EFC Trigger System

The EFC trigger provides two types of trigger information according to the energy and location of signals in the BGO crystals.

• Bhabha trigger:
  Coplanar forward/backward coincidence of energetic EM showers in EFC
  
• Two-photon trigger:
  Single EM shower together with some CDC tracks or ECL clusters.
  

The basic EFC trigger unit is a trigger cell. Two neighboring $\phi$ segments have three trigger cells according to different $\theta$ angles. In the innermost ring, a trigger cell is formed by two crystals. The other two trigger cells consist of four crystals each. The analog sum of crystal signals from the same trigger cell is fed into a constant fraction discriminator and gives the trigger output. Currently the trigger cell threshold is set at about 1 GeV. These trigger cells are then grouped into four sectors in $\phi$ in the forward and the backward EFC separately. The trigger logic is made by a logical 'OR' of all trigger cells inside each sector.

The logic output of one sector is used to form a back-to-back logic for a Bhabha trigger and also used as the gate signal for the MQT300A chips within the same sector. Another trigger scheme, called EFC-Tag, is formed by a logical 'OR' of the four forward EFC trigger sectors. It is combined with central tracking and calorimeter information to provide a tagged two-photon trigger. A schematic diagram of the EFC trigger logic is shown in Fig. .

Figure: Schematic diagram of the EFC trigger arrangement.