ZDC Vertex Efficiency

The ZDC vertex can be measured from the average time on the left and the average time on the right.

<Left> = Average left time

<Right> = Average right time

Vertex = (<Left> - <Right>)/2.0*c - offset

The current method for determining the average time is to take the average of at least the first two ZDC counters on either side.

The time for each counter is valid if the adc value >= 0, and 0 < time < 2000

If all three tubes are valid then

<Left> = (L1Time + L2Time + L3Time)/3

<Right> = (R1Time + R2Time + R3Time)/3

If only the 1^st and 2^nd tubes are valid then

<Left> = (L1Time + L2Time)/2

<Right> = (R1Time + R2Time)/2

Otherwise Vertex = 99999.

As a result, the vertex efficiency is affected. The plot below is shows the number of ZDC vertices determined using this method, labeled ZDC.

This method will give a vertex resolution of 4.2cm (1s), when comparing the ZDC Vertex to the projected vertices for each track.

To increase the efficiency, a new method is used.

If only the 1^st counter's time is valid then

<Left> = L1Time

<Right> = R1Time

If only the 1^st and 2^nd tubes are valid then

<Left> = (L1Time + L2Time)/2

<Right> = (R1Time + R2Time)/2

If all three times are valid then

<Left> = (L1Time + L2Time + L3Time)/3

<Right> = (R1Time + R2Time + R3Time)/3

Otherwise Vertex = 99999.

The plot below shows the efficiency between the old method and the new method.

The resolution will increase to 4.3cm (1s).