particle physics - What is Transverse Energy?

What is transverse energy? Why we use transverse total energy instead of energy and transverse momentum in place of Total momentum in the particle detectors?

Answer

Transverse momentum, $\vec{p}_T$, is the momentum of an object transverse to the beam. Transverse energy is defined as $E_T = \sqrt{m^2+p_T^2}$ for an object with mass $m$ and transverse momentum $p_T$.

The initial longitudinal momentum in a parton collision is unknown, because the partons that make up a proton share the momentum. We do know, however, that the initial transverse momentum was zero. So we look for missing transverse momentum, defined $E_T^\textrm{miss} = -\sum_i \vec{p}_T(i)$ for visible particles $i$. Finding missing transverse momentum would indicate that new, unaccounted for particle(s) had escaped the detector.

Confusingly, $E_T^\textrm{miss} = -\sum_i \vec{p}_T(i)$ is commonly called missing transverse energy or MET. Missing transverse energy is equivalent to missing transverse momentum only if the missing particle(s) were massless.

Also, events in which the products have large transverse momentum are more likely to be genuine, interesting events.