PERFORMANCE EVALUATION FOR THE BELLE II CENTRAL DRIFT CHAMBER AFTER LONG-SHUTDOWN-I USING 2024 DATA

Sumitted to PubDB: 2026-05-12

Category: Master Thesis

Tags: CDC

Principal Authors Anh-Ky Nguyen
Date 2026-05-12
Belle II Number BELLE2-MTHESIS-2026-039
Abstract The Belle II experiment at the SuperKEKB electron--positron collider in Tsukuba, Japan, is designed to probe the frontiers of particle physics by studying CP violation, rare $B$ meson decays, exotic hadron states, and other potential signs of new physics beyond the Standard Model. With an unprecedented integrated luminosity target of $50~\mathrm{ab}^{-1}$, Belle II requires extremely precise charged-particle tracking and momentum resolution in order to fully exploit the physics opportunities offered by high-intensity $e^{+}e^{-}$ collisions. Central to this capability is the Central Drift Chamber (CDC), which provides high-granularity measurements of charged-particle trajectories, transverse and longitudinal impact parameters, and contributes critically to vertex reconstruction. During Long Shutdown I (LS1), the CDC underwent a series of important maintenance and upgrade activities to address performance challenges observed during earlier phases of operation. These challenges included gas contamination in the detector volume, aging effects on sensing wires, and instabilities in the front-end electronics, all of which contributed to degraded tracking performance. Upgrades included the installation of oxygen removal systems with Nafion dryer and platinum catalyst, enhancements to gas circulation and monitoring infrastructure, and careful reconditioning of sensitive wire and readout components. These interventions were aimed at ensuring the long-term stability and optimal functionality of the CDC under the demanding conditions of the high-luminosity Belle II program. This report presents a detailed performance evaluation of the Belle II CDC using cosmic-ray and 2024 collision data collected after LS1. A split-track method was employed, in which tracks are divided into upper and lower segments, and differences in key track parameters are used to extract single-track resolutions. The analysis covers transverse impact parameter ($d_{0}$), longitudinal impact parameter ($z_{0}$), azimuthal angle ($\phi_{0}$), dip angle ($\tan\lambda$), and transverse momentum ($p_{T}$). The results show significant improvements compared to pre-LS1 performance, with impact parameter resolutions improving by $20$--$25\%$ and transverse momentum resolution reaching approximately $0.9\%$. These improvements confirm that the CDC has successfully recovered and even exceeded its design performance, ensuring that Belle II is fully prepared for its core physics program---precision CP violation measurements and searches for rare phenomena.
Institute IOP

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