Körrapidity

Körrapidity is a term used in particle physics to describe the rapidity of a particle produced in a high-energy collision, specifically in the context of a central rapidity region. Rapidity itself is a relativistic measure of velocity, defined as $y = \frac{1}{2} \ln\left(\frac{E+p_z}{E-p_z}\right)$, where E is the particle's energy and $p_z$ is its momentum along the beam axis. The rapidity is advantageous because it is additive for particles moving in the same direction, similar to how velocities add in non-relativistic mechanics.

The central rapidity region is the portion of the detector's acceptance where the beam particles have been significantly disrupted and have produced a shower of secondary particles. Particles with rapidities close to zero are typically considered to be in this central region. Körrapidity is often used in experiments to define a specific range of rapidity that is of interest for analysis, usually corresponding to the most central collisions or the region where the highest particle multiplicities are observed. This terminology helps to isolate and study particles that are produced from the core interaction rather than those that retain a significant fraction of the initial beam momentum. It is a way to focus on the particles emerging from the most energetic part of the collision.