Sector Antenna Clustering

An atennna-based jet clustering algorithm is under development. So far, only a set of elementary 3→2 clustering maps are provided.

The VinClu clustering map

AB->arb The VinClu clustering map is a 3->2 clustering map rather similar to that of the ARCLUS jet clustering algorithm. Its principal advantage is that it contains the exact inverses of the Vincia kinematics mappings, and hence it can be used to "undo" branchings and/or reconstruct the exact intermediate shower states from a given momentum configuration. The clustering is invoked by creating an instance of the class VinClu and using either of the two methods:

(Which to use depends on whether the momenta should be treated as massless or massive.) iMap is the index number of the kinematics map to be used (the numbering follows kineMapType), pIn is a vector of arbitrary size containing the input momentum configuration (in PYTHIA 8's Vec4 format), and the indices a, r, b represent the 3 particles to be clustered in the backwards reconstruction of AB->arb. For the variant with massive momenta, mA and mB should be used to specify the on-shell masses of A and B.

Technically, the clustering is done as follows. pClu is initially set equal to pIn. The reconstructed momenta of the parents, A and B, are then stored in pClu[a] and pClu[b], respectively, and finally pClu[r] is erased. (Note that this last step may cause the indices of A and/or B in pClu to be different from those of a and b in pIn. For the most common case, a colour-ordered clustering with r=a+1 and b=a+2, the indices of A and B after the clustering will be A=a and B=A+1.)