Juergen Fent, Barbara Dulny, Christian Kiesling, Achim Osthoff

Max Planck Institut Muenchen
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The physical idea of the  system is quite simple to formulate: It should find 
the localized regions of energy depositions corresponding to a typical radius,
 together with the Theta and Phi values of the shower center, where Theta is the
 pseudo-rapidity and Phi the azimuthal angle of  all the jet  centers in the calorimeter.
In a trigger application as well as in the refinement of the normal readout data this
 requires a very fast parallel processing algorithm.
The proposed system consists of well defined packages of processing units which 
work in a pipelined configuration.
In the first step the signals are converted to eight bit words, weighted and stored
 in circular memories for later readout.  
The next step realizes the basic fast parallel algorythm of „ energy bump" finding.
 Each of the towers interrogates its nearest neighbours ( known a priori by the fixed
 geometry of the towers) to determine whether any of them has more energy than
 the interrogating tower (called IT). If this is not the case the IT is a local energy
 maximum or „bump". The jet energy is then formed by adding all the energies of
 the neighbouring towers to the energy of the bump. Since this algorithm only 
interrogates the immediate neighbourhood, it is suited for parallel execution: All
 towers determine their bump property independently of the others. The result is a 
list of jets, defined by energy and coordinates.
For trigger purposes the jets  are sorted according to energy. Also this step is
 performed in a parallel algorithm. Finally  the trigger element generator
 discriminates individual jet energies, counts jets above certain thresholds and
 determines topological correlations on the basis of position information of the jets.
To show the possibilities of a jet system  on a running detector, an additional first
 level trigger  is at present designed for the  H1 detector at HERA.  It  will
 evaluate the sixteen largest jets in the whole volume  and sort them according to
 tranverse energy. The information transferred per jet is its transverse energy and
  the polar and azimuthal angle.
 The whole operation, starting from analog tower signals  is executed in six
 pipeline steps well within the first level trigger latency. It results in a computing
 power of  about 70 billion operations per second. The functionallity was already
 shown in an fully operational demonstrator system.
At present the system is investigated for its application in the ATLAS Hadronic
 Endcap Calorimeter HEC.  In the context of refinement of the data quality in
 energy and timing, especially for rather small and distributed signals a  very
 usefull application can be seen. 
The system can give additional input to the DSP calculations in the Readout
 Driver (ROD) system.  It can be shown that the timing information for lower
 energy  and distorted events can be reasonably enhanced.
As the ROD modules are housed in crates, the possibility of accessing and
 networking data from different Frontend Boards is possible.
On the other hand it can also deliver additional information to the level one
 trigger system.
As in the H1 case, the modules are built of a large array of high complex
 FPGA’s ( Field Programmable Gate Arrays) which form a  network, modelling
 the detector.



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A novel calorimeter data processing system is proposed as additional option in
 the LAr calorimeter data stream.  The concept is based on the very fast search of
 localized energy depositions ( „jets") in the calorimeter within the latency time of
 the first level of triggers. The aim of the new system is to provide optimal use of
 the calorimeter information and to cure limitations of  current trigger systems. As
 a result,the jet system demonstrates increased sensitivity to low energy
 depositions, originating, e.g. from charm final states, and provides new tools for
 efficient rejection of background using the information from the jets in the Theta - Phi plane.


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