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?

• The existing question What is p_T? (transverse momentum?) basically answers the "why" question and the "what" question as well by a simple extension. – dmckee Apr 15 '13 at 14:33
• Did you try to Google transverse energy? – Qmechanic Apr 15 '13 at 20:32
• I tried both the existing question as well as google but not able to understand well. – ramkrishna Apr 16 '13 at 3:50

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.