Cluster and tag the final state particles in an event represented by a Graphicle object. These clusters are formed by considering the topology of the directed acyclic graph (DAG) generating the event, tracking descendants of hard partons, and the momenta of the hard partons compared against the final state particles.

The steps defining this algorithm are summarised:

• Find descendants of all hard partons within DAG

• Remove final state radiation from descendants

• Where descendants of multiple hard partons annihilate color with each other, assign exclusive parentage of subsequent color neutral particles to closest hard parton in the pseudorapidity-azimuth (\(\eta-\phi\)) plane

• Where background is used to annihilate color of hard parton descendants, remove all final state particles beyond a distance of radius from the position of the hard parton in the \(\eta-\phi\) plane

Parameters:¶

graph : Graphicle¶

Full event record as a DAG.

radius : float¶

Radius in the \(\eta-\phi\) plane defining the enclosed clustering region around hard partons which are color-connected to the underlying event. See notes for more information.

Returns:¶

Flat MaskGroup, containing MaskArray instances which reconstruct the hard partons from the final state particles.

Return type:¶

MaskGroup[MaskArray]

Notes

Hard partons may be color connected with each other, or with the underlying event. eg. for a hierarchical clustering:

MaskGroup(agg_op=OR)
├── t
│   ├── b
│   └── W+
│       ├── c
│       └── s~
└── t~
    ├── b~
    └── W-
        ├── s
        └── c~

the quarks decaying from the W bosons are color-connected to each other, as they form from a color-singlet. This means they will almost certainly annihilate their color with each other during hadronisation. However, the top quarks are color connected to the underlying event, and therefore will almost certainly annihilate their color with partons that do not descend from the hard process. This is done via proxy of the bottom quark, which inherits its color from the top. This results in background radiation in the descendants tree of the bottom quark, spread over a wide region of the \(\eta-\phi\) plane. This is when the radius parameter is applied, excluding all final state descendants whose distance from the hard bottom quark exceeds the value passed, cleaning up the signal.