vendredi 4 août 2017

The ALPHA experiment explores the secrets of antimatter

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Aug. 4, 2017

Image above: Alpha Experiment (Image: Maximilien Brice/CERN).

In a paper published yesterday in Nature, the ALPHA experiment at CERN’s Antiproton Decelerator reports the first observation of the hyperfine structure of antihydrogen, the antimatter counterpart of hydrogen. These findings point the way to ever more detailed analyses of the structure of antihydrogen and could help understand any differences between matter and antimatter.

The researchers conducted spectroscopy measurements on homemade antihydrogen atoms, which drive transitions between different energy states of the anti-atoms. They could in this way improve previous measurements by identifying and measuring two spectral lines of antihydrogen. Spectroscopy is a way to probe the internal structure of atoms by studying their interaction with electromagnetic radiation.

In 2012, the ALPHA experiment demonstrated for the first time the technical ability to measure the internal structure of atoms of antimatter. In 2016, the team reported the first observation of an optical transition of antihydrogen. By exposing antihydrogen atoms to microwaves at a precise frequency, they have now induced hyperfine transitions and refined their measurements. The team were able to measure two spectral lines for antihydrogen, and observe no difference compared to the equivalent spectral lines for hydrogen, within experimental limits.

“Spectroscopy is a very important tool in all areas of physics. We are now entering a new era as we extend spectroscopy to antimatter,” said Jeffrey Hangst, Spokesperson for the ALPHA experiment. “With our unique techniques, we are now able to observe the detailed structure of antimatter atoms in hours rather than weeks, something we could not even imagine a few years ago.”

With their trapping techniques, ALPHA are now able to trap a significant number of antiatoms – up to 74 at a time – thereby facilitating precision measurements.  With this new result, the ALPHA collaboration has clearly demonstrated the maturity of its techniques for probing the properties of antimatter atoms.

The rapid progress of CERN’s experiments at the unique Antiproton Decelerator facility is very promising for ever more precise measurements to be carried out in the near future.

Image above: The ALPHA experiment is a successor of an earlier antimatter experiment, ATHENA. Set up in late 2005 with similar overall research goals as its predecessor, ALPHA makes, captures and studies atoms of antihydrogen and compares these with hydrogen atoms. Image: CERN.

Creating antihydrogen depends on bringing together the two component antiparticles, antiprotons and positrons, in a trapping device for charged particles. Since antihydrogen atoms have no electric charge, once they form they can't be confined in such a device. In the ATHENA experiment the antiatoms would drift naturally to the walls of the trap. Because these walls were made of ordinary matter, the contact caused the antiatoms to annihilate a few microseconds after they were created.

ALPHA is picking up from where ATHENA left off. ALPHA uses a different trapping method to hold the antihydrogen atoms, and will keep them for a longer period before they annihilate with ordinary atoms.

In June 2011, ALPHA reported that it had succeeded in trapping antimatter atoms for over 16 minutes: long enough to begin to study their properties in detail. This should give the physicists time to take measurements and to find more answers to the antimatter mystery.


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 22 Member States.

Related links:

Nature paper:

ALPHA experiment:

Large Hadron Collider (LHC):

For more information about European Organization for Nuclear Research (CERN), Visit:

Images (mentioned), Text, Credits: CERN/Stefania Pandolfi.

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