Event Trigger

Experiment E838 used three different types of triggers;

• beam trigger - prescaled beam counter trigger requiring two coincident hits in the beam telescope.
• $\gamma \gamma n$ trigger - two-photon trigger for measurement of the signal $\pi ^-p\rightarrow \gamma \gamma n$ events.
• $\pi ^o$ trigger - two-photon trigger for the $\pi^- p \rightarrow \pi^o n$; $\pi^o\rightarrow\gamma\gamma$ events. This data, which was the principal source of background in the large opening angle region, was taken periodically as a calibration source.

The beam trigger, also called R-RATE trigger, was used for sampling the particle composition of the beam (via time of flight (TOF) and pulse height) as recorded from the beam counters for a prescaled sample of the beam.

Table 3.3: The four basic levels for the $\pi ^-p\rightarrow \gamma \gamma n$ trigger.

Trigger Level	$\quad$ Trigger Logic	Applied In:
One	Hit multiplicity of	$coincidence$
	trigger scintillators
Two	$C$ & $D$ scintillator hit	$MLU$
	patterns
Three	Minimum number of drift	$OR$ $boards / AHC$
	chamber cells and clusters
	of cells fired: AHC cut
Four	Reject back-to-back	$OR$ $boards / SSP$
	drift chamber cell
	hits: $SSP$ cut

The trigger logic for measurement of the two-photon ( $\pi ^-p\rightarrow \gamma \gamma n$) reaction was changed from the earlier RMC set-up involving a single-photon reaction ( $\mu^-p\rightarrow\nu n\gamma$) experiments (26) in the following way;

The $\gamma \gamma n$ two-photon trigger comprised of four levels:

• Level one was derived from the coincidence of the hit multiplicity of the $A$, $A$', $C$ and $D$ counters. The two-photon trigger required $\overline{A}\cdot\overline{A'}\cdot\geq2C\cdot\geq3D$, which vetoed the event if the $A$, $A$' counters fired, signifying that the charged particle came from the target and not from a photon that converted into $e^+e^-$ pairs in the lead converter. Further, at least 2 hits in the $C$ counters and 3 hits in the $D$ counters were required.
• Level two was derived from the memory logic unit ($MLU$) which looked for and compared the $C$ and $D$ counter hit patterns at the hardware. Each pattern of fired $C$ counters defined a range of valid $D$ counters. The two-photon trigger required $\geq$3 valid $D$'s. A trigger file mC3_gC1_rD1_OC3 was employed which rejected back-to-back photon-pairs from $\pi^o\rightarrow\gamma\gamma$ events and required two or more $C$ counter hits (n$C\geq$2), three or more $D$ counter hits (n$D\geq$3) and as part of the $CD$ hit pattern requirement, required that at least three valid $D$ counters fire. The trigger file mC3_gC1_rD1_OC3 is named as such to reflect the $C$ counter hit multiplicity ($mC3$), required gap between $C$ hits ($gC1$), the range of the fired $D$'s ($rD1$), and the back-to-back veto from oppositely fired $C$'s ($OC3$).
• Level three was derived from the hardware unit Analog Hit Counter (AHC), with a requirement based on number of cells that fired and the cluster of such cells in the superlayers of the drift chamber. These were read from and derived in the 'OR boards' (26). During the first 45% of the data taking, a minimum of three cell hits in the superlayer 2, and at least three clusters of cells in the superlayers 3 and 4 together, were required to fire ( $AHC$ cut $\equiv$ (3,3) ). For the remaining 55% of the data taking, a (4,6) requirement was put in place.
• A fourth trigger level, called the SSP cut, was used that matched and rejected back-to-back pattern of hits in the drift chamber cells and clusters of cells due to two-photon events from $\pi ^o$ decay. The drift chamber layer 1 was divided into 7 sectors for the purpose of identifying the cell hits, and the back-to-back hits were identified and rejected. For example, events with hits in sector 4,5 were rejected if sector 1 was also hit.

The $\pi ^o$ two-photon trigger mC3_gC1_rD2 comprised of the first trigger level and recorded the back-to-back two-photon events. An account of the trigger and acceptance studies is given in Chapter 4.