Measurement of the material in the ATLAS Inner Detector using hadronic interactions for an improvement in the track reconstruction

The ATLAS is a high energy physics experiment at the Large Hadron Collider (LHC). The main purpose of this experiment is a discovery of the new particles and new phenomena. In the experiment, the inner detector plays a crucial role in the track reconstruction of charged particles, and it is essential to reconstruct tracks precisely since the accuracy of the track reconstruction strongly affects physics results. Since particles passing through the detector are affected by the material, a precise description of the material in the detector is required for the Monte Carlo simulation to obtain a good agreement in tracking between the data from the detector and the simulation. However, it is hardly possible to put an exact amount of the materials in the simulation just from the design in the engineering point of view, thus datadriven method is adopted for the measurement of the detector materials. Although photon conversions are traditionally used for the material measurement, the method using hadronic interactions is adopted for this analysis, and comparison of measured material in the inner detector between data and simulation is performed. The data used in this analysis were collected in March-June 2010 in proton-proton collisions at a center-of-mass energy of 7 TeV by the ATLAS detector. In this period, the integrated luminosity was 19 nb${}^{−1}$. Since the fake vertices accidentally reconstructed from the irrelevant tracks are expected to increase in recent high-luminosity runs, these low luminosity runs are intentionally used to retain the high purity of the reconstructed secondary vertices. The secondary vertices, which are decay points of primary particles generated in the collisions, are reconstructed from the secondary track candidates that are expected to emerge from the hadronic interactions with the detector material. Hence, it is necessary to reconstruct many secondary tracks which started at the points away from the proton-proton collision points. However, the standard reconstruction system in the ATLAS is not optimised for treating such secondary tracks, thus the retracking technique which retries the track reconstruction with the loose selection criteria is adopted. The combination of the retracking and the secondary vertex reconstruction provides much more hadronic interaction vertices than the past analysis, and the structure of the outer layers in the tracking system becomes clearly visible. On the other hand, as the purity of the reconstructed vertices decreases due to the combination of the irrelevant tracks, the study to improve the purity after the retracking is also performed. As a result of the analysis, 5-6% material uncertainty is estimated under the current geometry in the simulation, while 10% uncertainty on the amount of the material is applied for any physics analysis. In addition, the systematic uncertainty on the track reconstruction efficiency is evaluated in the assumption where an improved description of the material is considered in the simulation, and physics impacts are also investigated with the improved material estimation. This study indicates the possibility that the uncertainties on the jet energy scale and the fragmentation function would decrease by about 30% and 20%, respectively.