DT hit efficiency measurements with reduced high voltage

Efficiency measurements were carried out on the CMS Drift Tubes (DT) applying anode High Voltage lower than nominal. The "HV scans", performed periodically with cosmic rays, allowed to exclude detector ageing during Run 2, and also to assess the effect of lowering the Front End threshold. These studies allowed the optimization of HV and threshold settings, that in 2018 were tuned to protect the chambers as much as possible while preserving the previous efficiency. Measurements performed with pp collisions and a reduced HV applied to one layer per chamber, made visible the dependence of efficiency on track inclination. Since the DT chambers most exposed to radiation are also the ones where tracks are most inclined, this dependence has a positive effect in the prediction of future ageing scenarios.