5.2.1 Barrel Detector Algorithm

We have seen that the drift chambers are composed of Superlayers made of groups of four staggered layers of drift tubes packed together. This design assures the redundancy needed in the detector, but it was also chosen on purpose to allow the parent bunch crossing identification in the trigger.

Infact bunch crossing is detected using a generalized mean-timer method. Its principle is shown in Figure 13.

Figure 13: Exemplification of the generalized meantimer method.

The sketch on the left-hand side shows a particle crossing a group of four tubes and generating four drift times. The one on the right-hand side shows a group of four shift registers each one associated to a drift cell. The signals generated in each cell are shifted in the register: the four hits are aligned always at a fixed time TMAX, equal to the maximum drift time in the cell, after the muon crossing time, thus uniquely identifying the parent bunch crossing. The track inclination and the crossing position can be determined by the shift times ts = TMAX - td giving a rough track measurement.

The actual algorithm is described elsewhere[6] and ASIC prototypes were recently produced and tested. The bunch crossing tagging efficiency measured using the prototypes is shown in Figure 14 as a function of the incident angle for a sample of drift times patterns provided generating single muons inside the detector with full background simulation.

Figure 14: Bunch crossing detection efficiency for a set of ASICs recently tested. The abscissa scale is not linear.