Performance of production modules of the Belle II pixel detector in a high-energy particle beam

TL;DR

This paper evaluates the performance of the Belle II pixel detector modules in a high-energy particle beam, demonstrating their suitability for precise vertex measurements in the Belle II experiment.

Contribution

It provides the first detailed beam test results of the Belle II PXD modules, showing their spatial resolution and efficiency meet experimental requirements.

Findings

Spatial resolution depends on incidence angle and charge sharing.

Measured module performance aligns with Belle II specifications.

Modules exhibit high hit efficiency in beam tests.

Abstract

The Belle II experiment at the Super B factory SuperKEKB, an asymmetric $e^+e^-$ collider located in Tsukuba, Japan, is tailored to perform precision B physics measurements. The centre of mass energy of the collisions is equal to the rest mass of the $\Upsilon(4S)$ resonance of $m_{\Upsilon(4S)} = 10.58\,\rm GeV$. A high vertex resolution is essential for measuring the decay vertices of B mesons. Typical momenta of the decay products are ranging from a few tens of MeV to a few GeV and multiple scattering has a significant impact on the vertex resolution. The VerteX Detector (VXD) for Belle II is therefore designed to have as little material as possible inside the acceptance region. Especially the innermost two layers, populated by the PiXel Detector (PXD), have to be ultra-thin. The PXD is based on DEpleted P-channel Field Effect Transistors (DEPFETs) with a thickness of only $75\,\rm\mu m$. Spatial resolution and hit efficiency of production detector modules were studied in beam tests performed at the DESY test beam facility. The spatial resolution was investigated as a function of the incidence angle and improvements due to charge sharing are demonstrated. The measured module performance is compatible with the requirements for Belle II.