A differential description of colour singlet production at N3LO requires
a scheme to handle infrared divergences. I will discuss the idea behind
the method of "phase space slicing", where one uses a suitable
observable to subdivide the phase space according to the infrared
properties of the squared matrix elements. I will identify the necessary
building blocks for such a calculation and explain how to calculate one
such building block - the beam functions for zero-jettiness.