Information About

Pseudorapidity

\eta = -\ln\left(	an\left(rac{1}{2}	heta
ight)
ight),

where

heta

is the angle relative to the beam axis. It is numerically close to the Rapidity, y, defined in Special Relativity as

y = rac{1}{2} \ln \left(rac{E+p_L}{E-p_L}
ight)

when the particle is relativistic. Here,

p_L

is the component of the momentum along the beam direction. Notice that

\eta

does not depend on the mass of the particle - only on the polar angle of its trajectory.

Here are some representative values:

There is a symmetry about

heta = 90

degrees:

\eta

at

180-	heta

=

-\eta

at

heta

.

The rapidity (or pseudorapidity) is preferred in hadron colliders over the polar angle

heta

because, loosely speaking, particle production is constant as a function rapidity. Furthermore, the difference in the rapidity of two particles is independent of Lorentz Boost s along the beam axis.