Introduction and PhD motivation

As a new member of the group I would like to introduce myself and my background in physics: I am Andi, coming from Vienna, where I did my bachelors and masters degree in physics at the University of Vienna. Since 2019 I’ve been doing my PhD at the Stefan Meyer Institute for subatomic physics in Vienna as part of the ASACUSA-Cusp (Atomic Spectroscopy And Collision Using Slow Antiprotons) collaboration in the antiproton decelerator (AD) facility at CERN.

The ASACUSA-Cusp experiment aims to measure a property called the hyperfine splitting of antihydrogen ^[1]. Antihydrogen () is the antiatom of hydrogen, consisting of a positron (e⁺) and an antiproton (), i.e. the antiparticles of electrons and protons, respectively. An antiparticle has the same mass and same absolute charge value as the matter-particle, but opposite sign of the electric charge and magnetic moment. As an example the antiproton has a charge of -1 while the proton has a charge of +1, but both have the same mass. A fundamental property of antimatter is that if it comes close to its matter counterpart both will annihilate and produce a photon. According to the current model of particle physics, the Standard Model, matter-antimatter pairs can be produced by high-energy interactions (the famous formula for the equivalence of energy and mass E = m c² makes that possible). The energy available for the production of a particle-antiparticle pair must be at least the energy equivalent to the mass of the particles produced, for example to produce an electron-positron pair you need at least 2511 = 1022 keV of energy. However, they would annihilate again if not separated. This creation-annihilation processes occurred at short times after the Big Bang but must have stopped at some point as we are living in a universe built from matter. Which leads to the question: Where did the antimatter go?

As hydrogen is the most investigated element it opens the possibility to compare properties to antihydrogen to look for differences, which could give a hint for physics beyond the standard model (in the case of the ASACUSA experiment the framework used is called Standard Model Extension ^[2,3], but that goes deeply into quantum field theory and will not be further described), which might explain the missing antimatter in our universe.

Experimental setup

The experimental setup is sketched in Fig.1. (I will not go into detail about the spectroscopy setup as it was not used during my time in the collaboration, but for further information you can look up “Rabi experiment” or have a look at the original publication [4]). The positrons come from a ²²Na source, are moderated by a solid Ne moderator^[5], and are accumulated in a so-called Buffer Gas Trap ^[6]. This is done the same way as the Positronium experiments here at UCL, for more information click here. Antiprotons are produced at CERN via the process by shooting high energy protons at tungsten and iridium targets. The energy of the incoming proton must be higher than two times the rest mass of the proton to produce the additional proton-antiproton pair. As the particles have different charge, they can be separated easily by an electric field, protons going one way and antiprotons going the other. The antiprotons can be decelerated afterwards in two rings (the AD and ELENA) to about 100 keV.

The main process to form antihydrogen is called three-body recombination (). That means that a large number of positrons (a few million) and antiprotons (a few hundred of thousand) have to be close together. That is achieved in so-called Penning-Malmberg traps, which is a slightly modified version of a real Penning trap (but the same methods are used). A Penning-Malmberg trap consists of a stack of typically cylindrical electrodes on which different voltages can be applied, producing an on-axis potential (as indicated in Fig.2). Particles with energies lower than the potential barrier cannot escape axially. However, they are pushed towards the wall of the electrodes radially. To counteract this movement, a magnetic field parallel to the electrodes is applied (depending on the purpose of the trap the magnetic field can vary from a few mT up to several T). The particles have no option to escape from the trap, neither axially nor radially, hence they are trapped in a specific region.

By using this type of trap it is possible to accumulate high numbers of particles with the same charge (e.g. positrons). When a certain density is reached, the particles start to act as an ensemble, which is called a non-neutral plasma. For the production of antihydrogen typically non-neutral plasmas of positrons are needed. By having positrons and antiprotons in the same trap it possible to get an overlap of the positron and antiproton plasmas and the antihydrogen production reaction can start. As antihydrogen is electrically neutral, it can leave the trap and a measurement can be performed.

My previous work

During my PhD my focus lay in upgrading the positron system. I replaced the previous moderation system and buffer gas trap, commissioned an additional trap for accumulating a high number of positrons and optimised the systems. Additionally, I have participated in the design phase, assembly and testing of the new antihydrogen production trap. A further part of my work was the optimisation of the positron plasma before the production of antihydrogen in this trap.

As the AD is not operating all the time, i.e. there are periods when no antiprotons are delivered, the ASACUSA collaboration designed, built and commissioned a low energy proton source [8]. With this source it is possible to use the same apparatus to produce hydrogen when no antiprotons are available for optimising the production scheme. The installation, testing and optimisation of the proton source in the experiment was another part of my PhD.

Future project

As a research fellow I am working in the group of David to set up an experiment to measure the influence of gravity on Rydberg positronium. The first step will be to build a new positron beamline, similar to the one I used at CERN. In parallel we are designing the apparatus for bending positronium upwards using an electric field. This will be a first milestone for the experiment, but that will be expanded upon in a different blog entry.

References

[1] E. Widmann, R. Hayano, M. Hori, and T. Yamazaki, “Measurement of the hyperfine structure of antihydrogen”, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, vol. 214, pp. 31–34, 2004. Low Energy Antiproton Physics (LEAP’03).

[2] D. Colladay and V. A. Kostelecký, “Lorentz-violating extension of the standard model,” Phys. Rev. D, vol. 58, p. 116002, Oct 1998.

[3] V. A. Kostelecký and A. J. Vargas, “Lorentz and cpt tests with hydrogen, antihydrogen, and related systems,” Phys. Rev. D, vol. 92, p. 056002, Sep 2015.

[4] I. I. Rabi, S. Millman, P. Kusch, and J. R. Zacharias, “The molecular beam resonance method for measuring nuclear magnetic moments. the magnetic moments of ₃Li⁶, ₃Li⁷ and ₉F¹⁹,” Phys. Rev., vol. 55, pp. 526–535, Mar 1939.

[5] J. Mills, A. P. and E. M. Gullikson, “Solid neon moderator for producing slow positrons,” Applied Physics Letters, vol. 49, pp. 1121–1123, 10 1986.

[6] T. J. Murphy and C. M. Surko, “Positron trapping in an electrostatic well by inelastic collisions with nitrogen molecules,” Phys. Rev. A, vol. 46, pp. 5696–5705, Nov 1992.

[7] B. Kolbinger, et al., “Measurement of the principal quantum number distribution in a beam of antihydrogen atoms,” The European Physical Journal D, vol. 75, p. 91, Mar 2021.

[8] A. Weiser, A. Lanz, E. D. Hunter, M. C. Simon, E. Widmann, and D. J.Murtagh, “A compact low energy proton source,” Review of Scientific Instruments, vol. 94, p. 103301, 10 2023.