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: m T 2 = m 2 + p x 2 + p y 2 = E 2 − p z 2 {\displaystyle m_{T}^{2}=m^{2}+p_{x}^{2}+p_{y}^{2}=E^{2}-p_{z}^{2}} where the z-direction is along the beam pipe and so p x {\displaystyle p_{x}} and p y {\displaystyle p_{y}} are the momentum perpendicular to the beam pipe and m {\displaystyle m} is the (invariant) mass.
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: m T 2 = m 2 + p x 2 + p y 2 = E 2 − p z 2 {\displaystyle m_{T}^{2}=m^{2}+p_{x}^{2}+p_{y}^{2}=E^{2}-p_{z}^{2}} where the z-direction is along the beam pipe and so p x {\displaystyle p_{x}} and p y {\displaystyle p_{y}} are the momentum perpendicular to the beam pipe and m {\displaystyle m} is the (invariant) mass.
Source: Wikipedia "Transverse mass" · CC BY-SA 4.0
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