The figure is a Mandelstam Diagram. It depicts symbolically

an electron (e)
and a
proton (p)
colliding with one another. In

the resulting interaction the proton (p) is
deflected and converted

into a neutron (n)
while the electron is only deflected (e'). The

angle
through which they are scattered is represented by θ.

Simultaneously, new particles are created whose mass is M_{X}.

The square of the mass of the whole process is
represented

by s.
The square of the momentum transfered from the proton

and neutron is represented by t; from
the electron by Q^{2}.

Stanley Mandelstam first suggested the use of diagrams like
this

one
in 1958.