**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 x^{2}=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, exactly matching the particle but
with opposite charge. For the electron there should be an
"antielectron", for example, identical in every way but with a
positive electric charge. The insight opened the possibility of entire galaxies
and universes made of antimatter.

But when matter and
antimatter come into contact, they annihilate – disappearing in a flash of
energy. The big bang should have created equal amounts of
matter and antimatter. So why is there far more matter than antimatter in the
universe?

At CERN, physicists make antimatter to study in
experiments. The starting point is the Antiproton Decelerator, which slows down
antiprotons so that physicists can investigate their properties.

In the antimatter hall at CERN, numerous
experiments are using antiprotons from the Antiproton Decelerator to
investigate the properties of antimatter.

ACE brings together an international team of
physicists, biologists and medics to study the biological effects of antiprotons

AEGIS uses a beam of antiprotons from the
Antiproton Decelerator to measure the value of Earth's gravitational
acceleration

ATRAP compares hydrogen atoms with their
antimatter equivalents – antihydrogen atoms

ALPHA makes, captures and studies atoms of
antihydrogen and compares them with hydrogen atoms

ASACUSA compares matter and antimatter using
atoms of antiprotonic helium

ALPHA experiment
shows antihydrogen charge is neutral

21 Jan 2016 – ALPHA shows the most accurate
measurement yet of the electric charge of antihydrogen atoms in a new Nature
paper

BASE compares
protons to antiprotons with high precision

12 Aug 2015 – In a paper published today in Nature,
BASE reports the most precise comparison of the charge-to-mass ratio of the
proton to the antiproton