Three entries in Spring:Parton_List, RBUF(17), (18), and (19), have different meanings from the previous ones.
Helicities of the primary partons should be stored in
RBUF(17) = IHELwhen necessary as with tau leptons: IHEL=-1 for left-handed and +1 for right-handed 's. Notice that, when this entry is zero, taus decay as they are unpolarized.
Color singlet groups are distinguished by
RBUF(18) = ICFwhere ICF must have different values for different color singlet groups. This entry is meaningful only when RBUF(19) is nonzero. Notice also that the ICF's are not necessarily contiguous.
The parton showering is handled level by level, since showers from parent parton pairs might modify the momenta of their daughter partons. The level is defined by
RBUF(19) = ISLEV*100 + ISPAIRwhere partons with the same ISLEV are passed to LUFRAG at one time (that means they comprise a set of color singlet states and that there is no parent-daughter relation among the set) and partons with the same ISPAIR are paired in LUSHOW calls in LUFRAG. The ISPAIR's of the same ISLEV must be contiguous and must start from 1. On the other hand, the ISLEV itself is not necessarily contiguous, though it must be less than or equal to 10. Lepton pairs are required to have ISLEV = 0 and NDAU = 0 for proper polarization handling when the pair contains a tau. If you are satisfied with the default tau decay treatment, you do not have to distinguish leptons from quarks in the decay daughters from W or Z, etc.
Fig.3.1 shows an example of the color-singlet ID and showering-level setting (RBUF(18) and RBUF(19)) for .
Notice that the bosons have nonzero showering level flags so as to be paired with the b or in the parton-showering. When W momenta change after the parton-showering, the momenta of their daughter partons get modified, regardless of whether they are leptons or quarks. The daughter partons with the modified momenta are then passed to LUFRAG to process the parton-showering or fragmentation in the next level.