Higgs coupling grows with particle mass

Most forces play favorites by charge: the photon couples to anything electrically charged, and equally to a particle and its lighter cousin if they share the same charge. The Higgs is different in a beautiful and very telling way: it couples to particles in proportion to their mass. The heavier a particle, the more strongly it interacts with the Higgs; the lighter it is, the more feebly. A top quark, the heaviest particle we know, grabs the Higgs hard, while a light electron barely notices it. This is not a coincidence — it is the signature of a particle whose whole job is connected to mass.

The reason is built into the Higgs mechanism. A particle's mass is literally how strongly it couples to the Higgs field, multiplied by the field's vacuum value. So 'mass' and 'Higgs coupling' are two names for the same underlying quantity, just measured differently: a heavy particle is, by definition, one that interacts strongly with the Higgs. This makes the theory remarkably predictive. Once you know a particle's mass, you know exactly how strongly it should couple to the Higgs, with no freedom to adjust. For the force carriers the rule is slightly different (their coupling grows with the square of their mass), but the same theme holds: heavy means strongly coupled.

This pattern is one of the sharpest tests of whether the particle found in 2012 is the genuine Standard Model Higgs. Experimenters plot the measured coupling strength of each particle against its mass and check that they fall on the predicted straight line — and so far, from the heavy top quark down to the lighter tau lepton and bottom quark, they do, across orders of magnitude in mass. A measured coupling that fell off the line would be dramatic evidence for new physics. The honest limit: couplings to the lightest particles (the electron, the up and down quarks) are far too weak to measure directly yet, so the pattern is confirmed only for the heavier particles.