A muon is like an electron, but about 200 times heavier. When it stops in a piece of silicon, it gets pulled into the atomic nucleus like a nail to a magnet. The nucleus gains extra energy and starts to "boil": various particles fly out — protons, deuterons, tritons, and alpha particles. It's like a pot of boiling water with steam bubbles bursting out, except instead of steam, you get nuclear fragments.
Scientists from the collaboration at the RIKEN-RAL facility in Japan set up an experiment: they stopped muons in a silicon target and caught the emitted particles with special detectors. The detectors measured the particles' energies and identified what each particle was. For the first time, they managed to measure the spectrum of alpha particles in the low-energy region — previously, that was a gap.
The results were compared with two theoretical models. One model (MEM) describes the proton, deuteron, and triton spectra well, but for alpha particles at high energies it deviates from the experiment. The other model (PHITS) describes alpha particles at low energies reasonably well, but it significantly underestimates the yield of deuterons and tritons. This means that the models need to better account for how particles cluster together when they leave the nucleus.
The new data are like missing puzzle pieces. They show that different particles are emitted from the nucleus in different ways, and this cannot be described by a single simple formula. Physicists will have to refine the models so that they correctly predict the yields of all particles. This is important not only for fundamental science but also for applications, such as in radiation shielding calculations or in medical physics.
A muon lives only 2.2 microseconds, but in that time it manages to hit the nucleus and trigger a whole "fireworks" of particles.