Each of the matter particles we are familiar with has an opposite. For the electron there is the positron: its electrically positive mirror image. For the proton there is the antiproton: its electrically negative twin. These substances are referred to as antimatter, and although they can be created naturally, they do not exist for long. This is because antimatter is instantly annihilated when it comes into contact with its matter equivalent. Physicists are able to create anti-particles in powerful collisions inside particle accelerators, and thanks to the electrical charges on the positron and antiproton they can be contained using electric fields.
Just as the simplest matter atom is hydrogen, the simplest antimatter atom is anti-hydrogen. While hydrogen is composed of a single electron and proton, anti-hydrogen consists of a positron and antiproton. For years the ATHENA project at CERN has been able to create such anti-hydrogen by mixing these ingredients together. However, as hydrogen is electrically neutral, the electronic trap is unable to contain the newly formed atoms, which quickly drift away to be annihilated.
Now the successor to ATHENA, the ALPHA (Anti-hydrogen Laser PHysics Apparatus) project, has made capturing anti-hydrogen a reality. Prof. Mike Charlton of Swansea University, one of the lead scientists on the project, explained to Materials Today that achieving this goal was no simple matter. The anti-hydrogen atoms that were produced in ATHENA had a very high velocity, making trapping them impossible. Charlton reveals that the breakthrough was the development of “a new method of very gently driving the antiprotons into the positrons using a frequency sweep technique. This proved to be the vital element which gave us enough very low energy anti-hydrogen atoms for us to trap a few”.
While anti-hydrogen is electrically neutral, it does posses a magnetic moment, and so the key to trapping the anti-atoms was to use the new ALPHA trap, built using “state of the art superconducting materials to create a magnetic field”. The magnetic trap was specially designed to be able to be switched off in under 30 ms. According to Charlton this is critical; “When we switch the trap off any anti-hydrogen inside will escape and annihilate. We look for these annihilations in a narrow time window, which helps us to defeat the background due to cosmic rays striking our unique imaging detector”.
One of the experiments the teams hope to perform on the anti-hydrogen is to measure its emission spectrum. By comparing characteristic emission lines to those of hydrogen, the researchers will be able to test matter-antimatter symmetry. This is vital if we are to understand the dominance of matter over antimatter, as Charlton explains, “This is an example of a broken symmetry, and one which we are far from understanding. We hope that our work on anti-hydrogen might make a contribution”.