Axion research: CAST experiment sets new record for measuring accuracy

As Nature Physics reports, the CAST experiment has enhanced its measurement accuracy significantly - thus paving the road for a new generation of more sensitive experiments.

May 05, 2017

The universe needs new particles: Many phenomena cannot be satisfactorily explained on the basis of the elementary particles known today. One such new, hypothetical particle is the axion. The CAST experiment at CERN has been searching for axions from the Sun since 2003. Scientists from the Max Planck Institute for Physics (MPP) and the Max Planck Institute for Solar System Research (MPS) contribute to it. Although CAST has not discovered any axions, the most recent measurement phase has yielded promising results. As now reported by Nature Physics, CAST has significantly improved its measuring accuracy, thus paving the way for the next generation of more sensitive experiments.

Even if CAST has not detected any axions from the Sun, it has yielded findings that will benefit the next detector generation. “The final measuring phase of CAST proved to be particularly fruitful,” says Georg Raffelt, astroparticle physicist at the MPP. “In comparison to the first measuring phase, the CAST experiment was able to triple its sensitivity.”

In concrete terms, this means that scientists are able to delineate more precisely the energy range in which to search for axions. The final measuring phase between 2013 and 2015 used ultra-low-noise detectors and a new X-ray telescope – technologies that set the standard for follow-up experiments.

Axions are hard to detect, not least of all because their mass is unknown. “Current theories put the axion in an energy range between 40 and 400 microelectronvolts, making it around ten billion times lighter than the electron,” Raffelt explains. According to theoretical models, axions should appear throughout the universe; however, stellar plasmas are believed to be particularly strong sources. “Among these, the Sun is a very promising research object due to its close proximity to Earth”, says Prof. Dr. Sami K. Solanki, director at the MPS. Scientists assume that within our star the axions originate in the nucleus. There the electrical fields of the solar plasma transform light into axions.

So how can axions be detected? “On the basis of a special property of the particles,” Raffelt explains. “In a strong magnetic field, axions are transformed into electromagnetic waves, i.e., light, and vice versa.” The light particles can be captured and visualised with the help of suitable detectors. Solar axons would have an energy in the X-ray range consistent with the high temperature in the interior of the Sun.

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