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Preliminary results from the cosmic data taking of the BESIII cylindrical GEM detectors

BESIII is a multipurpose spectrometer optimized for physics in the tau-charm energy region. Both detector and accelerator are undergoing an upgrade program, that will allow BESIII to run until 2029. A major upgrade is the replacement of the inner drift chamber with a new detector based on Cylindrical Gas Electron Multipliers to improve both the secondary vertex reconstruction and the radiation tolerance. The CGEM-IT will be composed of three coaxial layers of cylindrical triple GEMs, operating in an Ar$\,$+$\,$iC$_4$H$_{10}$ (90:10) gas mixture with field and gain optimized to maximize the spatial resolution. The new detector is readout with innovative TIGER electronics produced in 110$\,$nm CMOS technology. The front end is a custom designed 64$\,$channel ASIC featuring a fully digital output and operated in trigger-less mode. It can provide analog charge and time measurements with a TDC time resolution better than 100$\,$ps, that will allow to operate in $μ$TPC mode. Before the installation inside BESIII, foreseen in 2021, a long standalone data taking is ongoing at the Institute of High Energy Physics in Beijing; currently, the first two cylindrical chambers are available for the test, and are used to complete the integration between the detector and the electronics and to assess the required performance. In this proceeding a description of the CGEM-IT project, the TIGER features and performance, and the results of the analysis of first cosmic ray data taking will be presented. Focus will be given on the strip analysis, from which it is possible to measure the basic properties of the detector, and the cluster analysis. The first preliminary results on efficiency and spatial resolution will be also presented.