Towards Increased Antihydrogen Trapping Rates In The Alpha Experiment Using Sympathetically Laser Cooled Positrons

This project is being undertaken on behalf of the ALPHA collaboration, a CERN-based collaboration that traps and studies antihydrogen in order to be able to compare its properties to those of hydrogen and gain a better understanding of the physics of antimatter. The motivation for this study comes from the Standard Model, which predicts equal amount of matter and antimatter after the Big Bang. This is at odds with what we observe in the universe, which is made almost entirely of matter. Antihydrogen is the ideal place to search for differences between matter and antimatter due to the depth of understanding that modern theoretical and experimental physics have given us of the hydrogen atom.

ALPHA has so far measured the resonance frequency [1] and lineshape [3] of the 1S-2S transition in antihydrogen, and found both properties to be in good agreement with the same properties in hydrogen, and is currently constructing an extension to their experimental apparatus called ALPHA-g, which aims to measure how antihydrogen interacts with gravity.

ALPHA creates and traps antihydrogen in an experimental apparatus called ALPHA-2 by isolating plasmas of its two constituent particles, the antiproton and the electron, in a charged particle trap called a Penning-Malmberg trap, a description of which is given in ref. [4]. Antihydrogen atoms are then trapped in a shallow magnetic minimum neutral trap which can trap antiatoms with a temperature less than 0.54K [4]. Since antihydrogen atoms are typically produced with a much higher temperature than this in ALPHA-2, only a small fraction of the antihydrogen produced is cold enough to be trapped. Practically, this means that for every cycle, where around 30,000 antihydrogen atoms are produced, only around 10 are trapped. This translates to around 5 antiatoms per minute. A more detailed description of this process is given in [2].

Improving this trapping rate would be beneficial to any future measurements performed by ALPHA, since it would allow measurements to be made more quickly, allowing for more data to be gathered in the limited time that ALPHA has available for trapping antihydrogen. This project is based on investigating a method of increasing antihydrogen trapping rate that involves introducing a plasma of laser cooled [5] Be+ ions into the positron plasma used for antihydrogen formation. The Be+ ions can be laser cooled to very low temperatures whilst being held in a mixed plasma of Be+ and positrons. The positrons will then cool via Coulomb interactions with the Be+ and reach some equilibrium temperature with the Be+. This has been demonstrated experimentally in ref. [6] and via simulation in ref. [7]. There is strong evidence that the temperature of the positron plasma used in antihydrogen formation affects both the number of antihydrogen atoms produced and the final temperature of the antihydrogen atoms, meaning that a colder positron plasma will lead to larger numbers of antihydrogen atoms trapped, potentially by orders of magnitude.

EPSRC Research Area: Antihydrogen