Is mass conserved in relativistic collision?

[DrGreg]

Dale, I am assuming you can add these just like regular vectors? But magnitudes would be done by squaring each term and adding them with the +,-,-,- or -, +,+,+ signatures? (basically a dot product)

And is the invariant mass such a magnitude?

Yes to all 3 questions.

But for the 2nd question, you also need to square-root the absolute value of the final answer (as I think you know but forgot to say!).