Unitarising Matrix Element + Parton Shower merging
  We revisit the CKKW-L method for merging tree-level matrix elements with parton showers, and amend it with an add/subtract scheme to minimise dependencies on the merging scale. The scheme is constructed to, as far as possible, recover the unitary nature of the underlying parton shower, so that the inclusive cross section is retained for each jet multiplicity separately. 
Leif Lonnblad and Stefan Prestel
hep-ph hep-ex