What is antimatter?
(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?
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
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
) to provide
antiprotons and a 22
Na 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
, together with a pulse of laser-excited
by shooting positrons
a nano-structured target) to make a pulse of horizontally-travelling antihydrogen atoms.
These atoms will pass through an instrument called a
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
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
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, 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.