What is antimatter?

In 1928, British physicist Paul Dirac wrote down an equation that combined quantum theory and special relativity to describe the behaviour of an electron moving at a relativistic speed. The equation – which won Dirac the Nobel prize in 1933 – posed a problem: just as the equation x2=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But classical physics (and common sense) dictated that the energy of a particle must always be a positive number.

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What is AEgIS and how does it work?

The primary scientific goal of the Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy (AEgIS) is the direct measurement of the Earth's gravitational acceleration, g, on antihydrogen. The AEgIS experiment is a collaboration of physicists from all over Europe. In the first phase of the experiment, we will use antiprotons from the Antiproton Decelerator, together with a pulse of laser-excited positronium atoms (obtained by shooting positrons onto a nano-structured target) to make a pulse of horizontally-travelling antihydrogen atoms.

Because the vertical deflection due to gravity is expected to minute (few microns), the beam of antiatoms will need to pass through a device that can superimpose a pattern with a structure of the same size (microns). Such an instrument is for example a moiré deflectometer. A system of gratings in the deflectometer will split an antihydrogen beam into parallel rays, forming a periodic pattern. Once the antiatoms arrive at the third plane of the deflectometer (which has no slits), they will annihilate upon contact with matter. Since areas behind the gratings are shadowed, while those behind the slits are not, the annihilation points reproduce the periodic pattern. The annihilation points are measured with a very precise detector combining silicon strips (to measure the time of arrival of each atom, together with its approximate point of annihilation) and photographic emulsion plates (to measure the annihilation point of each atom with high precision). From this pattern, we can thus measure how much the antihydrogen atoms of different velocities drop during their horizontal flight (also relative to beams of light, who do not drop on our scale), and thus determine the strength of the gravitational force between the Earth and the antihydrogen atoms.

Moiré deflectometers are not the only possible device; optical gratings, or working in a regime in which quantum interferences occur between different paths that an (anti)atom can take, are also possible approaches.

The AEGIS experiment aims to carry out the direct measurements of the gravitational interaction between matter (the Earth) and antimatter systems, among them antihydrogen atoms. Construction and commissioning of the main apparatus was completed at the end of 2014. Since then, the different steps leading to pulsed production of antihydrogen were developed and implemented.

One of the next steps of the AEGIS experiment is to commission the pulsed production of antihydrogen atoms, before working on forming a horizontally-travelling pulse of these atoms. Gravity measurements will follow subsequently.

Does antimatter fall with the same acceleration as matter?

The principle of universality of free fall (or Weak Equivalence Principle, WEP) states that all bodies fall with the same acceleration, independent of mass and composition. The WEP has been tested with very high precision for matter but never (directly) for antimatter.

The principal goal of the AEGIS experiment is to test the Weak Equivalence Principle with antihydrogen atoms at the European laboratory for particle physics (CERN), using the antiproton decelerator (AD) to provide antiprotons and a 22Na source to supply us with antielectrons, which we combine to form antihydrogen atoms. Direct tests with charged antiparticles are hopeless, given the extreme weakness of gravity in comparison with the other forces, while tests with (neutral) antihydrogen atoms are merely extremely difficult.

Our method requires forming many atoms of antihydrogen at once, giving them a horizontal velocity kick to get them flying in one direction, and finally measuring a tiny vertical shift induced by gravity, all of which require adapting state-of-the art techniques to our special requirements.

A very versatile experiment

The AEGIS experiment has many components that can work together but also individually, and has been designed to carry out many other experiments in addition to measureing the gravitational interaction between matter (the Earth) and antimatter (single antihydrogen atoms).

In addition to antiprotons (from the AD) and positrons (from our positron source), we also have electrons (from heated filaments) and protons (from a newly added proton source). The apparatus also includes a fork in the path the positrons take from their source to the main apparatus, allowing us to shoot pulses of positrons into an external test station, where the formation of positronium will be studied.

In the course of the coming years, the availability of a pulsed cold antihydrogen beam will allow us to carry out further experiments on antihydrogen as well: our highest priority among these is the spectroscopy (via microwaves) of antihydrogen atoms in flight, where the very well-defined magnetic fields through which they will be led to pass allows very precise studies of the atomic levels of antihydrogen atoms. But also spectroscopic measurements of the energy levels of positronium are possible in our secondary positron test set-up, as well as studies of antiproton-induced nuclear fragmentation, and many other experiments using the (anti)particles, detectors and techniques that we are working on.

An international, interdisciplinary and young team

The AEGIS collaboration consists of experts from many different fields of physics: particle physicists, laser experts, plasma physicists, cryogenics experts, physical chemists, molecular physicists, experts in trapping and laser cooling of atoms and ions, mechanical engineers, computer specialists, positron and positronium experts, material scientists and chemists, ... from Norway, Poland, France, the Czech Republic, India, Russia, Switzerland, Latvia and Italy.

Over half of the people working at the experiment at any one time are doctoral and master students; we have many possibilities for students to carry out their bachelor, master or PhD thesis with us in a number of cutting edge areas