CERN moves closer to antihydrogen spectroscopy

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Dec 9, 2010

Physicists at CERN have taken a big step towards making the first spectroscopic measurements on a beam of antihydrogen atoms. The antihydrogen atoms, which consist of an antielectron orbiting an antiproton, were made by members of the lab's ASACUSA group. The beams could be used to carry out the first detailed studies of the energy levels in antihydrogen.

Measuring in detail the energy levels in antihydrogen is important because the Standard Model of particle physics says they should be identical to those of hydrogen. Any slight differences in the "fine structure" of the levels compared to ordinary hydrogen could shed light on why there is so much more matter than antimatter in the universe.

The breakthrough comes just weeks after researchers in the ALPHA collaboration at CERN succeeded in trapping 38 antihydrogen atoms for about 170 ms. This was the first time antimatter atoms had been stored for long enough to measure their properties in detail and, taken together, the two results represent major advances in studies of antimatter.

The ASACUSA researchers, however, used an alternative technique for creating antihydrogen. Led by Yasunori Yamazaki of the RIKEN laboratory in Japan, they created their antiatom beams by combining antiprotons with positrons in a "cusp trap".

The trap comprises 17 successive ring-shaped electrodes and two magnetic coils, which are wired to create magnetic fields in opposite directions (see figure). A cloud of antielectrons (also called positrons) from a radioactive source is first sent into the trap, where it is held as a plasma. A cloud of antiprotons – created in a nearby accelerator – is then fired into the plasma to create the antihydrogen atoms.

Charged particles remain stuck in the trap, while neutral antihydrogen atoms are able to move further along the apparatus to a "field ionization trap". At this point, antihydrogen atoms in highly excited Rydberg states, in which the positron lies very far from the antiproton, are ionized and their antiprotons are trapped.

The trapped antiprotons are then released and quickly annihilate upon contact with the walls of the trap. Each annihilation event creates pions, which are easily spotted by a bank of detectors surrounding the trap. By comparing the number of antiprotons injected into the trap with the number of annihilations detected, the team estimated that about 7% of antiprotons combine to form antihydrogen.

The team is now trying to improve the way in which antihydrogen is extracted from the trap before passing it through a microwave cavity in which hyperfine transitions between atomic energy states should occur. Making precise measurements of these transitions, which have not yet been carried out, could be used to study a fundamental quantum transformation known as the charge-parity-time (CPT) operation.

When applied to a physical system, a CPT transformation converts every particle to its antiparticle, reflects each spatial co-ordinate, and reverses time. Although is currently no experimental evidence that the CPT symmetry is violated, it could show up as a slight difference in the frequency of hyperfine transitions in hydrogen and antihydrogen atoms. The discovery of such a violation could also help physicists understand why there is much more matter than antimatter in the universe.

The work is reported in Phys. Rev. Lett. 105 243401.

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Antihydrogen trapped at CERN

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Nov 17, 2010

Physicists at CERN in Geneva are the first to capture and store atoms of antimatter for long enough to study its properties in detail. Working at the lab's ALPHA experiment, the team managed to trap 38 anti-hydrogen atoms for about 170 ms. The next step for the researchers is to measure the energy spectrum of the atoms, which could provide important clues as to why there is much more matter than antimatter in the universe.

Antihydrogen is the antimatter version of the hydrogen atom and comprises a positron – or antielectron – and an antiproton. According to the Standard Model of particle physics, the energy levels of antihydrogen should be identical to those of hydrogen. Any deviations from this could help physicists identify new physics – and explain why there is much more matter than antimatter in the universe.

Although creating positrons and antiprotons is relatively easy, making antihydrogen is much harder. This form of antimatter was not isolated until 1995 – also in experiments at CERN. Making it stick around for long enough to study in detail is even more difficult. But in being able to trap antihydrogen atoms for 170 ms, the members of ALPHA, who come from 14 institutions in seven different nations, can now look forward to studying its atomic energy levels.

The experiment begins by making a cloud of positrons and a cloud of antiprotons. The antiprotons are created in an accelerator by smashing high-energy protons into a stationary target. The antiprotons are then slowed down and cooled in a series of steps involving a storage ring and electromagnetic traps. The positrons are produced by a radioactive source and then accumulated and cooled in a special trap.

The clouds are injected into a superconducting magnetic trap, where they mix for about 1 s to create antihydrogen. The charged positrons and antiprotons are then ejected from the trap, leaving behind neutral antihydrogen. While most of this antihydrogen is moving too quickly to be trapped, atoms with very little kinetic energy are held by a magnetic field gradient.

ALPHA researchers then detected the atoms by switching off the trap and setting the antihydrogen free to annihilate with surrounding matter. This created several charged particles including pions, which were spotted by a bank of detectors surrounding the trap. In total, the team has managed to see 38 annihilation events that are consistent with the release of antihydrogen that had been trapped for 170 ms.

The next step for the researchers is to use the antihydrogen to study a fundamental quantum transformation known as the charge-parity-time (CPT) operation. When the CPT transformation is applied to a physical system, three things happen: every particle is converted to its antiparticle; each spatial co-ordinate is reflected so that left becomes right, up becomes down and forward becomes backward; and time is reversed.

There is currently no experimental evidence that the CPT symmetry is violated, but it could show up as a slight difference in the frequency of certain atomic transitions in hydrogen and antihydrogen atoms. The discovery of such a violation could also help physicists understand why there is much more matter than antimatter in the universe.

"For reasons that no one yet understands, nature ruled out antimatter. It is thus very rewarding, and a bit overwhelming, to look at the ALPHA device and know that it contains stable, neutral atoms of antimatter," said ALPHA spokesperson Jeffrey Hangst of Aarhus University in Denmark. "This inspires us to work that much harder to see if antimatter holds some secret."

The work is described in Nature doi:10.1038/nature09610.

View the original article here

Antihydrogen trapped at CERN

To enjoy free access to all high-quality "In depth" content, including topical features, reviews and opinion sign up

Nov 17, 2010

Physicists at CERN in Geneva are the first to capture and store atoms of antimatter for long enough to study its properties in detail. Working at the lab's ALPHA experiment, the team managed to trap 38 anti-hydrogen atoms for about 170 ms. The next step for the researchers is to measure the energy spectrum of the atoms, which could provide important clues as to why there is much more matter than antimatter in the universe.

Antihydrogen is the antimatter version of the hydrogen atom and comprises a positron – or antielectron – and an antiproton. According to the Standard Model of particle physics, the energy levels of antihydrogen should be identical to those of hydrogen. Any deviations from this could help physicists identify new physics – and explain why there is much more matter than antimatter in the universe.

Although creating positrons and antiprotons is relatively easy, making antihydrogen is much harder. This form of antimatter was not isolated until 1995 – also in experiments at CERN. Making it stick around for long enough to study in detail is even more difficult. But in being able to trap antihydrogen atoms for 170 ms, the members of ALPHA, who come from 14 institutions in seven different nations, can now look forward to studying its atomic energy levels.

The experiment begins by making a cloud of positrons and a cloud of antiprotons. The antiprotons are created in an accelerator by smashing high-energy protons into a stationary target. The antiprotons are then slowed down and cooled in a series of steps involving a storage ring and electromagnetic traps. The positrons are produced by a radioactive source and then accumulated and cooled in a special trap.

The clouds are injected into a superconducting magnetic trap, where they mix for about 1 s to create antihydrogen. The charged positrons and antiprotons are then ejected from the trap, leaving behind neutral antihydrogen. While most of this antihydrogen is moving too quickly to be trapped, atoms with very little kinetic energy are held by a magnetic field gradient.

ALPHA researchers then detected the atoms by switching off the trap and setting the antihydrogen free to annihilate with surrounding matter. This created several charged particles including pions, which were spotted by a bank of detectors surrounding the trap. In total, the team has managed to see 38 annihilation events that are consistent with the release of antihydrogen that had been trapped for 170 ms.

The next step for the researchers is to use the antihydrogen to study a fundamental quantum transformation known as the charge-parity-time (CPT) operation. When the CPT transformation is applied to a physical system, three things happen: every particle is converted to its antiparticle; each spatial co-ordinate is reflected so that left becomes right, up becomes down and forward becomes backward; and time is reversed.

There is currently no experimental evidence that the CPT symmetry is violated, but it could show up as a slight difference in the frequency of certain atomic transitions in hydrogen and antihydrogen atoms. The discovery of such a violation could also help physicists understand why there is much more matter than antimatter in the universe.

"For reasons that no one yet understands, nature ruled out antimatter. It is thus very rewarding, and a bit overwhelming, to look at the ALPHA device and know that it contains stable, neutral atoms of antimatter," said ALPHA spokesperson Jeffrey Hangst of Aarhus University in Denmark. "This inspires us to work that much harder to see if antimatter holds some secret."

The work is described in Nature doi:10.1038/nature09610.

View the original article here