Interplane alignment of SiT detector in ATLAS Forward Proton detectors

The ATLAS Forward Proton (AFP) detectors open up new possibilities to expand the ATLAS physics reach and probe unique physics processes by measuring intact protons produced in diffractive and photon-induced processes. The AFP system is made up of devices known as Roman pots (stations) which are located at a distance of approximately 205 m and 217 m on both sides of the interaction point and uses the Roman pot devices to perform the measurement in the vicinity of the LHC beam. The near stations consist of only 3D Silicon Tracker (SiT) detectors that measure the positions of the protons, while the far stations contain also Time of Flight (ToF) detectors, which measure the arrival times of the protons. An AFP SiT detector consists of four planes of silicon pixel sensors. In order to ensure the best accuracy of the measurements, it is necessary to precisely understand the relative positions of these pixel planes, known as inter-plane alignment. The method applied in this study to obtain the parameters of the inter-plane alignment, is based on differences between the cluster positions measured separately in each pixel plane and the position of the track reconstructed from the measurement in all planes. The algorithm applies corrections to the alignment parameters iteratively, until convergence is reached. To stabilize the numerical solution, only the most relevant degrees of freedom are considered: the shifts in the x and y directions and the rotations around the z-axis.