Abstract
We re-examine the kinematic variable m\_T2 and its relatives in the light of
recent work by Cheng and Han. Their proof that m\_T2 admits an equivalent, but
implicit, definition as the `boundary of the region of parent and daughter
masses that is kinematically consistent with the event hypothesis' is
far-reaching in its consequences. We generalize their result both to simpler
cases (m\_T, the transverse mass) and to more complex cases (m\_TGen). We further
note that it is possible to re-cast many existing and unpleasant proofs (e.g.
those relating to the existence or properties of "kink" and "crease" structures
in m\_T2) into almost trivial forms by using the alternative definition. Not
only does this allow us to gain better understanding of those existing results,
but it also allows us to write down new (and more or less explicit) definitions
of (a) the variable that naturally generalizes m\_T2 to the case in which the
parent or daughter particles are not identical, and (b) the inverses of m\_T and
m\_T2 -- which may be useful if daughter masses are known and bounds on parent
masses are required. We note the implications that these results may have for
future matrix-element likelihood techniques.
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