No small matter: exploring the strange world of antimatter

Einstein's formula

The story of antimatter begins (again) with Einstein and his famous formula: E=mc2. It means that energy and mass are interchangeable - so mass can be transformed to energy (as in stars), or energy into mass. And this has huge consequences!

Dirac's equation

Paul Dirac, a famous British physicist, was first to find an equation uniting quantum physics with the theory of relativity. Surprisingly, it predicted something completely new: every particle has an anti-particle with the same mass, but opposite charge.

The discovery of antimatter

In 1932, four years after Dirac’s prediction, a positive electron was discovered in cosmic rays and called ‘positron’. In 1955, an ‘anti-proton’ was produced at the 'Bevatron' accelerator in Berkeley, which was specially built for this purpose.

Always 50:50

The collision of two energetic particles can create new particles, and part of the collision energy transforms into mass! However, this process always produces equal amounts of matter and antimatter, in perfect balance.

Destructive encounters

When a particle and its antiparticle collide, both disappear and their masses transform into energy. This energy can then produce new particles.

Cosmic observation

The AMS experiment on the International Space Station is looking for signs of heavy anti-nuclei in space that originate from hypothetical 'anti-stars'.. But so far, none has been found - it looks as if all antimatter from the Big Bang has disappeared.

The antimatter factory

The Antiproton Decelerator (AD) at CERN is a unique facility in the world. Here, scientists can produce antiprotons, slow them down and capture them. In a second step, antiprotons can then be combined with positrons to create antihydrogen atoms.

Production of antiprotons

The production of antiprotons starts in the AD target area with the collision of about 10 trillion protons with a block of metal (the ‘target’). This produces lots of secondary particles, among them about 100 million antiprotons that are injected into the AD.

Deceleration and trapping

The antiprotons coming from the target area travel at 96% of the speed of light. The AD slows them to 10%, in several steps. Then, they can be further slowed down in the ELENA deceleration ring to 1.5% of the speed of light.

How a trap works

Antiprotons are trapped using electric and magnetic fields. The magnetic field forces them into a spiral movement along the field lines, and the electric field between the three electrodes confines them in the middle of the trap.

How a trap works (end)

The incoming antiprotons from the AD are trapped by lowering and then quickly raising the voltage on the entrance electrode, thus trapping all antiprotons that are reflected by the electric potential at the exit electrode.

Positron sources

Positrons are produced in the decay of specific nuclei (e.g. Na-22, or by colliding high-energy electrons with a target. The positrons can then be accumulated in traps. Both methods are used by AD experiments.

Making antihydrogen

Large numbers of antiprotons and positrons can be stored and then mixed in a trap, producing slow moving antihydrogen atoms. This technique was pioneered by the ATHENA experiment at the AD, in 2002.

Cosmic conundrum

The extreme heat in the micro-seconds after the Big Bang produced a perfect balance between the creation and annihilation of particles and antiparticles. As the universe cooled down, most particles and antiparticles destroyed each other, producing lots of radiation. However, a tiny bit of matter survived - but how?

How did antimatter survive ?

In 1967, the Russian physicist Andreij Sakharov proposed a scenario how this could have happened: a small asymmetry in the decay of matter and antimatter particles may have produced a small surplus of matter over antimatter: one particle more in a billion would be enough.

Something instead of nothing

Without a tiny asymmetry between matter and antimatter there would be no solid bodies in the universe - no stars, no planets, no humans, only radiation. So we owe our existence to this difference - but where does it come from?

How do we know

We know about the destructive battle between particles and antiparticles because it left a characteristic signature: for each (surviving) proton or neutron, there are about 1 billion photons (light particles) in the universe - the relics of the matter-antimatter destruction.

Searching for a reason

Many theories exist about possible reasons for the matter-antimatter asymmetry, but none is experimentally proven. It could be the result of a process during the decay of massive particles (called CP violation), or because of a difference in particle versus antiparticle properties (called CPT violation).

Two different ways

Scientists at CERN explore all possibilities. The LHCb experiment compares the decay of so called b-quarks and their antiparticles. The experiments at the AD compare the properties of matter and antimatter with extreme precision.

LHCb Experiment

The LHCb experiment explores why we live in a Universe composed almost entirely of matter, but no antimatter. The experiment is installed at the LHC and compares very precisely how 'b quarks’ and ‘anti-b-quarks’ decay.

Precision, precision

Most scientists are convinced that particles and antiparticles have identical masses. But what if not? Only very precise measurements can tell if this is true.

Mass

The charge-to-mass ratio of protons and antiprotons have been compared very precisely by the ATRAP and the BASE experiments at CERN. The two masses agree to 70 parts per trillion - that is as if you measured the weight of an elefant to the precision of a small grain of sand.

Antihydrogen spectrum

The ALPHA experiment has succeeded in storing antihydrogen atoms for hours and comparing the energy levels of hydrogen and antihydrogen using light from an ultra-precise laser beam. The relative difference is less than 2 parts in a trillion - and the experiment will eventually reach a much higher precision.

Does antimatter fall down?

Three experiments at the AD (AEgIS, Alpha-G, Gbar) will test if antimatter atoms fall with the same acceleration towards the centre of the Earth. This will be the first time that the influence of gravity on antimatter will be tested. Most likely an anti-apple would fall like an apple - but who knows for sure until it has been measured?

PET scanner

The Positron Emission Tomograph (PET scanner) is using positrons emitted from an isotope injected into the blood stream. PET scanners are used to study brain functions or as a diagnostics tool for locating tumours.

Anti-efficiency

The efficiency of antimatter production is only about 1 in a billion. The main reasons are quantum physics - the production of antiprotons in particle collisions has a very small probability - and the limited efficiency of decelerating, trapping and storing antimatter.

The most expensive stuff

Even if the antiproton decelerator at CERN would work around the clock for the entire year, it would produce less than a billionth of a gram of antiprotons, for a cost of more than a million Euros. This makes antimatter the most expensive stufff on Earth - 10 trillion times more expensive than gold.

Antimatter bomb

Would 1 gram of antimatter make a powerful bomb, as described in the book ’Angels & Demons’? Yes, it would be as powerful as a small atomic bomb. But it would take a billion years to make it, and the cost of producing it would exceed a billion million Euros.

Can we store antimatter?

The TRAP experiment at CERN kept a single antiproton trapped for 57 days! During this time, scientists performed very precise measurements of the antiproton mass and charge, until the trap was switched off and the antiproton ... annihilated.

Transporting antimatter

An experiment at CERN named ‘PUMA’ plans to transport billions of antiprotons from the AD to another facility, called ISOLDE. The first antiproton transport will take place in 2022.

Bones and bananas

The body of a person weighing 80 kg emits about 180 positrons per hour! This comes from the decay of potassium-40, a naturally occurring isotope that is ingested by drinking water, eating food and breathing. For the same reason, bananas release one positron about every 75 minutes.