Initial studies of a short drift GEM tracking detector

In an effort to develop new tracking detectors for future experiments at RHIC, we have been developing a short drift GEM tracking detector that has the potential to provide a position resolution ~ 100 microns even at large angles. We have conducted new high precision studies of our prototype short drift GEM detector at the Fermilab test beam facility in October 2013 and February 2014 which confirm initial studies carried out at CERN in 2012. The detector is made up of a triple GEM stack with a 16mm drift gap and is read out using COMPASS style x-y strips, in conjunction with a SRS/APV-25 DAQ system. The charge deposited in the drift gap from incident particles is drifted onto the readout plane, where the charge arrival time is used to reconstruct particle position and angle. Although the old and new measurements show very nice agreement, the older measurements suffer from a relatively large uncertainty in the position of the reference track, resulting in the unfolding of an error about as large as the resolution of the GEM detector itself. This paper will describe the important aspects of the newer, higher precision measurement, and will report on improvements in the data analysis, which includes a method that uses the charge weighted centroid, together with the reconstructed vector to get a better measure of the position than either the centroid or vector alone.