Broken Symmetry

Everything we interact with in normal life is matter made out of electrons, protons, and neutrons. From certain physical processes and experiments, we also know that there is anti-matter consisting of anti-particles like anti-electrons and anti-protons with the only difference being their electrical charge. An electron is negative whereas its counterpart the positron is positive. Every particle has its anti-particle – a beautiful symmetry of the model. Following this symmetry, at the Big Bang particles and anti-particles should have been created in equal numbers. But why is then almost only matter in our universe today? The Belle II experiment at the accelerator SuperKEKB in Tsukuba, Japan, was built to answer this fundamental question.

Theories attribute the difference between particles and anti-particles to the violation of the so-called CP symmetry. Various physical processes breaking this symmetry have been already observed and analyzed at previous experiments but none of the results can explain fully the actual excess of matter in our universe. The Belle II experiment was designed to test these new theories, which include “new physics” beyond the Standard Model of particle physics, and resolve this fundamental question. The aim of the experiment is to analyze the particle/anti-particle system with extreme accuracy by minimizing both, the statistical and systematic errors of the measurement.

In the heart of the Belle II experiment, closest to the interaction point, is the PXD (PiXel Detector). It measures effectively the lifetime of particles and anti-particles generated at the interaction point of electrons and positrons with unprecedented accuracy. The HLL designed and built the DEPFET based pixel sensors and developed and assembled the modules. In collaboration with Universities from Germany, Spain, Czech Republic, Poland, and the institutions DESY and MPI for Physics, the HLL contributed substantially to design, construction, installation and operation of the entire pixel detector system. The first version of PXD was installed in Belle II 2019 and is since then taking data.

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High precision measurements with minimal material

Produced from 1000-fold purer silicon than traditional transistors or memory chips, the Belle II pixel detector (PXD) module integrates 200.000 DEPFET pixel cells on a surface of about 8 cm². DEPFET is an acronym for Depleted p-channel Field Effect Transistor. The DEPFET silicon technology allows for the detection of single photons or charged particles with highest efficiency and precision. The basic detection process is the same as in customary photo- or video-cameras. However, the primary signal from single particles is much smaller compared to the photon flux of visible light impinging on commercial cameras. Here the unique feature of the DEPFET pixel comes into play: the very small signal from a single particle hitting the pixel is amplified in the pixel itself. In this way, the DEPFET pixel combines the sensor material and the first amplification in one unit. Due to the arrangement of many DEPFET pixels to an array an image sensor is created, with which one can determine the passage of a particle through the sensor with an accuracy of about one hundredth of a millimeter (10 micrometers).

In order to process and read out the pixel information from the matrix as fast as possible, additional electronics is necessary. Realized in form of application-specific integrated circuits (ASICs), this electronics is directly attached to the sensor substrate. The signals of the pixel matrix are digitized and discriminated by these ASICs, and finally transferred to the data acquisition system with a rate of 50.000 pictures per second. Due to the enormous number of electrical connections on the sensor, joining the 200 000 pixels to the ASICs, the DEPFET matrix becomes a complex module with a maximum integration density. Despite of all its complexity, the matrix can be made extremely thin and light, with the crucial advantage that a particle track is not disturbed by the sensor material itself, thus enabling the most precise measurement of the particle’s origin (“vertex”).

For the realization of a pixel vertex detector, forming the innermost two layers of the Belle II detector, the HLL developed a unique technology, which allows producing extremely thin and highly integrated sensor modules. The sensitive part of the module, the DEPFET matrix, is thinned down by a anisotropic etching technique to 75 micrometers, corresponding to the thickness of a human hair. This thin foil of sensitive silicon and flexible in itself, is supported by a monolithically integrated frame, where the ASICs and other passive elements are mounted using flip-chip and SMD technologies. The power and data lines run through a flexible electrical circuit attached to the end of the module. Two such modules are glued together at the other end of the sensor, making up a self-supporting “ladder” with the flexible supply and readout circuits on either end.

This technology allows for the arrangement of the planar, thin DEPFET ladders in a cylindrical shape around the interaction point of the experiment, supported only at the ends, where the main ASICs are located. The support, which is outside of the acceptance volume of the detector, also serves as a cooling block for the ASICs, flushing liquid CO2 through specially arranged micro channels within the cooling block. The full PXD vertex detector is made of 8 ladders in the inner layer and 12 ladders in the outer layer, located 14 and 22 millimeters away from the interaction point. With this arrangement the required high precision measurements of particle tracks and their origins become reality.

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