Dark Matter remains obscure
It is one of the hidden secrets that literally surround the Universe. Experiments have shown no result so far because trying to capture particles that do not seem to interact with ordinary matter is no trivial exercise. The OSQAR experiment at CERN is dedicated to the search for axions, one of the candidates for Dark Matter. For its difficult challenge, OSQAR counts on one of the world’s most powerful magnets borrowed from the LHC. In a recent publication, the OSQAR collaboration was able to confirm that no axion signal appears out of the background. In other words: the quest is still on.
The OSQAR experiment installed in the SM18 hall. (Photo by F. Capello)
The OSQAR “Light Shining Through a Wall” experiment was officially launched in 2007 with the aim of detecting axions, that is, particles that might be the main components of Dark Matter. OSQAR uses the powerful LHC dipole magnet to intensify the predicted photon-axion conversions in the presence of strong magnetic fields. “According to the theory, axions have an extremely weak interaction with matter and are not stopped by optical barriers,” says Andrzej Siemko, the OSQAR Technical Coordinator. “From our recent measurements, we learned that our resolution is not yet good enough to allow us to indirectly detect the axions. Our aim for the next two years is to increase the production of axions by increasing the light intensity, and to improve further the sensitivity of our detection system.”
The OSQAR experiment is divided into two sections: the first is designed to produce axions, while the aim of the second is to detect them. Axions may be produced by converting photons generated by a light source – in this case a laser – with the aid of a very strong magnetic field. In the second section, called the “Regeneration Part”, a small fraction of these axions is converted back into photons that can be easily detected. “The OSQAR principle is rather simple: by shining light from one side of a wall, we expect to find this light on the other side,” explains Siemko.
“With respect to other experiments pursuing the same search, OSQAR can count on the most powerful magnetic field ever used for this kind of experiment,” adds Pierre Pugnat, OSQAR's spokesperson. “The state-of-the-art LHC dipole magnets – providing 9 T over 14 m long aperture – are used for the photon-axion-photon conversions in a vacuum or using a buffer gas at a specific pressure as an amplification medium. For the first time in 2010, this experiment has been operated with two aligned LHC magnets in the SM18 hall and the results have been recently submitted for publication. This translates into a higher sensitivity and a higher resolution.”
OSQAR is not the only research project looking for axions at CERN; CAST is the other one. “We can say that CAST and OSQAR are complementary: both are looking for axions but from slightly different points of view,” explains Siemko. “While CAST works as a sort of telescope that stares at the Sun, trying to record hypothetical axions produced in its nuclear reactions, OSQAR's experimental approach consists in producing the axions by itself.”
In addition to looking for axions, the members of the OSQAR collaboration have other challenging plans for their experiment. In the years to come, they will test some predictions of Quantum Electro Dynamics (QED) – the most precise theory developed by physicists so far. “In the 1930s, scientists predicted a very tiny effect at low energy, related to the light-by-light scattering, one of the most fundamental processes in QED,” explains Pierre Pugnat. “Nobody has directly measured this effect before, but OSQAR can reach the sensitivity to measure it for the first time. Indeed, particle physics is not just about high-energies – low energies still need further exploration! And it is quite fascinating to have the possibility to investigate the theories postulated by the fathers of modern physics, trying to prove if they were right or not.”
The search for Dark Matter is proving tough for scientists and the quest is far from over. But OSQAR scientists are confident that if axions exist, as theory seems to indicate, their experiment will be able to find them.
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