Speaker
Description
The High-Luminosity phase of the CERN Large Hadron Collider will pose new challenges for the detectors. The Electromagnetic Calorimeter (ECAL) of the CMS experiment will be equipped with a completely new readout electronics to cope with increase in the number of pp collisions per bunch crossing, as high as 200, and higher noise induced by radiation. Two on-beam vertical integration tests were performed at the CERN H4 facility using near-final components, installed in an ECAL Supermodule identical to the 36 Supermodules the barrel is made of. The data acquisition chain and the results of the test beam will be presented.
Summary (500 words)
The CMS Electromagnetic Calorimeter is composed, in the barrel region, of 61200 PbWO4 scintillating crystals read by avalanche photodetectors (APD).
The new ECAL readout electronics for HL-LHC is composed of a front-end and a back-end system. The former is made of the VFE card, serving 5 crystals, and FE card, connected to the 5 VFE cards that compose the "readout tower" alongside the LVR card for power supply. On the VFE card, the signal from the APD is amplified by the CATIA transimpedence amplifier ASIC via two independent gain channels and sampled by the LiTE-DTU ASIC at 160 MHz. The digitised samples, after compression and digital gain selection, are sent via optical connection to the backend though the FE, which comprises 4 lpGBT chips. The lpGBT ASIC (Low Power GigaBit Transceiver) is a new 65nm-CMOS radiation tolerant serialiser/deserialiser device designed for HL-LHC applications. On the backend, the custom-designed Barrel Calorimeter Processor (BCP) cards implements trigger and readout algorithm by use of large FPGAs.
The use of lossless compression algorithms, needed to optimise bandwidth allocation, poses non-trivial channel synchronisation and alignment problems. To ease this task, the LiTE-DTU can receive a start-of-orbit signal (BC0) and tag the corresponding sample. The BC0-marked sample can then be used by the BCP to align the channels. We have studied the problem extensively using pulse injection from an external signal generator and by use of the monitoring system that injects laser light in the ECAL crystals.
Numerous link stability studies were conducted. In particular, the LiTE-DTU implements the PLL block from the lpGBT chip to generate the 1.28 GHz clock that underlies the transmission. Error transmission rates were studied as a function of the PLL capacitor setting.
The ECAL barrel is composed of 36 supermodules of 1700 channels. A spare supermodule was equipped with 200 channels of near-production electronics, while a first version of the BCP provided back-end readout. This system was used for the tests above and for two test-beam campaigns at the CERN H4 facility in November 2022 and July 2023. Electron beams of 20 to 200 GeV permitted a full assessment of the performances in terms of electronics ( noise, error rate, functionality) and physics (energy and time resolution). A detailed analysis of the data acquired in these campaigns will be presented.
