KATRIN measures the tritium beta decay spectrum near its endpoint — the region where the neutrino mass imprints itself on the electron energy distribution. The experiment's primary goal is measuring the neutrino mass. But the same spectrum is sensitive to something else: if a heavier sterile neutrino exists, it produces a kink in the spectrum at an energy determined by its mass.
The TRISTAN upgrade (arXiv:2603.23256) replaces KATRIN's focal plane detector with a high-rate silicon detector array designed to measure the full beta spectrum, not just the endpoint. This extends the search from the eV-scale (where KATRIN currently operates) to the keV-scale, where sterile neutrinos are candidates for dark matter.
The sensitivity analysis quantifies exactly what KATRIN+TRISTAN can see: for each sterile neutrino mass, the minimum mixing angle that would produce a detectable spectral distortion. The reach covers a parameter space that is theoretically motivated (keV sterile neutrinos from resonant production in the early universe) and experimentally unexplored.
The through-claim: the same experimental apparatus, with a detector upgrade, searches two different mass scales for two different physics goals. The tritium source doesn't change; the information was always in the spectrum. The original detector threw away most of the spectrum to focus on the endpoint. The upgrade recovers the discarded information, and that information constrains a different particle.