The calorimeter trigger hardware project
In close collaboration with physicists from several European countries, the Mainz particle physics group has developed and built a hardware-based trigger sub-system for the ATLAS detector at CERN. The Jet and Energy Processor (JEP) analyzes energy data coming from the ATLAS calorimeters via several stages of analogue and digital circuitry, and it calculates in real-time the number of particle jets with an energy exceeding programmable thresholds. It also determines the total transverse energy deposited in the calorimeters. Further it interprets the measured scalar energies as vectors pointing to the position of the energy deposit.
After a vector summation on this quantity, due to energy imbalance, any single particle with large transverse momentum that goes undetected will show up as a "missing energy" vector, pointing to the direction of escape.
The results of the calculations are made available both to the central trigger processor, alerting the whole of the ATLAS detector, and to the 2nd level trigger computer systems in charge of having a closer look at the usefulness of the triggered event.
The JEP is a single rack with electronic modules (left image), living in two separate shelves. Each shelf, a "9U VME crate" is equipped with several types of electronic modules. The majority of those is the Jet and Energy modules (JEMs) (right image), of which 16 are deployed in each shelf. They are Mainz designed, custom made electronic modules. Their computing power is provided by programmable logic devices (FPGAs) and is optimised for massive parallelization of fast arithmetic algorithms. While the capabilities are limited to integer summation, subtraction, bit shift, and memory lookup operations, a very large number of such operations can be performed within the LHC's bunch clock period of 25ns. This allows for dead-time free, low latency, pipelined operation required for triggering the ATLAS detector.
The modules have been built and tested in Mainz and are now being operated at CERN, in the ATLAS electronics cavern. The Mainz particle physics group has responsibility for the maintenance, the optimization and the eventual upgrade of these modules. A new incarnation of the ATLAS trigger electronics is due, once the LHC will be brought up to higher luminosities, which is currently projected to occur in 2014.
Increasing interaction rates will require the trigger algorithms to become more selective. To cope with the harsher requirements, new electronics modules will have to be built. Mainz is actively contributing to the development of new trigger electronics and the simulation of new algorithms.