Background studies in the CMS DT chambers during LHC Run 2

Characterising background in terms of trends vs LHC instantaneous luminosity and of spatial distributions, helps to understand its sources and to devise mitigation measures to prevent detector early ageing, especially in view of the Luminosity increase expected at High Luminosity LHC. During LHC Run2, the background in the CMS Drift Tubes (DT) was monitored by means of both online anode current measurements and offline analysis. For the offline analysis a method was developed to classify background and signal objects, where objects are either single hits or reconstructed track segments. Both rates and currents were observed to depend linearly on LHC instantaneous luminosity: the slopes of these trends were used as a measure of background, depending only on the chamber position. A Zero Bias data sample was used to measure both background and signal rates, however the same method can also be applied to derive conservative background estimates using samples muon enriched skimmed events. The observed spatial distribution of background supports the hypothesis of two different components: one coming directly from the interaction point, the other coming from outside the detector, through a so called neutron gas setting up in the cavern.