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Invisible particles may give themselves away by a tiny kink in the energy of electrons.
Radioactive tritium decays, emitting an electron. Its energy is almost always the same, but sometimes, just a little less. This shortfall could mean that alongside the ordinary neutrino, another, hidden particle was born.
The Standard Model describes three types of neutrinos, but scientists suspect there is a fourth — the sterile one. It takes part in no interactions except gravity, so it cannot be caught directly. But it can be inferred from a kink in the electron spectrum: like a missing step in an otherwise even staircase.
The KATRINexperiment has already measured the mass of the ordinary neutrino, and now it is switching to the search for the sterile one. For this, they are building the TRISTAN detector — an array of silicon drift detectors capable of counting electrons at an enormous rate.
In four months of operation, KATRIN could spot a sterile neutrino admixture even if it is only one part per million. That is tens of times better than previous searches.
If the kink is found, it will be the first real trace of a particle that could make up dark matter, the invisible substance that holds galaxies together.
