The
transverse mass is a useful quantity to define for use in
particle physics as it is invariant under
Lorentz boost along the z direction. In
natural units it is:
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=3965)
- where the z-direction is along the beam pipe and so
and
are the momentum perpendicular to the beam pipe and
is the (invariant) mass.