The design and expected performance of the ALICE ITS3 upgrade

TL;DR

The paper presents the design and feasibility of ALICE ITS3, a major upgrade that replaces the inner three tracking layers with six stitched, bent wafer-scale MAPS on carbon-foam spacers to achieve a very low material budget of 0.09% $X_0$ and a closest approach of 19 mm to the IP. The ITS3 concept promises a ~2× improvement in pointing resolution at low $p_T$, enabling enhanced heavy-flavor and low-mass dielectron measurements, as well as more precise QGP temperature determinations. The work details sensor R&D progress (MOSS, MOST, MOSAIX prototypes) using 65-nm TPSCo MAPS on 300 mm wafers, demonstrations of high yield, irradiation tolerance, and bending performance, and outlines the path to final production before Run 4, with a vision for 50 μm-thin, ~40 mW/cm$^2$ sensors for ALICE 3 and beyond. Overall, ITS3 represents a feasible, high-impact advancement in vertexing and low-momentum tracking for heavy-ion physics and precision electroweak probes at the LHC.

Abstract

During the LHC Long Shutdown 3 (2026-29) ALICE will replace its three innermost tracking layers by a new detector, the "ITS3". It will be based on newly developed, wafer-scale monolithic active pixel sensors, which are bent into truly cylindrical layers and held in place by light mechanics made from carbon foam. Unprecedented low values of material budget (0.09\% $X_0$ per layer) and proximity to the interaction point (19 mm) lead to a factor two improvement in pointing resolutions for particles from very low $p_{\mathrm{T}}$ (O(100 MeV/$c$)), achieving, for example, 20 $μ$m and 15 $μ$m in the transversal and longitudinal directions, respectively, for 1 GeV/$c$ particles. After a successful R&D phase (2019-2023), which demonstrated the feasibility of this innovative detector and led to the Technical Design Report (https://cds.cern.ch/record/2890181/), the final sensor and mechanics are being developed right now. This contribution will review the conceptual design and the main R&D achievements, as well as the current activities and road to completion and installation. It includes a projection of the improved physics performance, in particular for heavy-flavor mesons and baryons, as well as for thermal dielectrons that will come into reach with this new detector installed.

Figures (9)

• Figure 1: Left: The ALICE ITS3 upgrade will have three layers. Each layer consists of two stitched and bent 27 cm-long sensors. Right: The process of stitching allows for a repeated sensor unit (RSU) and a left endcap (LEC) and a right endcap (REC) to be made into one wafer-scale sensor. Figures are taken from its3tdr.
• Figure 2: The ITS3 will allow for measurements of B-meson decays with much better precision than the ITS2 today. Figure taken from Ref. its3tdr.
• Figure 3: Tracking efficiency and impact parameter resolution for primary particles in Pb-Pb collisions as measured using the present ITS2 and the ITS2 with the ITS3 layers. Especially the pointing resolution improves by a factor 2 over almost all momenta. Figure taken from Ref. its3tdr.
• Figure 4: The QGP temperature can be measured from thermal dielectrons. The ITS3 reduces the systematic error on the temperature by a factor two compared with what the ITS2 can achieve today. Figure taken from Ref. its3tdr.
• Figure 5: Full-scale engineering model of an ITS3 detector including a dummy sensor and flexible PCBs for powering and readout. The wire bonding of such a bent sensor, shown in the figure, needs special care and tools.