Test beam studies of Gas Electron Multiplier (GEM) detectors for the upgrade of CMS endcap muon system

The High Luminosity LHC (HL-LHC) will provide exceptional high instantaneous and integrated luminosity. The forward region $\mid \eta \mid \geq 1.5$ of the CMS detector will face extremely high particle rates in tens of $KHz/cm^{2}$ and hence it will affect the momentum resolution and longevity of the muon detectors. To overcome these issues the CMS collaboration has decided to install new large size rate capable Triple Gas Electron Multiplier (GEM) detectors in the forward region of CMS muon system. The first set of Triple GEM detectors will be installed in the GE1/1 region $(1.5 \leq \eta \leq 2.2)$ of muon endcap during the LS2 of the LHC and the next one will be installed in the GE2/1 region $(1.6 \leq \eta \leq 2.5)$, during the LS3. Towards this goal, full-size CMS Triple GEM prototype chambers have been fabricated and put under the test beam at the CERN SPS test beam facility. The GEM detectors were operated with two gas mixtures $Ar/CO_{2}$ (70/30) and $Ar/CO_{2}/CF_{4}$ (40/15/45). In 2014 and 2016, good quality data was collected during test beam campaigns. In this talk, the performance of the detectors will be summarised based on their tracking efficiency, time resolution, space resolution, cluster size, etc.