Investigating the impact of 4D Tracking in ATLAS Beyond Run 4

This document presents the first investigation of the usage of precision timing information in the ATLAS tracker beyond the HL-LHC Run 4. The Inner Detector of the ATLAS Experiment will be upgraded to a full-silicon Inner Tracker (ITk) to cope with the extreme conditions of the High-Luminosity phase of the Large Hadron Collider, currently foreseen to start with Run 4 towards 2029. ATLAS will also be installing a High-Granularity Timing Detector (HGTD) in the forward pseudorapidity region. The HGTD will help mitigate the effects of pile-up in the forward region by distinguishing between collisions occurring close in space but well-separated in time. Due to the high radiation dose in proximity of the interaction point, the two innermost pixel layers of the ITk are designed to be replaced after 2000 $fb^{-1}$. This represents a unique opportunity to bring in technological innovation and expand the physics potential of HL-LHC by including fast-timing through 4-dimensional (4D) tracking in the ATLAS barrel region. While HGTD will provide unique handles to improve the reconstruction of physics objects in the forward region, its capability is limited by its reduced $\eta$ acceptance. There are also compelling physics reasons to consider fast-timing in the central region. In particular, barrel timing information can significantly improve the identification of b-jets, enhancing the prospects to observe Di-Higgs. This note documents the main physics impacts that a 4D tracking upgrade beyond Run 4 could have in ATLAS. The studies are based on full simulated Monte Carlo samples, but use a simplified, and idealistic, model for track-time resolution. The goal is to assess early the physics merits of timing information in the central pseudorapidity region, before a dedicated long-term simulation effort is potentially launched as a second step.