Total Energy Trigger

Total energy information is one of the most effective triggers to identify hadronic events in high energy $e^+e^-$ annihilations. The advantage of the total energy trigger is that the trigger logic itself can be simply built as a fast trigger in regard to the trigger timing latency in data acquisition and the deposit energy of beam backgrounds in the calorimeter is usually very soft.

Analog-sum signals of STM, called GATESUM, are further processed in an analog adder (AAA) when the deposit energy in TC is above the threshold value. This arrangement minimizes incoherent electronics noise contribution from each TC. AAA first adds up to five GATESUM signals depending on the cylindrical $\phi$-direction and the other analog adder (AAB) finally produces an entire $\phi$-ring analog sum signal. As shown in Fig. , the $\phi$-rings depending on the $\theta$-direction consists of 12 units in the barrel and 5 units in the end-cap regions, i.e. 3 in the forward and 2 in the backward end-cap. The scheme of two stage analog summation simplifies the complexity of cabling between the detector and the electronics hut located about 30 m away. The current analog driving method was adopted to reduce the signal attenuation and ground loop effects.

Figure: Calorimeter division for the selection of Bhabha events in the $\theta$-direction. Here each unit represents one $\phi$-ring summation.

Bhabha events are triggered using back-to-back conditions in the center of mass frame. Table  shows 11 types of Bhabha triggers based on each $\phi$-ring summation. The neighboring combinations are slightly overlapped with the next $\phi$-ring in order to save Bhabha events passing through the boundary region. These logics provide the trigger efficiency greater than 99 in real data taking runs.

Table: Various energy cuts to trigger Bhabha events. F, C and B represent the Forward end-cap, Barrel and Backward end-cap calorimeters, respectively. indicates Backward gap and Forward gap.

Type	Logic	Threshold (GeV)
1	F1+F2+B1+B2	9.0
2	F2+F3+B1+B2+C11	9.0
3	F2	6.5
4	F3+C10+C11	9.0
5	C1+C9+C10	9.0
6	C1+C2+C9	9.0
7	C2+C8+C9	9.0
8	C3+C7+C8	9.0
9	C4+C6+C7	9.0
10	C5+C6	9.0
11	C10	3.5

Figure  (a) shows the singles rate of the total energy trigger for cosmic rays as a function of threshold energy. Electronics noise and cosmic ray contributions are clearly separated around 200 MeV, and the rms noise level is about 50 MeV. For the physics trigger of total deposit energy, the threshold of about 1 GeV has been used stably.

Figure: (a) Singles rates of the total energy sum (E) as a function of threshold energy. Electronics noises and cosmic rays are clearly separated around = 200 MeV. (b) Singles rates as a function of the number of isolated clusters(ICN).