I consider the laboratory system. The four momentums in this reference system are respectively:
##p^\mu = \big(\sqrt{|p|^2+m^2}, 0, 0, |p| \big)##
##p'^\mu= \big(m, 0, 0, 0 \big)##
##k^\mu = E\big(1, 0, 1, 0\big)##
##k'^\mu = E'\big(1, 0, -\sin \varphi, \cos \varphi \big)##
I used conservation...