The Optical Search for QED Vacuum Bifringence, Axions and Photon Regeneration (OSQAR) experiment at CERN searches for hypothetical particles called axions, and studies the properties of a vacuum. According to some theories, axions could be components of dark matter, and they could help to explain why there is more matter than antimatter in the universe today.

OSQAR is set up in CERN’s magnet-testing facility on the border of France and Switzerland. It makes use of two superconducting dipole magnets of the type used in the Large Hadron Collider that contain a vacuum chamber measuring 55 metres long by 40 millimetres across.

“Light shining through a wall”

OSQAR looks for axions and axion-like particles by exposing a laser beam containing photons (particles that make up visible light) to a 9 Tesla magnetic field. This field – the strongest ever used in an axion search – causes some of the photons in the laser to turn into axions.

The OSQAR researchers shine the laser into a vacuum chamber containing a barrier that stops photons but lets axions pass through. If they glimpse light on the other side of the barrier, the researchers deduce that axions have travelled through the barrier, and turned back into detectable photons on the other side. Physicists know this type of experiment colloquially as "light shining through a wall". The stronger the electromagnetic field, the greater the chance of an axion occurring. This makes the especially powerful LHC magnets ideal for the OSQAR experiment.

Solving some of the great mysteries in physics

Some theorists think that axions were produced during the big bang, and that they are still being produced by the Sun. Axions have a tiny mass, weighing in at 500 million times lighter than an electron. They have no electric charge and interact minimally with normal matter, making them difficult to observe. Axions could be a component of the mysterious dark matter that makes up 26% of the universe. They are also a major component of string theory, which says that elementary particles like quarks and gluons are connected via oscillating strings.

Axions might also help solve a puzzle in the Standard Model known as the "strong CP problem". After the big bang, there were equal amounts of matter and antimatter in the universe.  But we now live in a universe made of matter, a proof that somehow matter prevailed. According to CP symmetry, the same physical laws apply to particles and antiparticles, even when observed in a mirror. But, if this symmetry is broken, more particles would have survived from the early Universe than antiparticles and explain why matter prevailed over antimatter. So far, all observed violations of CP symmetry are related to weak interactions. However, some physicists believe axions could be created if the strong interaction could also violate CP symmetry.

Vacuum properties

OSQAR also studies the properties of a vacuum when it is exposed to an electromagnetic field and laser. In the presence of the two, the vacuum is thought to alter the path of light. Understanding these properties will help future research that relies on vacuums.

OSQAR started taking data in 2006. It complements the CERN Axion Solar Telescope (CAST), which is searching for axions produced by the Sun.