Antihydrogen Trapped In The Alpha Experiment

In 2010 the ALPHA collaboration succeeded in trapping antihydrogen atoms for the first time.[i] Stored antihydrogen promises to be a unique tool for making high precision measurements of the structure of this first anti-atom. Achieving this milestone presented several substantial experimental challenges and this talk will describe how they were overcome. The unique design features of the ALPHA apparatus will be explained.[ii] These allow a high intensity positron source and an antiproton imaging detector similar to the one used in the ATHENA[iii] experiment to be combined with an innovative magnet design of the anti-atom trap. This seeks to minimise the perturbations to trapped charged particles which may cause particle loss and heating[iv]. The diagnostic techniques used to measure the diameter, number, density, and temperatures of both plasmas will be presented as will the methods developed to actively compress and cool of both plasma species to sizes and temperatures [v], [vi], [vii] where trapping attempts with a reasonable chance of success can be tried. The results of the successful trapping experiments will be outlined as well as some subsequent experiments to improve the trapping rate and storage time. [i] 'Trapped antihydrogen' G.B. Andresen et al., Nature 468, 673 (2010) [ii]'A Magnetic Trap for Antihydrogen Confinement' W. Bertsche et al., Nucl. Instr. Meth. Phys. Res. A566, 746 (2006) [iii] Production and detection of cold antihydrogen atoms M. Amoretti et al., Nature 419, 456 (2002). [iv]' Antihydrogen formation dynamics in a multipolar neutral anti-atom trap' G.B. Andresen et al., Phys. Lett. B 685, 141 (2010) [v]' Evaporative Cooling of Antiprotons to Cryogenic Temperatures', G.B. Andresen et al. Phys. Rev. Lett 105, 013003 (2010) [vi]'Compression of Antiproton Clouds for Antihydrogen Trapping' G.B. Andresen et al. Phys. Rev. Lett 100, 203401 (2008) [vii] 'Autoresonant Excitation of Antiproton Plasmas' G.B. Andresen et al., Phys. Rev. Lett. 106, 025002 (2011) Organizer: Ferdinand Hahn PH/DT Detector Seminar webpage.

In 2010, the ALPHA collaboration achieved a first for mankind: the stable, long-term storage of atomic antimatter, a project carried out a the Antiproton Decelerator facility at CERN. A crucial element of this observation was a dedicated silicon vertexing detector used to identify and analyze antihydrogen annihilations. This thesis reports the methods used to reconstruct the annihilation location. Specifically, the methods used to identify and extrapolate charged particle tracks and estimate the originating annihilation location are outlined. Finally, the experimental results demonstrating the first-ever magnetic confinement of antihydrogen atoms are presented. These results rely heavily on the silicon detector, and as such, the role of the annihilation vertex reconstruction is emphasized.

The advent of high-precision antihydrogen spectroscopy has opened up the possibility of direct tests with unprecedented accuracy of some of the most fundamental principles of physics, notably Lorentz and CPT symmetry and the Einstein equivalence principle. This book reviews these principles, emphasising their interconnections in quantum field theory and general relativity and the special role of antimatter, and explores how they may be tested in current and forthcoming experiments on antihydrogen. Original research results relevant to the experimental programme of the ALPHA collaboration at CERN are presented, together with the implications for antihydrogen of proposed theories featuring novel `fifth-force' interactions.

The asymmetry between matter and antimatter in the universe and the incompatibility between the Standard Model and general relativity are some of the greatest unsolved questions in physics. The answer to both may possibly lie with the physics beyond the Standard Model, and comparing the properties of hydrogen and antihydrogen atoms provides one of the possible ways to exploring it. In 2010, the ALPHA collaboration demonstrated the first trapping of antihydrogen atoms, in an apparatus made of a Penning--Malmberg trap superimposed on a magnetic minimum trap. Its ultimate goal is to precisely measure the spectrum, gravitational mass and charge neutrality of the anti-atoms, and compare them with the hydrogen atom. These comparisons provide novel, direct and model--independent tests of the Standard Model and the weak equivalence principle. Before they can be achieved, however, the trapping rate of antihydrogen atoms needs to be improved. This dissertation first describes the ALPHA apparatus, the experimental control sequence and the plasma manipulation techniques that realised antihydrogen trapping in 2010, and modified and improved upon thereafter. Experimental software, techniques and control sequences to which this research work has contributed are particularly focused on. In the second part of this dissertation, methods for improving the trapping efficiency of the ALPHA experiment are investigated. The trapping efficiency is currently hampered by a lack of understanding of the precise plasma conditions and dynamics in the antihydrogen production process, especially in the presence of shot--to--shot fluctuations. This resulted in an empirical development for many of the plasma manipulation techniques, taking up precious antiproton beam time and resulting in suboptimal performance. To remedy these deficiencies, this work proposes that simulations should be used to better understand and predict plasma behaviour, optimise the performance of existing techniques, allow new techniques to be explored efficiently, and derive more information from diagnostics. A collection of numerical models for Penning--Malmberg trap plasmas are introduced, which are designed to simulate a major subset of the plasma manipulation techniques used in ALPHA, targeted at the plasma conditions available therein, and with near--real--time experimental usability in mind. The first of these is a zero--temperature plasma solver, which exploits the water bag model to compute the density and potential of a cold, stationary plasma with a given radial profile and electrode excitations. It is suited to analysing slow (or stationary) processes, where the variations applied are on a much slower time scale than the typical time between collisions in the plasma. The density and electric potential output by the solver inform the programming of the electrode voltages, which is of particular value when plasma bunches need to be weakly confined in shallow wells. The second numerical model developed for this work is a radially--coupled Vlasov--Poisson solver, which evolves the axial phase space distribution of a plasma under the influence of (time-dependent) electrode excitations, from a given initial state. It takes into account the plasma self--field and the radial variations in potential and density, and assumes that radial transport is negligible. This model simulates processes where the dynamic behaviour of the plasma is critical to their outcome. It allows for tests of plasma manipulation techniques over a wide range of tunable parameters and plasma conditions prior to an actual experiment, potentially reducing the need for empirical tuning. The third numerical model is an azimuthally averaged, energy--conserving Fokker--Planck solver for a discrete, non-regular grid distribution. It simulates the effects of weakly magnetised collisions on the bulk parallel and perpendicular velocity distributions of a plasma, as the particles collide among themselves. The collision coefficients are analytically calculated by azimuthally averaging the derivatives of the Rosenbluth potentials. This model is applicable to plasmas where self--collisions of antiprotons have a non-negligible effect, possible examples of which include the antiproton--positron mixture which exists during antihydrogen formation, and the antiproton cloud captured from the Antiproton Decelerator, the source of ALPHA's antiprotons. The fourth numerical model is an azimuthally averaged Fokker--Planck model for intermediately magnetised collisions. It generalises the preceding model to study Fokker--Planck--type collisions of electrons, positrons and antiprotons in magnetic fields of arbitrary strength. Unlike the previous model, analytic solutions for collisions in arbitrarily strong magnetic fields are not known. The collision coefficients are therefore computed numerically via an adaptive Monte Carlo averaging of the colliding particles' changes in parallel and perpendicular velocities, over their impact parameter and their velocity phase angles. The collision process itself is simulated via a variable--time--stepping Boris particle pusher. This model is applicable to a wide range of processes involving cooling and thermalisation, which are critical to the ALPHA experiment. The water bag and Vlasov models are employed to simulate the excitation of antiprotons during the antiproton--positron mixing process, which produces antihydrogen atoms and determines whether they can be confined by the magnetic minimum trap. The agreement between the simulation and experimental measurements, analytic predictions and other existing simulations is demonstrated. The simulation is then used to optimise the excitation under various plasma conditions, and novel excitation techniques are proposed and explored. The models developed throughout this work lay the foundation for a systematic analysis of the plasma phenomena in the experiment. Future work includes extending the result of the mixing simulation to study collisional and recombination effects, as well as applying the models to other processes in the experiment. It is also of interest to apply the collisional formulations in this work to particle--in-cell (PIC) models and to explore three--dimensional plasma effects.

This book contains the proceedings of the Fourth Meeting on CPT and Lorentz Symmetry, held at Indiana University in Bloomington on August 8-11, 2007. The Meeting focused on experimental tests of these fundamental symmetries and on important theoretical issues, including scenarios for possible relativity violations.Experimental subjects covered include: astrophysical observations, clock-comparison measurements, cosmological birefringence, electromagnetic resonant cavities, gravitational tests, matter interferometry, muon behavior, neutrino oscillations, oscillations and decays of neutral mesons, particle-antiparticle comparisons, post-Newtonian gravity, space-based missions, spectroscopy of hydrogen and antihydrogen, and spin-polarized matter. Theoretical topics covered include: physical effects at the level of the Standard Model, General Relativity, and beyond; the possible origins and mechanisms for Lorentz and CPT violations; and associated issues in field theory, particle physics, gravity, and string theory. Contributors consist of the leading experts in this very active research field.

This book contains the proceedings of the Fourth Meeting on CPT and Lorentz Symmetry, held at Indiana University in Bloomington on August 8-11, 2007. The Meeting focused on experimental tests of these fundamental symmetries and on important theoretical issues, including scenarios for possible relativity violations. Experimental subjects covered include: astrophysical observations, clock-comparison measurements, cosmological birefringence, electromagnetic resonant cavities, gravitational tests, matter interferometry, muon behavior, neutrino oscillations, oscillations and decays of neutral mesons, particle-antiparticle comparisons, post-Newtonian gravity, space-based missions, spectroscopy of hydrogen and antihydrogen, and spin-polarized matter. Theoretical topics covered include: physical effects at the level of the Standard Model, General Relativity, and beyond; the possible origins and mechanisms for Lorentz and CPT violations; and associated issues in field theory, particle physics, gravity, and string theory. Contributors consist of the leading experts in this very active research field. Sample Chapter(s). Improved Tests of Lorentz and Cpt Symmetry Using Noble-Gas Masers (447 KB). Contents: Improved Tests of Lorentz and CPT Symmetry using Noble-Gas Masers (A Glenday et al.); Rotating Experiments to Test Lorentz Invariance in the Photon Sector (M E Tobar et al.); Perspectives on Lorentz and CPT Violation (V A Kosteleckcents); Lorentz Violation in a Diffeomorphism-Invariant Theory (R Jackiw); Studies of CPT Symmetry with ASACUSA (R S Hayano); Torsion Balance Test of Preferred-Frame and Weak Coupling to Polarized Electrons (B R Heckel et al.); Seeking a Solution of the Pioneer Anomaly (M M Nieto & J D Anderson); Preliminary Results from a Test of CPT and Lorentz Symmetry using a K- 3 He Co-magnetometer (T W Kornack et al.); Ives-Stilwell for the New Millennium (M A Hohensee et al.); Data Tables for Lorentz and CPT Violation (V A Kosteleckcents & N Russell); and other papers. Readership: Theoretical and experimental physicists with interests in relativity, spacetime symmetries, and underlying unified t

This book contains the proceedings of the Fourth Meeting on CPT and Lorentz Symmetry, held at Indiana University in Bloomington on August 8-11, 2007. The Meeting focused on experimental tests of these fundamental symmetries and on important theoretical issues, including scenarios for possible relativity violations.Experimental subjects covered include: astrophysical observations, clock-comparison measurements, cosmological birefringence, electromagnetic resonant cavities, gravitational tests, matter interferometry, muon behavior, neutrino oscillations, oscillations and decays of neutral mesons, particle-antiparticle comparisons, post-Newtonian gravity, space-based missions, spectroscopy of hydrogen and antihydrogen, and spin-polarized matter. Theoretical topics covered include: physical effects at the level of the Standard Model, General Relativity, and beyond; the possible origins and mechanisms for Lorentz and CPT violations; and associated issues in field theory, particle physics, gravity, and string theory. Contributors consist of the leading experts in this very active research field.

Positrons can be used to study metallic defects. Positron annihilation experiments have been carried out to identify the defects in complex oxides. Positrons have also been used to study the Bose–Einstein condensation (BEC). Ps-BEC can be used to measure antigravity using atomic interferometers. This Special Issue hopes to bring awareness of the various aspects of positron interactions to the larger physics communities. We invite authors to submit articles from all areas of physics.

This volume provides detailed insight into the field of precision spectroscopy and fundamental physics with particles confined in traps. It comprises experiments with electrons and positrons, protons and antiprotons, antimatter and highly charged ions together with corresponding theoretical background. Such investigations represent stringent tests of quantum electrodynamics and the Standard model, antiparticle and antimatter research, test of fundamental symmetries, constants and their possible variations with time and space. They are key to various aspects within metrology such as mass measurements and time standards, as well as promising to further developments in quantum information processing. The reader obtains a valuable source of information suited for beginners and experts with an interest in fundamental studies using particle traps.