Optimization of the reconstruction algorithm in triple-GEM detector

Gas detectors are very light instruments used in high energy physics to measure the particle properties: position and momentum. Gas Electron Multiplier (GEM) technology has been invented by F. Sauli in 1997 and in the past tens of years the knowledge of this Micro Pattern Gas Detector (MPGD) increased. A design with a triple-GEM has been used in several experiments in high energy physics such as TOTEM and COMPASS. This technology allows to achieve good spatial resolution performances and can be used to create large area detector with a shapeable surface, e.g. the KLOE2 Inner Tracker or the upcoming upgrade of the BESIII Inner Tracker. A triple-GEM uses three stages of GEMs, with high electric field, to amplify the number of the electrons produced by the primary ionization of the charged particle passing in the gas, with a total gain of about 104. The signal is then collected on a segmented anode and dedicated algorithms are used to reconstruct the charged particle position. The measurement of both the time and the charge information on the anode strip allows to use two algorithms: the Charge Centroid and the micro-Time Projection Chamber readout. The CC is strongly performing with orthogonal tracks while the μTPC gives its best results if magnetic field or if non-orthogonal tracks are present. These reconstruction methods are anti-correlated and the combination of the two is needed to keep the spatial resolution stable between the regions with different performances. A beam test with planar triple-GEM has been performed and in this presentation the merging algorithm of the CC and μTPC as a function of the charge and the multiplicity of the signal will be shown: a stable spatial resolution of about 130 μm has been achieved. In addition to this, the impact of the transversal and longitudinal diffusions on the reconstruction in μTPC mode will be shown, as well as the inter-strip capacitance effect.