3.3 Calorimeter Trigger Algorithms

The calorimeter trigger receives ET and fine grain profile information from the calorimeter electronics and finds isolated and non-isolated electrons or photons, t candidates, jets and missing ET. The geometry of calorimeters and their mapping to the trigger electronics and algorithms has been chosen to provide the best performance, i.e., meet the DAQ requirement for its output rate and high efficiencies for discovery physics, while keeping the system size and costs at a reasonable level. The calorimeter trigger algorithms are implemented in the Trigger Primitives Generator (TPG), Regional Calorimeter Trigger (RCT) and the Global Calorimeter Trigger (GCT). A block diagram of these subsystems and other trigger subsystems that they directly interacts with are shown in Fig. 3.3 and discussed in the following sections.
 
Fig. 3.3:  The calorimeter trigger system overview.
3.3.1 Geometry and Definitions
Trigger Tower

The trigger tower (h,f) dimension results from a compromise between the background rate of the electron/photon trigger, which increases with the cell size, and the number of trigger channels, which must be as small as possible for cost reasons. In total the CMS calorimeter trigger has 4176 towers, corresponding to 2448, 1584 and 144 towers respectively in the barrel, end-cap and forward calorimeters (Fig. 3.4).
 
Fig. 3.4:  Layout of the calorimeter trigger towers in the r-z projection.

Each ECAL half-barrel is divided in 17 towers in h and 72 towers in f, so that the calorimeter trigger tower in the barrel has dimensions Dh.Df=0.087x0.087. In the barrel the trigger tower is formed by 5x5 crystals.

The ECAL trigger towers in the barrel are divided in strips. Each trigger cell has 5 h-strips (one crystal along h and five crystals along f). The strip information allows for a finer analysis of the lateral energy spread of electromagnetic showers. The strips are arranged along the bending plane in order to collect in one or two adjacent strips almost all the energy of electrons with bremsstrahlung and converted photons (Fig. 3.5).
 
Fig. 3.5:  Calorimeter trigger tower layout in one ECAL half barrel supermodule. The trigger towers are organized in calorimeter regions of 4x4 towers. Tower 17 is integrated with the endcap towers 18, 19 and 20 in a calorimeter trigger region.

In the ECAL endcap where the crystals are arranged in a x-y geometry, the trigger towers do not follow exact (h,f) boundaries (Fig. 3.6). The trigger tower average (h,f) boundaries are DhxDf=0.087x0.087 up to 2. The h dimension of trigger towers grows with h as indicated in Fig. 3.4 and Table 3.1. The number of crystals per trigger tower varies between 25 at 1.5 and 10 at 2.8.

In the barrel and in the endcap, the boundaries of ECAL and HCAL trigger towers follow each other. Each trigger tower corresponds to the (h,f) size of an HCAL physical tower, except for h>1.74 where the HCAL tower has twice the f dimension of the trigger tower. In this region, the HCAL tower energy is divided in equal amount and assigned to two trigger towers that are contained in it.

The HCAL barrel trigger towers are formed by the sum of the first two longitudinal segments (the Outer HB is not included in the trigger). The endcap towers are formed by two or three segments. In the barrel-endcap transition region, barrel and endcap segments are added together (see Table 3.1).

The trigger segmentation of the forward hadron calorimeter (HF) is not required to have small f binning, since this detector does not participate in the electron/photon trigger. However, we do need seamless coverage for jet and missing ET algorithms. Therefore we keep 18 HF f divisions which exactly match the trigger boundaries of the 4x4 trigger tower regions in the HB and HE. As shown in the Fig. 3.7 the HF readout towers are combined 3hx2f groups to form more coarse trigger towers. The resulting HF segmentation of 4hx18f is used in the jet and missing transverse energy trigger. The f divisions are exactly four times the towers of HB/HE and the h divisions are approximately the size of outer HE divisions. The overlapping jet trigger extends seamlessly to |h|=5. Missing ET is computed using f divisions of 0.348 for the entire (h,f) plane.
 
Fig. 3.6:  Calorimeter trigger tower layout in the ECAL endcap
 
Fig. 3.7:  Calorimeter trigger tower layout in the HF.
Table 3.1: Characteristics of the Calorimeter Trigger towers. Towers 1-28 have Df=0.087. HCAL towers 21-28 have Df=0.174 and are split into two trigger towers each of Df=0.087. Towers 29-32 have Df=0.348.
Tower # in h
Dh
h max
ECAL crystals
HCAL long. segments
1-14
0.087
n x 0.087
5x5 barrel 
2 (HB0,1)
15
0.087
n x 0.087
5x5 barrel
3 (HB0,1,2)
16
0.087
n x 0.087
5x5 barrel
3 (HB0,1,2) + 2 (HE0,1) 
17
0.087
n x 0.087
5x5 barrel
2 (HE0,1)
18-20
0.087
n x 0.087
endcap
2 (HE0,1)
21
0.09
1.83
endcap
2 (HE0,1), split
22
0.1
1.93
endcap
2 (HE0,1), split
23
0.113
2.043
endcap
3 (HE0,1,2), split
24
0.129
2.172
endcap
3 (HE0,1,2), split
25
0.15
2.322
endcap
3 (HE0,1,2), split
26
0.178
2.50
endcap
3 (HE0,1,2), split
27
0.15
2.65
endcap
3 (HE0,1,2), split
28
0.35
3.00
endcap
3 (HE0,1,2), split
29
0.500
3.50
-
HF
30
0.500
4.00
-
HF
31
0.500
4.50
-
HF
32
0.500
5.00
-
HF
Calorimeter Regions

The trigger towers are organized in calorimeter regions, each one formed by 4x4 trigger towers (Fig. 3.5). The HF towers 29-32 in Table 3.1 are themselves treated as regions and their Df division matches the 4x4 regions in the barrel and endcap. These calorimeter regions form the basis of the jet and energy triggers. The dimensions of the calorimeter region are adequate to the jet trigger algorithm, which is based on sliding windows of 3x3 calorimeter regions (12x12 trigger towers). The h-f indexes of the calorimeter regions are used to identify the location of L1 calorimeter trigger objects (electron/photons and jets) in the upper stages of the trigger chain.

For further information on the exact mapping of trigger towers please see:
http://www.hep.wisc.edu/wsmith/cms/calgeo/cable.html


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