Studies of the ATLAS Inner Detector material using $\sqrt{s}=$13 TeV $pp$ collision data

The ATLAS Inner Detector comprises three different technologies: the Pixel detector (Pixel), the silicon strip tracker (SCT), and the transition radiation drift tube tracker (TRT). The material in the ATLAS Inner Detector is studied with several methods, using the $pp$ collision sample collected at $\sqrt{s}=$13 TeV in 2015. The material within the innermost barrel regions of the ATLAS Inner Detector is studied using reconstructed hadronic interaction and photon conversion vertices from samples of minimum bias events. It was found that the description of the Insertable B-Layer, which is the new, innermost Pixel layer installed in 2014, in the geometry model was missing some material components. After updating the model, data and simulation show good agreement at the barrel region. The Pixel services (cables, cooling pipes, support trays) were modified between the Pixel and SCT detectors in 2014. The material in this region is also studied by investigating the efficiency with which tracks reconstructed only in the Pixel detector can be matched to tracks reconstructed in the Pixel and SCT detectors. The track extension efficiencies in samples of minimum bias events in data and simulation are observed to agree well in the barrel region. Some discrepancies are observed in the Pixel service region at forward pseudorapidities. These studies have led to several improvements in the description of the material in the ATLAS Inner Detector simulation.