A heavy stone falls into a pond, and ripples spread across the water with a distinctive pattern — from that pattern, you can infer what lies beneath the surface. Physicists study particle decays in much the same way: rare transformations in which a heavy particle suddenly becomes a lighter one, emitting light or a pair of charged particles. In a new study, this process has been calculated for the first time for a particle called the xi-baryon — a heavyweight in the subatomic world.

The xi-baryon contains one heavy b-quark (a particle that, for example, makes up the proton but is much heavier) and two lighter quarks. When the b-quark transforms into another, lighter quark, the whole particle changes: instead of the xi-baryon, you get another particle — the lambda-baryon. Such transformations are described by complex formulas that depend on how charges and currents are distributed inside the particle. These "formulas" are called form factors, and it is precisely these that the authors calculated using a method known as lattice QCD. Lattice QCD is a computational approach in which space and time are divided into tiny cells, like a grid, and the equations are solved on this grid using supercomputers.

The calculations showed how often the xi-baryon decays with the emission of light (a photon) or with the creation of a pair of particles — a muon and an antimuon (heavy relatives of the electron). These decays are extremely rare: roughly once in a billion cases. But it is precisely this rarity that makes them valuable. If an experiment finds that the decay occurs more or less often than the current theory (the Standard Model) predicts, that would be a sign of new particles or forces we don't yet know about.

The result is a map that experimentalists at the Large Hadron Collider and other facilities can use to search for deviations from predictions. Just as the pattern of ripples from a stone can tell you about the depth of a pond, the rare decays of the xi-baryon can help us peer beyond the known physics.