What is antimatter?

Annihilation of antiprotons in emulsion
(From the CERN web site) 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.

Dirac interpreted the equation to mean that for every particle there exists a corresponding antiparticle, ... .

Does antimatter fall with the same acceleration as matter?

Do matter and antimatter feel gravity in the same manner?
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 provide antielectrons, which we combine to form antihydrogen atoms. 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.

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.

These atoms will pass through an instrument called a moiré deflectometer. A system of gratings in the deflectometer splits the antihydrogen beam into parallel rays, forming a periodic pattern. Once the antiatoms arrive at D, 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.

The AEGIS experiment aims to carry out the first direct measurement of a gravitational effect on an antimatter system. Construction of the main apparatus was completed at the end of 2012; setting up with electrons and positrons has been going on since then. Between December 2012 and August 2014, no antiprotons were available at CERN since all accelerators were being refurbished, but since August 2014, antiprotons are once again available.

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.

A very versatile experiment

Annihilation of antiprotons in emulsion
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, Russia, the UK, Germany, France, the Czech Republic, Austria, Switzerland 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.