Development and data analysis of a position detector for AEḡIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy)

Abstract

AEḡIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is an antimatter experiment based at CERN, the European Organization for Nuclear Research, whose goal is to carry out the first direct measurement of the Earth’s gravitational acceleration on antimatter. The outcome of such measurement would be the first precision test of the Weak Equivalence Principle in a completely new area. According to WEP, all bodies fall with the same acceleration regardless of their mass and composition. AEḡIS will attempt to achieve its aim by measuring the gravitational acceleration (ḡ) for antihydrogen with 1% relative precision. The first step towards the final goal is the formation of a pulsed, cold antihydrogen beam, which will be performed by a charge exchange reaction between laser excited (Rydberg) positronium and cold (100 mK) antiprotons. The antihydrogen atoms will be accelerated by an inhomogeneous electric field (Stark acceleration) to form a beam whose free fall due to Earth’s gravity will be measured with a moiré deflectometer coupled to a hybrid position detector. This detector will consist of an active silicon part, where the annihilation of antihydrogen takes place, followed by an emulsion part. The work in this thesis is part of the R&D efforts for the construction of the silicon position detector. The results presented here are from beam test studies of low energy antiproton annihilations in silicon sensors. The outcome of these tests defined the basis for the final design parameters for the silicon position detector. This thesis is based on three papers. The first paper reports on the results of the very first study of low energy (0-700 keV) antiproton annihilations in a segmented silicon detector. The results include cluster and energy deposition studies, as well as a first comparison with simulation models for low energy antiproton annihilation in silicon. The second paper presents the results of a study on the signatures of an annihilation event in different silicon sensors which were designed to detect minimum ionizing particles or slow charged hadrons. The characteristics of the clusters due to antiproton annihilations were investigated for silicon detectors with various geometries. The correlation of the clusters charateristics, such as the released energy and the size provided a better overview of the performance of the microstrip technology when compared to pixel detector technologies. The third paper compares two different silicon sensor technologies (MAPS and 3D pixel) used for the detection of low energy antiprotons in order to study the impact of the thickness of the detector on the cluster characteristics, as well as to estimate the achievable position resolution. Comparison with simulation models are also reported, which proved to account for the intrinsic technological differences in the two sensors. The work and the results in this thesis work have contributed to a better understanding of the annihilation process in silicon and proved that silicon detectors can be used for direct detection of low energy antiprotons. A first comparison between experimental data and Monte Carlo simulation results for low energy antiproton annihilation is also reported, suggesting areas where the improvement of simulation models is possible.