Latest feasibility studies of LAPPD as a timing layer for the LHCb Upgrade 2 ECAL

The target of the LHCb experiment Upgrade 2 is to operate with an instantaneous luminosity a factor seven higher than the current one to reach the ultimate precision in several domains of its physics program. This objective challenges the development of subdetectors able to cope with the high-occupancy regime foreseen. The time-of-arrival of the particles at the various subdetectors is a promising new feature. Simulation studies show that, with a time resolution of about 10-20 ps, it will be possible to exploit the time separation of the primary proton-proton collisions and effectively mitigate the pileup. Concerning the LHCb Upgrade 2 electromagnetic calorimeter, the "Large Area Picosecond Photo Detector" technology (LAPPD) is currently a candidate to constitute a timing layer placed at the shower maximum. The LAPPD is the largest microchannel-plate photomultiplier ever built, entirely made with inexpensive materials. This paper depicts the status of the art of the ongoing R&D campaign. In particular, four LAPPD models have been characterized so far: the Gen-I with stripline readout and the Gen-II with external pixelated readout, both with 10 or 20 mu m pore size. A time resolution close to the target was measured with test beams at DESY (electrons from 1 to 5.8 GeV) and SPS (electrons from 20 to 100 GeV). The radiation hardness of the MCP layers was stressed and verified up to 1016 protons/cm(2) at CERN IRRAD facility and 300 C/cm(2) using a UV lamp in the laboratory. The performances at high rates were investigated with two lasers (lambda = 405 nm): they will be crucial for the upcoming development steps.