Data analysis, simulations, and reconstruction of antiproton annihilations in a silicon pixel detector

There are strong theoretical arguments in favor of the gravitational acceleration being identical for matter and antimatter, as anything else would violate the weak equivalence principle. The weak equivalence principle is the cornerstone of general relativity, and today there are no experiments contradicting it. However, it has never been experimentally verified that the gravitational acceleration of matter and antimatter is indeed identical. The AEgIS experiment at CERN aims at measuring the gravitational acceleration of antimatter to a precision of 1% by determining the fall of antihydrogen over the length of around 1 m. The proposed method will make use of a position sensitive detector to measure the annihilation point of antihydrogen. Such a detector must be able to tag an antihydrogen atom, measure its time of arrival, and reconstruct its annihilation point with high precision in the vertical direction. This thesis presents a detector response model for antiproton annihilations in a silicon detector equipped with the Timepix3 readout, in order to evaluate the possibility of using such a detector in the AEgIS experiment. Antiprotons from the Antiproton Decelerator at CERN were used to obtain data of direct annihilations on the surface of a Timepix3 detector. These data were used to develop and verify the detector response model. The work presented here includes all steps from data collection, simulation, and verification of the simulation. Clear tagging criteria for annihilation clusters were found, and a tagging efficiency of 50 ± 10%is achieved. By using the annihilation products to reconstruct the annihilation point a position resolution of 22 μm is achieved on a subset of the annihilation clusters. This thesis also includes a full simulation of the GRACE beam line that was build to improve the experimental conditions for testing detectors. The GRACE beamline can select out only the low energy antiprotons and direct them towards the detector. The simulation of the GRACE beamline evaluates the flux and energy of the antiprotons, and in most cases reproduced the energy distribution and flux within ± 30 %

Study of antiproton annihilation in silicon with a hybrid pixel detector using the TimePix3 readout

The main goal of the AEgIS experiments is to measure the gravitational force for anti-hydrogen, testing Einstein's weak equivalence principle, which states that all bodies falls with the same acceleration, independently from their mass and composition. The measurement will be done using an anti-hydrogen beam sent trough a classical moire deflectometer. To measure the deflection of the beam from a straight path a position sensitive silicon detector followed by an emulsion detector and a scintillating fiber time-of-flight detector will be used. We present here a study performed using a novel hybrid pixel detector, employing the Timepix3 readout chip to tag and spatially resolve antiproton annihilations in silicon. In autumn 2014 we performed a test-experiment on a secondary beam line to the AEgIS experiment, where a pulsed beam of anti-protons of energy 5.3 MeV was delivered from the Antiproton Decelerator of CERN accelerator complex. Taking advantage of the high spatial resolution, ToA capabilities and extended energy range of the Timepix3, this study investigates unique features of antiproton annihilation events in silicon. We are for the first time able to set clear criteria to characterize an antiproton annihilation using a silicon detector

Treatment of non-Gaussian noise in invariant mass calculations

The Gaussian Sum Filter is a track reconstruction algorithm for treating energy loss by bremsstrahlung, and produces non-Gaussian estimates for the track parameters. This thesis explores a method of propagating these non-Gaussian errors into a non-Gaussian estimate of the invariant mass. It is tested if the method can be used to improve the invariant mass resolution in ATLAS, and if it gives a good description of the errors on the invariant mass. The result showed that the invariant mass resolution is not improved, but a large improvement in the description of errors is found

Comparison of planar and 3D silicon pixel sensors used for detection of low energy antiprotons

The principle aim of the AEḡIS experiment at CERN is to measure the acceleration of antihydrogen due to Earth’s gravitational field. This would be a test of the Weak Equivalence Principle, which states that all bodies fall with the same acceleration independently of their mass and composition. The effect of Earth’s gravitational field on antimatter will be determined by measuring the deflection of the path of the antihydrogen from a straight line. The position of the antihydrogen will be found by detecting its annihilation on the surface of a silicon detector. The gravitational measurement in AEḡIS will be performed with a gravity module, which includes the silicon detector, an emulsion detector and a scintillating fibre time-offlight detector. As the experiment attempts to determine the gravitational acceleration with a precision of 1 %, a position resolution better than 10 μm is required. Here we present the results of a study of antiproton annihilations in a 3D silicon pixel sensor and compare the results with a previous study using a monolithic active pixel sensor. This work is part of a larger study on different silicon sensor technologies needed for the development of a silicon position detector for the AEḡIS experiment. The 3D detector together with its readout electronics have been originally designed for the ATLAS detector at the LHC. The direct annihilation of low energy antiprotons (∼ 100 keV) takes place in the first few μm of the silicon sensor and we show that the charged products of the annihilation can be detected with the same sensor. The present study also aims to understand the signature of an antiproton annihilation event in segmented silicon detectors and compares it with a GEANT4 simulation model. These results will be used to determine the geometrical and process parameters to be adopted by the silicon annihilation detector to be installed in AEḡIS

Positron bunching and electrostatic transport system for the production and emission of dense positronium clouds into vacuum

We describe a system designed to re-bunch positron pulses delivered by an accumulator supplied by a positron source and a Surko-trap. Positron pulses from the accumulator are magnetically guided in a 0.085 T field and are injected into a region free of magnetic fields through a μμ-metal field terminator. Here positrons are temporally compressed, electrostatically guided and accelerated towards a porous silicon target for the production and emission of positronium into vacuum. Positrons are focused in a spot of less than 4 mm FWTM in bunches of ∼8 ns FWHM. Emission of positronium into the vacuum is shown by single shot positron annihilation lifetime spectroscopy

Towards the first measurement of matter-antimatter gravitational interaction

The AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is a CERN based experiment with the central aim to measure directly the gravitational acceleration of antihydrogen. Antihydrogen atoms will be produced via charge exchange reactions which will consist of Rydberg-excited positronium atoms sent to cooled antiprotons within an electromagnetic trap. The resulting Rydberg antihydrogen atoms will then be horizontally accelerated by an electric field gradient (Stark effect), they will then pass through a moiré deflectometer. The vertical deflection caused by the Earth's gravitational field will test for the first time the Weak Equivalence Principle for antimatter. Detection will be undertaken via a position sensitive detector. Around 103 antihydrogen atoms are needed for the gravitational measurement to be completed. The present status, current achievements and results will be presented, with special attention toward the laser excitation of positronium (Ps) to the n=3 state and the production of Ps atoms in the transmission geometry

Imaging a positronium cloud in a 1 Tesla

We report on recent developments in positronium work in the frame of antihydrogen production through charge exchange in the AEgIS collaboration [1]. In particular, we present a new technique based on spatially imaging a cloud of positronium by collecting the positrons emitted by photoionization. This background free diagnostic proves to be highly efficient and opens up new opportunities for spectroscopy on antimatter, control and laser manipulation of positronium clouds as well as Doppler velocimetry

Imaging a positronium cloud in a 1 Tesla

We report on recent developments in positronium work in the frame of antihydrogen production through charge exchange in the AEgIS collaboration [1]. In particular, we present a new technique based on spatially imaging a cloud of positronium by collecting the positrons emitted by photoionization. This background free diagnostic proves to be highly efficient and opens up new opportunities for spectroscopy on antimatter, control and laser manipulation of positronium clouds as well as Doppler velocimetry

Compression of a mixed antiproton and electron non-neutral plasma to high densities

We describe a multi-step “rotating wall” compression of a mixed cold antiproton–electron non-neutral plasma in a 4.46 T Penning–Malmberg trap developed in the context of the AE¯gIS experiment at CERN. Such traps are routinely used for the preparation of cold antiprotons suitable for antihydrogen production. A tenfold antiproton radius compression has been achieved, with a minimum antiproton radius of only 0.17 mm. We describe the experimental conditions necessary to perform such a compression: minimizing the tails of the electron density distribution is paramount to ensure that the antiproton density distribution follows that of the electrons. Such electron density tails are remnants of rotating wall compression and in many cases can remain unnoticed. We observe that the compression dynamics for a pure electron plasma behaves the same way as that of a mixed antiproton and electron plasma. Thanks to this optimized compression method and the high single shot antiproton catching efficiency, we observe for the first time cold and dense non-neutral antiproton plasmas with particle densities n ≥ 1013 m−3 , which pave the way for an efficient pulsed antihydrogen production in AE¯gIS