Neutrinos are the most elusive of all known particles. Every second, trillions of neutrinos from the Sun fly through your body, but you never notice: they barely interact with matter. To catch even one, physicists build giant detectors — pools of water or liquid argon deep underground. When a neutrino does collide with an atomic nucleus, it knocks out a particle, and by tracking that particle, scientists can figure out what happened.
The precision of such measurements hinges on knowing one quantity: how strongly a neutrino "pushes" a proton or neutron. This quantity is called the axial form factor, and until now it has been determined with an uncertainty of up to 20%. Too crude for the needs of modern physics. The authors applied a method previously used for other particles: lattice calculations — where space and time are divided into tiny cells and the interaction is modeled on a supercomputer. This calculation yields a mathematically rigorous result with full accounting of all uncertainties.
The scientists averaged several such calculations made by different groups and compared them with data from older experiments where neutrinos scattered off hydrogen and deuterium. The agreement was good but not perfect: lattice calculations give slightly lower values than some experiments. This discrepancy is not an error but a reason to dig deeper. Perhaps some subtle effects were not accounted for in the experiments, or the theoretical model needs refinement.
Now physicists have a more reliable foundation for analyzing data from future giant detectors such as DUNE in the US or Hyper-Kamiokande in Japan. The more precisely we know how neutrinos interact with matter, the better we can solve their biggest mysteries: why there are so many of them, how they change flavor on the fly, and why the universe has more matter than antimatter.
Neutrinos can pass through a light-year-thick layer of lead without hitting a single atom.