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4 July, 2019
First results are in for the KWISP detector’s hunt for hypothetical dark-energy particles from the Sun
Image above: The KWISP detector is looking for hypothetical “chameleon” particles that could be causing the universe to expand at an accelerating rate. (Image: NASA, ESA, H.Teplitz and M.Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), Z. Levay (STScI).
Astronomers observe that the universe is expanding at an accelerating rate, but what causes this acceleration is unknown. A form of energy known as dark energy is the most popular explanation, and is keeping scientists the world over occupied searching for it. Now, a team of researchers working with the KWISP detector at CERN has presented the first results of their search for hypothetical “chameleon” particles from the Sun that could make up dark energy.
Like their reptilian namesakes, chameleon particles would change depending on their surroundings. In regions of high density, such as on Earth, their mass would be large, and as a result their force would act over short distances. By contrast, in regions of low density, such as in empty space, their mass would be extremely small and their force would be long-ranged. This changing behaviour makes chameleon particles good candidate particles for dark energy, but it also makes them difficult to search for on Earth.
Enter KWISP, a unique detector recently installed at the CAST experiment to sense the force exerted on a thin membrane by a stream of chameleon particles from the Sun. On hitting the membrane, such a hypothetical stream would move it from its normal rest position by less than a proton’s radius – about a quadrillionth of a metre. This tiny displacement would be revealed by light from a laser beam that travels through a special optical configuration that includes the membrane.
The new results from KWISP were obtained with data taken at CAST in July 2017 for about 90 minutes, during which the experiment tracked the Sun. The data were part of a 10-day data-taking campaign to test KWISP. To increase the chances of finding solar chameleons, the team added two elements to the KWISP detector before taking the data: a mirror system to focus the incoming stream of solar chameleons, and a so-called mechanical chopper, placed between the mirror system and the detector, to modulate the force exerted by the stream in a way that maximises the detector’s sensitivity to the particles.
The researchers observed no signal of solar chameleons, but the data allowed them to derive an upper limit on the force exerted on the membrane by the particles of 44 ± 18 piconewtons – about the weight of a single human cell.
Together with theoretical calculations of the number of solar chameleons expected to reach the detector, this upper limit allowed bounds to be placed on the strength of the interactions of solar chameleons with matter and light. These bounds are complementary to those obtained from other experiments, such as the GridPix detector at CAST, which looks for X-ray photons from solar chameleons.
CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.
The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 23 Member States.
KWISP first results: https://arxiv.org/abs/1906.01084
CAST experiment: https://home.cern/science/experiments/cast
GridPix detector at CAST: https://arxiv.org/abs/1808.00066
For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/
Image (mentioned), Text, Credits: CERN/Ana Lopes.
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