Development of the GEM tracker for the J-PARC E16 experiment

The J-PARC E16 experiment has been proposed to measure the mass spectrum of ϕ mesons in nuclear matter to study the origin of QCD mass. The ϕ mesons are identified in the e+e− decay channel and the mass of these ϕ mesons is reconstructed using the momenta of the e+ and e− pairs. A tracking detector for this experiment is composed of a position sensitive device that employs Gas Electron Multipliers (GEMs) with a two-dimensional readout board. The ionization electrons are amplified with a triple-GEM stack. The required position resolution is 100μm up to an incidence angle of 30°. Position resolutions and efficiencies of the tracking device are evaluated using a π− beam with a momentum of 1.0 GeV/c at the J-PARC K1.1BR beam line. A position resolution of better than 100μm is obtained by calculating the center of gravity of the charges on the readout strips when tracks arrive perpendicularly at the detector plane. Timing information of charge clusters is essential to improve the position resolutions for the inclined tracks. The arrival times of the charge clusters are derived from the wave forms collected by flash ADC modules. Using the arrival times of the clusters, the obtained position resolution is better than 100μm for the track inclinations of 15° and 30°. The detection efficiency is also investigated as a function of the GEM gains, and the operational voltage is optimized