ALICE ITS3 Upgrade: first tests of MOSAIX prototype

Abstract

The Inner Tracking System of the ALICE detector at CERN’s Large Hadron Collider is to be upgraded by replacing the three innermost layers with novel, wafer-scale stitched sensors. By bending the silicon sensors around the beam pipe, an almost fully self-supporting structure is created, enabling the reduction of each layers material budget from the current 0.36 % X_0 to 0.09 % X_0. The reduction of the material in the active detection area, in combination with a reduced distance to the interaction point and an increased intrinsic resolution, enables a track impact parameter resolution increase by a factor of two, most notably for low momentum particles.
In the search for the optimal chip design, several prototypes are fabricated and extensively tested. As part of the characterization process of the (baby)MOSS prototype, a cosmic muon telescope - the Lund Telescope - was assembled and commissioned. The optimal operational configuration was identified and data acquisition was conducted for several detection thresholds over extended periods. The temperature of the detector is continuously monitored and the fake-hit rate is periodically measured.
The first testbeam with the MOSAIX sensor - the fully functional successor of the MOSS - was conducted at the T10 beam-line facility of CERN’s Proton Synchrotron. Tracking was used to perform detailed alignment of the newly built ALPIDE telescope and the functionality of the MOSAIX chip was confirmed from reconstructed hitmaps and correlations of hit coordinates.

Popular Abstract

Humanity has always been fascinated by extremes; in the realm of physics this has resulted in the study of phenomena at scales both larger and smaller than anything one could imagine, the beginning and end of the universe, and events unfolding at extreme energy densities.

But how does one go about studying such phenomena? One approach is to try and recreate these conditions by accelerating atomic nuclei to very high energies. By colliding nuclei traveling in opposite directions, an enormous amount of energy is squeezed into a tiny region of space. For an incredibly short time period, conditions comparable to those a few microseconds after the Big Bang are created. A spray of particles bursts outward in all directions, like debris from a microscopic explosion. To make sense of this, various questions need to be answered: What particles were created? In which direction are they moving? And how fast?
The detector used for such a meticulous task has many different components, each one optimized for some specific purpose.
With the development of new technologies, new ways of optimizing components are introduced. A currently ongoing development is the upgrade of the innermost sub-system of the ALICE detector at CERN, the world’s most prominent organization for particle research.
In practice, there are many different stages of development; ideas are formed and formulated; simple prototypes are designed and fabricated; these prototypes are distributed to various facilities and tested; the findings are used for the development of more complex prototypes and the process repeats.

The progression of this formidable task was followed for a few months with a focus on testing two different prototypes. The first prototype was tested by utilizing continuously arriving cosmic particles. These particles- known as muons- are very similar to electrons, but about 200 times heavier and effortlessly pass through us every second of the day. A cosmic ray telescope- called
Lund telescope- consisting of five detector planes was constructed and optimized. It has a size similar to a shoe box and control over different settings and operation modes is provided via a software. When these cosmic visitors pass through the telescope, they can be detected and their tracks can be reconstructed. These measurements are then used to evaluate key detector
properties, such as how often particles are successfully detected or how well real particle signals can be distinguished from random noise.

The second prototype was tested at a beamline facility at CERN’s Proton Synchrotron. There, high-energy particle beams are directed towards the detector. As they pass through it, they deposit small amounts of energy, which can then be recorded and analyzed. The goal is to understand the detector’s specific characteristics and determine its optimal operating conditions.
Once the scientific community feels confident that the optimal setup and operational mode has been found, the upgraded detector component will be implemented along with many other upgrades of CERN’s accelerators and detectors. This will happen during a three year long shutdown, starting in the middle of 2026.
Let’s see if the improved detectors can help us unravel more of the mysteries the universe has in store for us.

• author: Hebel, Marie-Philine Lieselotte
• supervisor: David Silvermyr; Iaroslav Panasenko
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: ALICE, ITS3, MOSS, babyMOSS, MOSAIX, Lund Telescope
• language: English
• id: 9247239
• date added to LUP: 2026-07-29 08:50:36
• date last changed: 2026-07-29 08:50:36