Upgrade of the ATLAS Tile Calorimeter for the High Luminosity LHC

The Tile Calorimeter (TileCal) is the hadronic calorimeter covering the central region of the ATLAS experiment. TileCal is a sampling calorimeter with steel as absorber and scintillators as active medium. The scintillators are read-out by wavelength shifting fibers coupled to photomultiplier tubes (PMTs). The analogue signals from the PMTs are amplified, shaped, digitized by sampling the signal every 25 ns and stored on detector until a trigger decision is received. The High-Luminosity phase of LHC (HL-LHC) expected to begin data taking in 2026 requires readout electronics suitable for a 1 MHz trigger rate, higher ambient radiation, and for better performance under high pileup. Both the on- and off-detector TileCal electronics will be replaced during the shutdown of 2024-2025. PMT signals from every TileCal cell will be digitized and sent directly to the back-end electronics, where the signals are reconstructed, stored, and sent to the first trigger level at a rate of 40 MHz. This will provide better precision of the calorimeter signals used by the trigger system and will allow the development of more complex trigger algorithms. Changes to the electronics will also enhance the data integrity and reliability of the system. Three front-end board options were built and investigated both in laboratory and in several beam test campaigns. The final version has been chosen after evaluating the results. A hybrid Demonstrator for the new readout architecture and compatible with the present system has been developed, adopting the new chosen front-end option. The Demonstrator is undergoing extensive testing and is planned for future insertion in ATLAS.