Front-end Intelligence for triggering and local track recognition in Gas Pixel Detectors

The combination of gaseous detectors with pixel readout chips gives unprecedented hit resolution (improving from O(100 um) for wire chambers to 10 um), as well as high-rate capability, low radiation length and giving in addition angular information on the local track. These devices measure individually every electron liberated by the passage of a charged particle, leading to a large quantity of data to be read out. Typically an external trigger is used to start the read-out. We are investigating the addition of local intelligence to the pixel read-out chip. A first level of processing detects the passage of a particle through the gas volume, and accurately determines the time of passage. A second level measures in an approximate but fast way the tilt-angle of the track. This can be used to trigger a third stage in which all hits associated to the track are processed locally to give a least-squares-fit to the track. The chip can then send out just the fitted track parameters instead of the individual electron coordinates. This self-triggering capability could have a major application in the level-1 track trigger proposed for the ATLAS upgrade for the sLHC. I will briefly summarise the track trigger requirements for the ATLAS Upgrade and highlight the advantages of a gaseous detector for this application, followed by discussing one approach to the local intelligence needed to realise such a trigger.