pseudorapidity

At a collider, one of the simplest things you can measure about a particle is the angle at which it flies away from the beam line. A particle shooting out sideways, at ninety degrees to the beams, is easy to catch; one skimming along close to the beam pipe is hard to see and easy to miss. Pseudorapidity, written with the Greek letter eta, is just a convenient relabelling of that angle, chosen so that the numbers behave nicely in the relativistic world of the collision.

It is defined purely from the angle to the beam: a particle going straight out sideways has pseudorapidity zero, and as the particle tips toward the beam direction, eta climbs — about 1 at thirty-some degrees, about 2.4 near where many detectors stop, and toward infinity for something running right along the beam. Pseudorapidity is the simple-to-measure cousin of true rapidity: for fast particles whose energy dwarfs their mass — which is nearly all of them at a high-energy collider — the two are almost identical, and eta has the great virtue of needing only an angle, not the particle's mass or energy.

Eta is the everyday coordinate of collider experiments. Detector coverage is quoted in eta, particles are grouped into regions like central and forward by their eta, and the standard angular map of a collision uses eta and the azimuthal angle around the beam. Because differences in true rapidity are unchanged by boosts along the beam — the very boosts that randomly shake hadron collisions — eta inherits nearly that same convenience, which is why it is plastered all over experimental papers and detector diagrams.