The DEA cut is done in two steps: first, tag the DEA segments, then, either pass or veto the event.
The tagging of the DEA segments is done in the function deaCut_tagDEA(). It determines which segments of DEA are hit by charged tracks by swimming the tracks stored in GROUP_TrackPositions to the DEA front and back faces and checking which DEA segments are hit. The result is encoded into an 8-bit bitmap (4 bits for the front and 4 bits for the back of DEA, each of the 4 bits corresponding to the top, left, right and bottom DEA segments). This 8-bit bitmap is then stored into the bits 8 through 15 of the DEA bitmap of GROUP_VETO_BITMAP.
The results of the tagging can be seen with the event display program- the tagged segments are plotted in a special color (green by default).
The decision to pass or veto the event is made in the function ProcessDEACuts. This function looks at all the DEA segments that were not hit by charged tracks. If any one of them has a hit (presumably from a photon), the event is vetoed.
The vetoing decision rule for one segment is given by in table 1.
| DEA hit | no DEA hit | |
| track hit | keep | keep |
| no track hit | veto the event | keep |