Background in the CMS drift tubes: measurements with LHC collision data and implications for detector longevity at HL-LHC

Drift Tubes (DT) are installed in the barrel region of the CMS muon spectrometer. They are used for offline tracking of muons and provide standalone trigger capabilities. Though an upgrade of the DT system electronics is foreseen for the High Luminosity LHC (HL-LHC), the present DT chambers will not be replaced, hence they will be called to operate enduring integrated doses far beyond what they were initially designed for. Together with accelerated aging studies, accurate measurements of the background-induced hit rates and currents observed over Run-2 are critical to assess the longevity of the DT chambers and to make projections of the expected performance of the system throughout HL-LHC. This report presents the state of the art of the studies on the background affecting the DT system, and connects them to results from longevity studies performed at the CERN high-intensity gamma irradiation facility (GIF++). Moreover, it describes all measures that have been put in place, by the end of Run-2 and over LS2, to mitigate aging and, where possible, reduce background level, with the aim of maximizing the performance of the DT muon detector throughout HL-LHC.