Spatial resolution of a GEM readout TPC using the charge dispersion signal

A large volume Time Projection Chamber (TPC) is being considered for the central charged particle tracker for the detector for the proposed International Linear Collider (ILC). To meet the ILC-TPC spatial resolution challenge of ~100 microns with a manageable number of readout pads and channels of electronics, Micro Pattern Gas Detectors (MPGD) are being developed which could use pads comparable in width to the proportional-wire/cathode-pad TPC. We have built a prototype GEM readout TPC with 2 mm x 6 mm pads using the new concept of charge dispersion in MPGDs with a resistive anode. The dependence of transverse resolution on the drift distance has been measured for small angle tracks in cosmic ray tests without a magnetic field for Ar/CO2 (90:10). The GEM-TPC resolution with charge dispersion readout is significantly better than previous measurements carried out with conventional direct charge readout techniques.