Fly High Engineering has deep knowledge about Data Acquisition (DAQ) systems, including the trigger systems of very complex and highly reliable instruments, such as the huge underground particle detector ATLAS of the Large Hadron Collider (LHC) at CERN, the Alpha Magnetic Spectrometer (AMS) installed on the International Space Station, and the X-ray imaging spectrometer STIX of the ESA-NASA Solar Orbiter mission.
The goal of the trigger system is to select the “events” that the DAQ system should record, while ignoring uninteresting events, which are usually much more numerous than the interesting ones. Interesting events are defined by listing several criteria they must meet. For example, the traffic police use speed cameras to take pictures of cars exceeding the speed limit, which are a minority of all vehicles traveling along that stretch of road. This is quite a trivial example of trigger system, however. Most scientific, medical, and industrial DAQ systems must cope with much bigger complexity. Therefore, it is usually better to think in a sort of reverse logic and focus on the reasons why a certain event should not be recorded. In the simple example above, the speed camera is sensitive to all vehicles, but it spends most of time ignoring those with speed below a certain threshold. Similarly, ATLAS must ignore the vastly bigger fraction of uninteresting events caused by multiple proton collisions at each bunch crossing, with 40MHz bunch crossing rate, and save only about 600 events per second (about 10 parts per million). This number is set by the capacity of the storage built at CERN for this experiment.
The recorded events are then skimmed further by performing a number of “selection cuts” during the offline data analysis step. For example, hundreds of trillions of events have been collected by ATLAS and CMS during LHC Run 1 from 2010 to 2012, before they could collect few hundreds of events (about 1 part per trillion!), necessary to achieve enough statistical evidence to claim the discovery of the Higgs boson.
Before addressing such complex systems again, let us review the main types of trigger conditions. The simplest one is the requirement that a certain value exceeds a predefined threshold, like the speed camera of the police. Voltage protection is also based on this approach, to make another common example. The assumption here is that, when the quantity exceeds the threshold, this is caused by some physical phenomenon that we want to study further (e.g., identify the license plate of the speeding car) or prevent (e.g., damage of electrical components). Sometimes, the direction in which the threshold is passed (increasing or decreasing signal) is also important, as implemented in the edge triggering of oscilloscopes.
On the opposite side, we might be interested in an unbiased, random trigger, with which we can accumulate events that are studied to characterize the environment or situations in which interesting events are supposed to occur. This trigger may be generated with a clock, for example to acquire events falling in predefined time intervals, random generator, or even a counter (in the assumption that interesting events occur randomly).
Interesting events may also be characterized by specific patterns, like the waveforms that led to the discovery of the first black-hole merge by LIGO, or the four-lepton pattern of the Higgs decay into two Z bosons. The challenge is how to detect such patterns real-time and with as little contamination as possible from uninteresting events.
Complex instruments like particle detectors adopt a multi-level approach to select interesting events. Traditionally, three trigger levels are identified. The first level (L1) needs to cope with the highest event rate; therefore, it is based on hardware logics implemented with ASICs and/or FPGAs. To keep the pace of input events, L1 typically exploits only local data, meaning the chips use input from a single type of detector to take a decision. The second trigger level (L2) is then used to reduce the event rate further, by combining information coming from different subdetectors. The third and last trigger level (L3) is finally capable of combining information from all subdetectors to detect the most complex patterns. Keep in mind that, while the trigger system is processing the event, the detector is “blind”, i.e. it is not sensitive to further events. This “dead time” ends when the DAQ system is reset, following rejection by the trigger or recording of the event.
The input rate to the ATLAS L1 trigger is the LHC bunch crossing rate (40 MHz) and its output is 100 kHz. This is further reduced by L2 down to about 1 kHz, which is then halved by the L3 (called “Event Filter” in ATLAS, which groups L2 and L3 into the software-based “High-Level Trigger”). Random triggers are also employed, to get unbiased estimates of the probability of different classes of events. ATLAS has a “trigger menu” consisting of several entries, because one may want to record events featuring one or more high-energy electrons, photons, or muons. Alternatively, a number of “jets” and/or a significant amount of missing transverse-momentum may be required by various physics analyses. Therefore, a further challenge is how to balance the relative ratios of different “menu entries” with the rigid constraint given by the final event rate on storage.
AMS-02 has only two trigger levels, which would correspond to L1 and L3 of the traditional nomenclature. The first decision is taken by the Time-Of-Flight (TOF) system, selecting the charged particles crossing the inner bore of the magnet. A quick veto may be set by the ACC (Anti-Coincidence Counters) and by a lookup table implemented in a FPGA. At the next level, information from the tracker, calorimeter, and Transition Radiation Detector, is used to refine the decision taken with the TOF and ACC. The orbit inclination of the International Space Station is 51.6°, which means that the AMS-02 latitude oscillates between 51.6°N and 51.6°S. Consequently, the flow of particles through it varies, with maximum of 2-3 kHz input rate reached closer to the poles and minimum below 500 Hz (depending on the solar activity) at the equator. However, the energy spectrum also changes with latitude, with less energetic particles dominating in polar regions, where the Earth magnetic field does not reflect them back to space. The bandwidth allocated to AMS-02 fixes the maximum rate of recorded events to a level below 100 Hz. Therefore, the challenge is to make smart use of such bandwidth by recording the most interesting events, like heavy ions (a small minority of all particle flux). As most cosmic-ray events are protons with relatively low energy, whenever the trigger recognizes this class, a pre-defined suppression factor may be applied by recording only one out of N events with a random choice algorithm. On the other hand, when the TOF detects a particle with charge 2 (a helium nucleus) or higher, there is no further suppression by the other trigger level.
In summary, a trigger system is always necessary to start the data acquisition process. The design of the trigger is the result of a trade-off between different requirements and constraints and is one of the most delicate aspects of the experiment design. The goal is to collect as many interesting as possible with the little fraction of “background” events compatible with the data acquisition capacity. Get in touch with Fly High Engineering to know more.