The Second Level Trigger

The SLT has access to a more complete and precise set of data than the FLT by virtue of the longer timescale on which it operates. It is currently envisaged[41] _{that the}

Global Second Level Trigger (GSLT) box[42] [43]_{will make an event decision available}

to components around 7 ms after the beam crossing. Unlike the FLT, the SLT is

asynchronous: different parts of the system are at any given moment analyzing data which was not all acquired at the same time.

4.3.2.1

The algorithm for the CTDSLT[44] [45] [46] [47] [48] _{proceeds in two stages: segment}

finding[49] _{and track finding}[50]_{. Segment finding is the grouping of hits in an eight-}

wire cell to produce small portions of tracks: these are then combined to form a complete track. The pulse heights from the DSPs (section 3.2.3.1) will enable electron tracks to be identified when the events are fully reconstructed because their characteristic dE/dx differs from that of other charged particles.

Drift times are the input to the CTDSLT which resides on a network of transputers. These are microprocessors with four bidirectional communication channels which mean that a wide range of topologies are available. They have their own language (occam[51]_{) which is deigned to fully exploit the inherent parallelism of the networks.}

For applications in the CTDSLT, factors of four improvements in time requirements have been measured using occam[52] _{as compared to more conventional languages.}

In axial SLs only, hits in each cell are examined to find track segments. Each cell is considered in turn, and the ‘single cell mask’ is stepped around the whole chamber.

4.3 The Trigger

4.3 The Trigger

‘Roads’ are defined so that the drift time at the nest wire is predicted from the previous hit on a segment. The gradient, intercept, variance and the mean z and r coordinates are passed on to the track finding stage.

The track finding sorts segments in overlapping octants making use of their angular values to consider groups likely to be on the same track. Three-dimensional tracks are formed from z-by-timing data associated with rφsegments via a straight line fit inrz. The CTDSLT will send two tables of results to the GSLT. Exit point and direction and pt will be available with error estimates for each track that has been found. Also

the charge and origin will be known. The vertex for the event as a whole is calculated, as is the total number of tracks found together with an estimate of how many tracks were missed (from the number of unused segments).

The present design of the FTDSLT envisages a tree search method which will be implemented in online memory. It will identify coordinate outputs from the chamber corresponding to straight tracks from the interaction region.It will require one cell hit in each layer: this corresponds to a polar angle requirement of 7◦ ≤θ ≤ 30◦. The FTDSLT should find all such tracks with momentum over1 GeV/ccoming from within

20 cmof the vertex.

4.3.2.2

As is common in the SLT as a whole, transputer networks are used for readout and triggering[53]_.

Timing of energy deposition in the calorimeter is very precisely measured at the second level. Because the distance from the interaction region is not the same for the FCAL and the RCAL there will be a 2 ns difference in arrival times for good physics events. More importantly, most beamgas events originate from upstream of the interaction point at negative z-coordinates. These are expected to produce a difference in arrival times of12 ns[54]. This permits discrimination between physics and background. Prior to this enhancement of capability, the design called for those calorimeter towers around the active beampipe region to be disbarred from setting isolated electron bits because of the intolerable leakage rate that would result. With

4.3 The Trigger

this timing information however it appears that this restriction may be relaxed thus improving efficiency. In addition, events with unphysical longitudinal momentum will be vetoed.

4.3.2.3

Other components are in communication with the GSLTB. It is clearly to be expected that the quality and quantity of information available at the second level will in general be superior to that at the FLT.

GFLTB The GFLTB sends the results of its calculations to the GSLTB along with component data and the FC information.

BAC Eight-bit 10MHz FADCs sum charges over two successive beam crossings. Two networks of transputers will be used: one will be in communication with the GSLT and the other with the EVB[55]_{. If an energy threshold is met, cluster data}

will be sent to the GSLT. Also, a muon trigger is formed from coincidence logic in the bottom yoke where there are no muon chambers. The data should in general be more precise than that from the BACFLT.

BMUON Coordinates of found muons should be available.

FMUON The FMUSLT will make an estimation of momentum from the sagitta of the particle found at the first level using the LT planes.

LPS A bit will be sent to the GSTLB to confirm or negate the LPSFLT. Further, a measurement of the proton momentum is made and is expressed as a fraction of the beam momentum. Horizontal and vertical projections of the transverse momentum of the proton are supplied.

LUMI The measurements made at the first level remain available. Further, the location of electromagnetic shower centres is measured and also photon shower centres if the bremsstrahlung flag is up.

4.3 The Trigger