# Missing transverse energy, exact definition

This might seem basic, but it is a bit confusing. You hear about missing transverse energy a lot in SUSY searches due to the LSP which cannot be detected.

Let's say I have the 4-vector for the LSP. How does one go about calculating missing transverse energy the 4-vector?

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 I am assuming I perform the calculation theoretically. Then, I can get the exact 4-vector of the outgoing LSP. How would I convert that to the MET variable? How is MET defined? Intuitively, I would just use px py components, but it is missing transverse ENERGY, not missing transverse momentum. – user788171 Oct 23 '12 at 21:18 @user788171 I assume you would simple project the energy along the momentum vector, but you say "in SUSY searches" which implies that someone must be thinking about an experimental component, and that introduces some additional difficulties as I've outlines. – dmckee♦ Oct 23 '12 at 21:25 To leading order I think you just assume the mass of the particle is negligible so that $E = pc$. There is certainly precedent for particle physicists saying "energy" when they mean "momentum." – David Zaslavsky♦ Oct 24 '12 at 3:20