Transverse 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:

This definition of the transverse mass is used in conjunction with the definition of the (directed) transverse energy

with the transverse momentum vector ${\displaystyle {\vec {p}}_{T}=(p_{x},p_{y})}$. It is easy to see that for vanishing mass (${\displaystyle m=0}$) the three quantities are the same: ${\displaystyle E_{T}=p_{T}=m_{T}}$. The transverse mass is used together with the rapidity, transverse momentum and polar angle in the parameterization of the four-momentum of a single particle:

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