07 January 2013

New Method To Cool Down Antimatter Using Lasers Proposed

Researchers have proposed using lasers to cool down highly energetic anti-hydrogen atoms to better study its properties.

Antimatter is matter that carries an opposite charge and spin. An electron's antimatter counterpart is the positron. The electron is negatively charged while the positron is positively charged. The antimatter counterpart of a positively charged proton is a negative charge antiproton.

When antimatter collides with matter, they destroy each other and release energy in a process called annihilation. It must be noted that despite having opposite charges and quantum spin, both particles still carry positive energy.

In 1929, Dmitri Skobeltsyn and Chung-Yao Chao were the first to observe positrons in separate experiments. They noted that the observed particles behaved like electrons but curved in the opposite direction in an applied magnetic field. Neither scientists pursued the anomaly.

It was in 1932, that the positron was officially discovered by Carl D. Anderson, who also coined the term "positron". He was awarded the Nobel Prize for Physics in 1936 for the discovery.

In 1995, physicists at CERN for the first time produced nine anti-hydrogen atoms. This was done by combining one positron and one antiproton. This was followed up by Fermilab which produced 100 antihydrogen atoms.

The resulting antihydrogen atoms proved to be too energetic or "hot" for it to be observed and studied carefully. Subsequent experiments like CERN's Antiproton Decelerator in 1999 managed to lower its energy levels but was still difficult to study.

In 2005, CERN formed the ALPHA collaboration (Antihydrogen laser physics apparatus) whose primary goal is to create less energetic ("cold") antihydrogen atoms that are better suited to study

Cooling Down Antimatter

Researchers have proposed a method for cooling trapped antihydrogen which they believe could provide 'a major experimental advantage' and help to map the mysterious properties of antimatter that have to date remained elusive.

The new method, developed by a group of researchers from the USA and Canada, could potentially cool trapped antihydrogen atoms to temperatures 25 times colder than already achieved, making them much more stable and a lot easier to experiment on.

The suggested method, which has been published today, 7 January 2013, in IOP Publishing's Journal of Physics B: Atomic, Molecular and Optical Physics, involves a laser which is directed at antihydrogen atoms to give them a 'kick', causing them to lose energy and cool down.

Antihydrogen atoms are formed in an ultra-high vacuum trap by injecting antiprotons into positron plasma. An atomic process causes the antiproton to capture a positron which gives an electronically excited antihydrogen atom.

Typically, the antihydrogen atoms have a lot of energy compared to the trapping depth which can distort the measurements of their properties. As it is only possible to trap very few antihydrogen atoms, the main method for reducing the high energies is to laser cool the atoms to extremely low temperatures.

Co-author of the study, Professor Francis Robicheaux of Auburn University in the USA, said: "By reducing the antihydrogen energy, it should be possible to perform more precise measurements of all of its parameters. Our proposed method could reduce the average energy of trapped antihydrogen by a factor of more than 10.

The ultimate goal of antihydrogen experiments is to compare its properties to those of hydrogen. Colder antihydrogen will be an important step for achieving this."

This process, known as Doppler cooling, is an established method for cooling atoms; however, because of the restricted parameters that are needed to trap antimatter, the researchers need to be absolutely sure that it is possible.

"It is not trivial to make the necessary amount of laser light at a specific wavelength of 121 nm. Even after making the light, it will be difficult to mesh it with an antihydrogen trapping experiment. By doing the calculations, we've shown that this effort is worthwhile," continued Professor Robicheaux.

Through a series of computer simulations, they showed that antihydrogen atoms could be cooled to around 20 millikelvin; trapped antihydrogen atoms so far have energies up to 500 millikelvin.

Video: What is Antimatter?

In 2011, researchers from CERN reported that they had trapped antimatter for over 1000 seconds – a record. A year later, the first experiments were performed on antihydrogen whilst it was trapped between a series of magnets.

Even though the processes that control the trapping are largely unknown, the researchers believe that the laser cooling should increase the amount of time antihydrogen can be trapped for.

"Whatever the processes are, having slower moving, and more deeply trapped, antihydrogen should decrease the loss rate," said Professor Robicheaux.

Colder antihydrogen atoms could also be used to measure the gravitational property of antimatter. "No one has ever seen antimatter actually fall in the field of gravity," said co-author Dr Makoto Fujiwara of TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics. "Laser cooling would be a very significant step towards such an observation."


Institute of Physics
Journal of Physics B: Atomic, Molecular and Optical Physics
Auburn University
TRIUMF: Canada's National Laboratory for Particle and Nuclear Physics
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