Changes between Version 6 and Version 7 of DevelopmentPage/CutTools

Timestamp:

Mar 3, 2010, 4:56:07 PM (15 years ago)

Author:

Valentin Hirschi

Comment:

--

DevelopmentPage/CutTools

@@ -31 +31 @@
 == First optimizations to be brought ==
 
-These are the most significant optimizations which can be brought once the raw procedure proved to work.
+These are the most significant optimizations which can be brought only '''once the raw procedure was proved to work'''.
 
 === 1) Reusing wavefunctions entering the loop ===
 
-We will refer to the tree structures attached to loops as Feynman Diagram structures, abbreviated FD-structures. The FD-structures contain external particles and one off-shell current which feeds the loop. 
+We will refer to the tree structures attached to loops as Feynman Diagram structures, abbreviated FD-structures. The FD-structures contain external particles and one off-shell current which feeds the loop.
+CutTools calls the amplitude of the cut-diagrams many times for the same phase-space point but with different loop momenta ''q''. Since the FD-structure are independent of the loop momentum, recycling them for each computation of the amplitude with a different loop momentum would save a lot of time. The gain in efficiency can be rather significant here since the number of evaluation of the cut-diagram amplitude per phase-space point is typically between 10 and 20.
+
+=== 2) Summing color-ordered structures attached to a loop before calling CutTools to compute it. ===
+
+What characterizes a loop for CutTools is only a set of momenta and masses for the loop lines. It is then possible to call CutTools with loop amplitudes computed with the sum of the matching color-ordered structures attached to the loop instead of calling CutTools for each structure within this sum.
+This might bring a very significant improvement for large multiplicities processes and small loops, for which there are many color ordered substructures contributing to the same structure attached to the loop. Recognizing these matching structures and organizing the computation with smart calls to CutTools is not straight-forward and will only be implemented in a second stage as an optimization update. But keep in mind that for each loop computation, CutTools calls the cut-diagram amplitude up to 20 times so it is worth trying to spare any single call to CutTools.