The ATLAS detector is a large, general-purpose particle detector designed to observe high-energy particle collisions on the Large Hadron Collider at CERN. This study uses electrons and photons from Run 2 proton-proton collision data (2015 - 2016) to check for differences between real and simulated detector material in the region before the first layer of the electromagnetic (EM) calorimeter. The main probe is the ratio of energies deposited in the first and second layers of the EM calorimeter.
The measured material differences are compared against results from similar studies performed using Run 1 data. Deviations between Run 1 and Run 2 results are observed, primarily in regions where detector hardware was upgraded before Run 2. The material differences are well accounted for by combining the existing Run 1 material systematic uncertainties with additional Run 2 uncertainties related to the new inner tracking layer (the IBL) and the modified PP0 service region.