Investigation of Fast Timing Capabilities of Silicon Sensors for the CMS High Granularity Calorimeter at HL-LHC

The High Granularity Calorimeter (HGCAL) is the technology choice of the CMS collaboration for the endcap calorimetry upgrade planned to cope with the harsh radiation and unprecedented in-time event pileup projected at the High Luminosity-LHC era. In this context, profiting from fast-timing information (~tens of picoseconds) embedded in the calorimeter would represent a unique capability for resolving information from individual collisions at the HL-LHC. This will enhance the reconstruction and physics capabilities of the CMS detector in terms of pileup mitigation and particle identification. The HGCAL is realized as a sampling calorimeter, including 40 layers of silicon pad detectors with pad areas of 0.5 â?? 1.0 cm^2 and three active thicknesses 320, 200 and 120 μm. Prototype p-in-n and n-in-p 5x5mm^2 silicon pads, with thicknesses of 285, 211 and 133μm, were tested with high-energy electrons at the CERN SPS. We present the motivation for this study including the concept and use of fast-timing in the HGCAL, and the measured intrinsic timing capabilities for electromagnetic showers and minimum-ionising particles.