The ATLAS Level-1 Trigger
The LHC is designed to collide bunches of about 1011 protons at a frequency of 40 MHz. At design parameters, this corresponds to 20 inelastic proton-proton collisions every 25 ns. The Level-1 trigger has to reduce the interaction rate of 40 MHz down to 100 kHz, by selecting those events which contain traces of interesting physics. Because of the limited size of the on-detector data buffers, a decision has to be taken at maximum 2.5 us after each event takes place.
To perform this task, the Level-1 trigger searches for highly energetic particles in the calorimeters and the muon system of the ATLAS detector. It consists of three different subsystems. The Level-1 Calorimeter Trigger (L1Calo) analyzes the energy depositions in the calorimeters to find electrons, photons, taus and jets. It also computes global sums of total and missing energy. The Level-1 Muon Trigger (L1Muon) uses data from the muon spectrometers to locate muon candidates. The information from these two subsystems is combined in the Central Trigger Processor (CTP), which makes the final Level-1 trigger decision. Only events which contain particles that pass configurable energy- or momentum-thresholds are accepted.
The L1Calo System
The L1Calo system uses reduced granularity data to find objects in the calorimeters of the ATLAS detector. It is divided into several different processors, all of which are fully implemented as special hardware and process the incoming data in parallel:
- The PreProcessor (PPr) receives ~7200 pre-summed analogue signals from the calorimeters for each bunch crossing of the LHC. It was developed at the KIP in Heidelberg. The PPr digitizes these signals at a frequency of 40 MHz, the same as the LHC bunch crossing frequency. Afterwards it determines the amount of energy and the bunch crossing of each measured energy deposition. The resulting energy values for the different channels are then routed to the two following processors.
- The Cluster Processor (CP) uses a sliding window algorithm to search for energy depositions that originate from electrons, photons or hadronically decaying taus.
- The Jet/Energy Processor (JEP) finds jet-candidates and measures global energy sums covering the whole of the calorimeter, i.e. the total transverse energy and the missing transverse energy. Like the CP, the JEP uses sliding window algorithms to fulfill its task.
The L1Calo PreProcessor
The contribution of the KIP ATLAS group to the trigger system is the construction and operation of the L1Calo PreProcessor. To handle the input of over 7000 analogue calorimeter channels, the PPr system is divided into ~120 PreProcessor modules (PPM) in eight VME crates. Each PPM is capable of processing 64 channels in parallel. The main processing tasks are carried out by Multichip Modules (MCM), of which there are 16 on each PPM. Each MCM is capable of handling four channels.
There are several chips soldered on each MCM, including the following:
Four Flash ADCs digitize the incoming analogue calorimeter signals at a rate of 40 MHz with 10 bit resolution.
A PHOS4 chip controls the digitization strobes of the ADCs in steps of 1 ns to ensure a precise digitization of the maximum of the analogue calorimeter signals. This way the highest possible energy resolution can be guaranteed
A custom ASIC implements the main functions of the MCM. It analyzes the digitized signals to extract the corresponding transverse energy and to synchronize the signals from different channels to the same LHC bunch crossing (bunch crossing identification, BCID). The ASIC was developed in cooperation with the ASIC laboratory at the KIP.
Three LVDS Serializers combine the parallel data streams of the different channels to a single serial stream for transmission to the object finding processors CP and JEP.
During the running of the LHC, the KIP group was heavily involved in monitoring and maintaining the L1Calo system. This task includes the calibration of PPr parameters for energy determination, the optimization of the fine timing as well as the development of monitoring tools for the different variables. Technical support for the installed PPMs was also provided.
The new Multichip Module
In Run-I of the LHC (2010-2012), the PreProcessor system performed its task very well and with high efficiency. The LHC upgrade during the long shutdown until 2015 will increase the number of particles colliding at the same time ('pile-up') and thus lead to harsher conditions for the trigger system. Therefor an upgraded replacement module for the MCM is being developed, the 'new MCM' (nMCM). Its main features are 2 dual-channel Flash ADCs which work at a higher frequency than the previous ones (80 MHz instead of 40 MHz), as well as a modern FPGA to replace the ASIC. As a new feature, the nMCM includes an on-board signal generator, to enable independent tests of the boards functionality. Prototypes were already produced and successfully tested. The production of the final nMCMs and their tests are scheduled for late 2013, with installation and commissioning at CERN taking place in early 2014.
These hardware changes allow a more precise processing of the digital calorimeter signals. Thus, the upgrade poses an opportunity to develop and implement new and enhanced trigger algorithms on the nMCM. For example, the KIP ATLAS group studies algorithms for the bunch crossing identification (BCID) for saturated signals. By using the higher digitization frequency of the nMCM, a maximum efficiency can be reached even for the highest possible energies. Possible ways to dynamically correct for pile-up induced fluctuations of the signal pedestal are also being analyzed.