where is the angle between the particle momentum and the beam axis. Inversely,
In terms of the momentum, the pseudorapidity variable can be written as
where is the component of the momentum along the beam axis. In the limit where the particle is travelling close to the speed of light, or in the approximation that the mass of the particle is nearly zero, pseudorapidity is numerically close to the experimental particle physicist's definition of rapidity,
This differs slightly from the definition of rapidity in special relativity, which uses instead of . However, pseudorapidity depends only on the polar angle of its trajectory, and not on the energy of the particle.
In hadron collider physics, the rapidity (or pseudorapidity) is preferred over the polar angle because, loosely speaking, particle production is constant as a function of rapidity. One speaks of the "forward" direction in a hadron collider experiment, which refers to regions of the detector that are close to the beam axis, at high
The rapidity as a function of pseudorapidity is given by
The difference in the rapidity of two particles is independent of Lorentz boosts along the beam axis.
Here are some representative values:
0° ∞ 180° −∞ 0.1° 7.04 179.9° −7.04 0.5° 5.43 179.5° −5.43 1° 4.74 179° −4.74 2° 4.05 178° −4.05 5° 3.13 175° −3.13 10° 2.44 170° −2.44 20° 1.74 160° −1.74 30° 1.32 150° −1.32 45° 0.88 135° −0.88 60° 0.55 120° −0.55 80° 0.175 100° −0.175 90° 0
Pseudorapidity is odd about degrees. In other words, .
Conversion to Cartesian Momenta 
Hadron colliders measure physical momenta in terms of transverse momentum , polar angle in the transverse plane and pseudorapidity . To obtain cartesian momenta (with the -axis defined as the beam axis), the following conversions are used:
- Introduction to High-Energy Heavy-Ion Collisions, by Cheuk-Yin Wong, See page 24 for definition of rapidity.