Pedestal Leveling: Preparing the LDMX Hadronic Calorimeter for Data Acquisition

The Light Dark Matter Experiment (LDMX) is a planned electron-on-target experiment at SLAC National Accelerator Laboratory designed to search for signatures of Dark Matter particle production through the measurement of missing momentum in fixed-target collisions. The hadronic calorimeter (HCAL) is one of the detectors in LDMX. When a particle hits a scintillator in the HCAL, a photon is produced and detected by a photomultiplier, generating a pulse that needs to be digitized. The digitization is performed by the HGCROC, a front-end readout chip originally designed for the CMS experiment. The HGCROC performs an analog-to-digital conversion (ADC) and transmits the data to other electronics in the HCAL. 

This talk will describe the readout chain and the configuration of the front-end chip prior to data acquisition, which was the focus of my summer research. Specifically, I will discuss the process of leveling the “pedestal” measurements across the chip’s 72 channels, which are distributed among two electrical links. The process of leveling pedestals begins with taking a pedestal run, which measures the baseline ADC count of each channel with no charge being injected into the chip, thus reflecting electronic noise. Then, channel- and link-specific parameters are adjusted to shift the ADC response until the final pedestal run shows uniform ADC values across all channels. This work is very timely as we prepare to test the readout electronics in a beam test later this Fall.