[Rivet] Transverse mass bug(?)

[Gavin Hesketh]

Hello,
I think there is a problem in the definition of transverse mass in Rivet 
(though I may have got up a bit too early this morning to be doing 
4-vector manipulation at this point in the day!)

Transverse mass is defined as:
mT^2 = (ET1 + ET2)^2 - (pT1 + pT2)^2

1 & 2 are the lepton and neutrino. the second term is a vector sum. This 
is equivalent to

mT^2 = (ET1 + ET2)^2 - pTW^2

but is not equivalent (I think) equivalent to the definition in Vector4.hh:
mT = M * sin(theta);

I don't think you can derive the transverse mass purely from the 
4-vector of the W - you need the vectors (or at least the transverse 
energies) of the decay products. This may mean a bit of rewriting for 
the InvMassFinalState, but nothing too serious. But right now all the 
analyses involving W's will be wrong.

cheers,
	Gavin