CERN Accelerating science

We have every reason to believe that equal amounts of matter and antimatter were produced in the early universe. Moreover, theory predicts that the laws of physics make no distinction between the two. In this light, the fact that the observable universe is overwhelmingly dominated by matter is inexplicable. ALPHA is an international project located at CERN involving approximately 40 physicists from 15 different institutions in 7 countries. The primary goal of the collaboration is to study the antihydrogen atom at the highest level of precision possible, and thereby enable comparisons between hydrogen and antihydrogen. Through these comparisons it hopes to improve our understanding of the distinction between matter and antimatter, and perhaps shed some light on the puzzle of why we live in a matter dominated universe. The hyperfine energy intervals of ground-state hydrogen and antihydrogen represent an opportunity for a precision comparison. A discrepancy between the energy levels of these two atomic systems would indicate a major revolution in physics, and in our understanding of the universe. [...]

Antihydrogen, the bound state of a positron and an antiproton, is the simplest pure anti-atomic system and an excellent candidate to test the symmetry between matter and antimatter. This thesis focuses on the magnetic confinement of antihydrogen and the first ever resonant interaction with trapped antihydrogen, as performed by the ALPHA collaboration. The ALPHA apparatus and the techniques that have been developed to form, trap, probe, and detect antihydrogen atoms will be described in detail. The first successful demonstration of trapped antihydrogen will then be described. In the initial demonstrations, 38 trapped antihydrogen atoms were detected after being confined for at least 172 ms. Since then, over 400 antihydrogen atoms have been trapped and confinement times of 1000 s (over 15 minutes) have been demonstrated. Spectroscopy of these trapped antihydrogen atoms is the next major step forward. As an initial proof-of-principle demonstration, ALPHA induced and observed resonant positron spin flip (PSR) transitions between the ground states of antihydrogen. [...]

The ALPHA experiment is an international effort to produce, trap, and perform precision spectroscopic measurements on antihydrogen (the bound state of a positron and an antiproton). Based at the Antiproton Decelerator (AD) facility at CERN, the ALPHA experiment has recently magnetically confined antihydrogen atoms for the first time. A crucial element in the observation of trapped antihydrogen is ALPHA’s silicon vertexing detector. This detector contains sixty silicon modules arranged in three concentric layers, and is able to determine the three-dimensional location of the annihilation of an antihydrogen atom by reconstructing the trajectories of the produced annihilation products. [...]

One proposed technique for trapping anti-atoms is to superimpose a Ioffe-Pritchard style magnetic-minimum neutral trap on a standard Penning trap used to trap the charged atomic constituents. Adding a magnetic multipole field in this way removes the azimuthal symmetry of the ideal Penning trap and introduces a new avenue for radial diffusion. Enhanced diffusion will lead to increased Joule heating of a non-neutral plasma, potentially adversely affecting the formation rate of anti-atoms and increasing the required trap depth. We present a model of this effect, along with an approach to minimizing it, with comparison to measurements from an intended anti-atom trap.

Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only approximately 0.5 K deep for ground-state antihydrogen atoms, much shallower than the energies of particles stored in the plasmas. [...]

Evaporative cooling has proven to be an invaluable technique in atomic physics, allowing for the study of effects such as Bose-Einstein condensation. One main topic of this thesis is the first application of evaporative cooling to cold non-neutral plasmas stored in an ion trap. We (the ALPHA collaboration) have achieved cooling of a cloud of antiprotons to a temperature as low as 9 K, two orders of magnitude lowerthan ever directly measured previously. The measurements are well-described by appropriate rate equations for the temperature and number of particles. The technique has direct application to the ongoing attempts to produce trapped samples of antihydrogen. In these experiments the maximum trap depths are ex tremely shallow (~0.6 K for ground state atoms), and careful control of the trapped antiprotons and positrons used to form the (anti)atoms is essential to succes. Since 2006 powerful tools to diagnose and manipulate the antiproton and positron plasmas in the ALPHA apparatus have been developed and used in attempts to trap antihydrogen. [...]

This thesis details the development and commissioning of the ALPHA antihydrogen trapping apparatus. It discusses the history of antimatter physics that led to and enabled the design of the apparatus. It discusses the importance of antihydrogen trapping in testing one of the basic assumptions of the Standard Model of particle physics (that of CPT invariance). It goes on to discuss the design and construction of the apparatus. Finally, it presents results that demonstrate antihydrogen formation in the new magnetic field configurations that together constitute a magnetic minimum trap for neutral antihydrogen. This is an important preliminary result for any antihydrogen trapping apparatus, and confirms that the ALPHA apparatus does present a potential route towards laser spectroscopy of antihydrogen.

This thesis describes several models of antihydrogen formation as well as the commissioning of the ALPHA antihydrogen during the 2006 Antiproton Decelerator (AD) physics run. Three models are given to describe the short-time production of antihydrogen, including the Simple Temperature Dependant Model, Inverse Velocity Model, and Scaled Inverse Velocity Model. All three models are compared to results from the ATHENA experiment. After an introduction to the ALPHA apparatus and some of the techniques used to produce antihydrogen the commissioning process is described, focusing on the optimization of the antiproton capture, cooling, and manipulation. Also included is an appendix describing in detail the ALPHA data acquisition system as of the end of the 2006 physics run