The trace of a short-lived particle is like a signature left in the detector. The particle is born and decays in an instant, but from its trace you can reconstruct its character — whether it's compact like a stone or fluffy like a snowball. That's exactly what this work is about: the authors propose a new way to read such signatures.

They noticed that each particle has an angle that links its mass and lifetime. This angle dictates what phase its trace should have in the experiment. The phase is a wave shift, as if two identical waves met but one arrived a fraction of a second late. Previously, the phase was often fitted to the data, but now it can be calculated in advance, without any models.

A test on the N(2190) particle showed that the sign of the phase in old data was flipped. After correcting the error, the physicists obtained a phase of −28°±10°, which exactly matched the prediction. It's like reading a signature on a document in a mirror for a long time, then flipping it over and seeing that it perfectly matches the original.

Now scientists have a simple and rigorous test: is it a compact particle or a molecule made of smaller parts? This will help sort out the zoo of new particles discovered at accelerators. And perhaps find ones we never even suspected.