). Since then, COHERENT has observed the phenomenon with different detectors and improved the initial precision.

Now, a new study published in Physical Review Letters, led by a team from IFIC, shows for the first time that COHERENT data can also be used to analyze important aspects related to neutrino production, including possible new interactions. In particular, the team shows that it is possible to study muon decay by observing how the emitted neutrinos and antineutrinos scatter off atomic nuclei through this “knock” process known as CEvNS. Using COHERENT experimental data, the Michel parameters — which describe the energy distribution of the antineutrino — have been extracted for the first time. These parameters were introduced 75 years ago for electrons and 30 years ago for neutrinos, and are now extended for the first time to antineutrinos.

The study was carried out by an IFIC team composed of Sergio Cruz-Alzaga, Martín González-Alonso, and Suraj Prakash, in collaboration with Víctor Bresó-Pla, a postdoctoral researcher at Harvard University, who obtained his doctorate at IFIC in 2023 and conducted part of this research while at the University of Heidelberg. “In the first publication we have focused on muon decay, assuming lepton flavor conservation, with the aim of presenting the idea and showing its usefulness,” the authors state. In addition, in a second study published in the Journal of High Energy Physics (JHEP), the team expands the analysis through an exhaustive examination of all possible interactions.

“Our approach can also be applied to recent CEvNS measurements with reactor and solar neutrinos to study their respective production processes, although this will require a specific analysis due to the particularities of each case,” the authors note.

Taken together, these studies reveal an interesting connection between neutrino physics and flavor physics, expanding the use of CEvNS measurements as a new tool for fundamental physics. Moreover, CEvNS measurements open the door to practical applications such as remote monitoring of nuclear reactors and the development of compact neutrino detectors.

References:

V. Bresó-Pla, S. Cruz-Alzaga, M. González-Alonso and S. Prakash, “Muon-Decay Parameters from COHERENT,” Phys. Rev. Lett. 135 (2025) 13, 13.
https://doi.org/10.1103/mlhl-v822

V. Bresó-Pla, S. Cruz-Alzaga, M. González-Alonso and S. Prakash, “EFT analysis of New Physics at COHERENT with Dirac neutrinos,” JHEP 12 (2025) 007.
https://doi.org/10.1007/JHEP12(2025)007