The weight of antimatter is a crucial missing measurement in our picture of the natural world. It is important in two ways: 1. The predominance of matter created in the Big Bang demands some form of mismatch in properties between matter and antimatter. Many experiments have sensitively compared their charge, magnetic moment, nuclear bonding and decay behaviour, yet no significant mismatch has been found to date to explain the cosmic matter dominance. One of the last unexplored domains is gravitational behaviour. 2. Our understanding of the subatomic world is wholly incompatible with General Relativity, the dominant phenomenon on astronomical scales. New ideas on a unified theory on atomic and gravitational interactions may require antimatter to respond uniquely to gravity. Measuring such behaviour experimentally will provide vital evidence to accept or reject these ideas, and further the development of a unified view of nature.
Experimentally, weighing antimatter has been difficult because electrical influences on the charged, energetic antiparticles commonly created in accelerators massively overwhelm their gravitational response. Their short life in these machines also leaves no time for observation. The antihydrogen trapping technology developed by the world-leading ALPHA collaboration has, however, completely altered this picture, by generating antimatter that has low energy, long lifetime and immunity to electric forces. The new ALPHA-g experiment is designed to leverage this new technology, and weigh antimatter by letting antiatoms escape through the bottom and top of a tall magnetic confinement system. By precisely controlling the magnetic field of the openings, the escape bias induced by gravity can infer antihydrogen weight to within 1%. This constitutes the most sensitive antimatter gravity measurement ever made, and a significant breakthrough in subatomic and fundamental physics. Its results have potential to revolutionise our understanding of matter and antimatter, natural forces and the process of creation.
In this presentation, we outline the basic principles and experimental design of the ALPHA-g experiment, with emphasis on technical challenges involved in the experiment.