10 - H1 - Desy
10 - H1 - Desy
10 - H1 - Desy
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3.2 <strong>H1</strong> detector 49<br />
rates exceed the ep interaction rates typically by two orders of magnitude, so the efficient<br />
trigger system is needed for successful experimental running.<br />
The <strong>H1</strong> trigger system consists of four levels (L1 - L4), of which all but L3 was in operation<br />
during HERA II data taking period. The <strong>H1</strong> trigger system, consists of several levels,<br />
denoted L1 to L4. Only after an event has been accepted by all operating systems, it is<br />
written to tape and analysed by the offline reconstruction. An illustration of the data<br />
flow through the <strong>H1</strong> trigger system is shown in figure 3.<strong>10</strong>.<br />
Subdetector−<br />
Data<br />
Dead−time<br />
free<br />
~<strong>10</strong>MHz<br />
Data pipelines<br />
L1<br />
Hard<br />
wired<br />
Logic−<br />
circuits<br />
2.3µs<br />
stop<br />
Pipelines<br />
L1KEEP<br />
max.<br />
1 kHz<br />
L2<br />
Neuronal<br />
Network<br />
Topolog.<br />
Trigger<br />
22µs<br />
Dead−<br />
time<br />
~2% ~7%<br />
read−out<br />
Pipelines<br />
L2KEEP<br />
max.<br />
200 Hz<br />
L2REJECT<br />
L3<br />
Processor−<br />
Farm<br />
~<strong>10</strong>0µs<br />
L3KEEP<br />
continue<br />
Read−Out<br />
max.<br />
50 Hz<br />
L3REJECT<br />
asynchronous Phase<br />
asynchronous<br />
Event Buffer<br />
L4/5<br />
Processor−<br />
Farm<br />
full Event−<br />
reconstruction<br />
~<strong>10</strong>0ms<br />
L4KEEP<br />
L4REJECT<br />
<strong>10</strong> Hz<br />
write<br />
on Tape<br />
reject<br />
Trigger Data<br />
clear Pipelines & restart<br />
Figure 3.<strong>10</strong>: The designed data flow through the <strong>H1</strong> trigger system.<br />
The first level trigger, L1, consists of around 200 trigger elements (TE) providing fast<br />
information from different detector subsystems. The central trigger logic combines TEs<br />
into 128 subtriggers, the majority of which are designed to select a variety of physics<br />
processes, although some are used to monitor background and trigger efficiencies. An<br />
event is kept if at least one of 128 subtriggers (s0-s127) give a positive decision. If a<br />
specific subtrigger has a too large rate it is consequently prescaled While the trigger<br />
decision is being taken, the readout is stored in the peaplines, which enables the L1<br />
trigger to be free of a deadtime. The L1 decision is taken within 2.3 µs and the output<br />
of the L1 trigger system is of the order of 1 kHz.<br />
The L2 system decision is derived within 22 µs from one of two independent hardware<br />
systems, a topological trigger (L2TT) and a neutral network trigger (L2NN) which both<br />
combine the information of several subsystems. A positive decision on L2 stops the<br />
pipepines and the whole event is read out. The readout process takes typically 1 − 2<br />
ms during which no further data can be collected and is considered as a detector dead<br />
time. The designed output rate of the L2 trigger is predicted as 200 Hz, but since the L3<br />
trigger level was not in operation during the most of the HERA II running period, the L2<br />
actually must decrease the rates down to 50 Hz.<br />
The L4 trigger decision is based on a full though simplified event reconstruction and<br />
decided in approximately <strong>10</strong>0 ms. Basing on calculated so called L4 statements, selected<br />
events are grouped into L4 classes. The physics event classes are selected based on a<br />
presence of a hard scale, like sufficiently large Q 2 or transverse momentum, or by the<br />
output of dedicated physics finders like ’open charm’ or ’open beauty’. Events that can
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