ALICE ITS3 WP4 Research

The ALICE experiment at CERN investigates the Quark-Gluon Plasma (QGP), a state of matter that mimics the conditions of the early universe. The key to understanding QGP lies in observing heavy particles like Charm and Beauty quarks that penetrate through it. Just like throwing a heavy bowling ball through a foggy room reveals the fog's density by its trail, these heavy quarks serve as probes to understand QGP properties.

However, these particles have extremely short lifetimes and decay in an instant. To capture their precise positions just before decay, we need highly accurate detectors placed as close as possible to the collision point.

This is the role of the Inner Tracking System (ITS). The next-generation ITS3 introduces a revolutionary idea: rolling paper-thin silicon sensors into cylindrical shapes. A flat sheet of A4 paper is floppy and can't support anything, but when rolled into a tube, it becomes sturdy enough to hold a book. This geometric stiffness allows the sensors to maintain their shape without separate support structures or cooling pipes.

This dramatically reduces the material budget (approximately 0.05% X₀ per layer). This minimizes multiple scattering effects in low-momentum particles, allowing particles to fly through undisturbed for more accurate trajectory measurements.

As a researcher at Pusan National University's HIPEx Lab, I participated in ALICE Collaboration's ITS3 WP4 (Mechanics & Engineering) research, conducting performance verification experiments on these innovative ITS3 sensors.

The main research task was to verify that silicon sensors don't break or degrade in performance when bent to the actual design specifications (radius 18mm, 24mm). Specifically, I experimentally investigated whether the mechanical stability of the silicon lattice and electrical response characteristics of the pixels are maintained when curvature is applied to ALPIDE sensors.

When sensors are forcibly bent (stressed), they may malfunction. The key verification items were:

• Threshold distribution: Whether the signal detection baseline remains consistent
• Noise level: Whether unwanted noise signals increase
• Fake hit rate: Whether false signal rates stay within acceptable limits

Initial measurements showed no significant performance degradation due to bending. However, this research was interrupted due to mandatory military service obligations, and a final conclusion was not reached. The collected data may serve as foundational material for future research.