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Laser Spectroscopy Precisely Measures Antiproton
Aug 2011
GARCHING, Germany, Aug. 1, 2011 — A new laser spectroscopy measurement providing the most accurate weight of antimatter yet reveals the mass of the antiproton (the proton's antiparticle) down to 1.3 parts per billion.

It is widely believed that, at the beginning of the universe, matter and antimatter were created in equal amounts during the Big Bang. Now an international team of physicists is developing concepts to explain why the visible universe seems to be made entirely out of matter and what happened to all the antimatter that supposedly was created. According to the current theories of particle physics, matter and antimatter should have exactly the same properties and, when they interact, should in effect annihilate each other or cancel each other out.

Dr. Masaki Hori's independent antimatter spectroscopy research group, which is associated with Max Planck Institute of Quantum Optics (MPQ), conducted the experiment, which was carried out in the antiproton decelerator at CERN, a particle physics laboratory near Geneva, as part of the lab's Atomic Spectroscopy and Collisions Using Slow Antiprotons experiment (ASACUSA). Other institutes involved in the experiment were Tokyo University in Japan, the University of Brescia in Italy, the Stefan Meyer Institute in Austria and the KFKI Research Institute in Hungary.

“Imagine measuring the weight of the Eiffel tower,” Hori said. “The accuracy we’ve achieved here is roughly equivalent to making that measurement to within less than the weight of a sparrow perched on top. Next time it will be a feather.”

An antiproton (black sphere) trapped inside a helium atom is probed by two laser beams. (Image: MPQ)

Physicists believe that antimatter atoms should weigh exactly the same as their matter counterparts. If scientists were to experimentally detect any deviation, however small, it would indicate that this fundamental symmetry is broken. “Small” is the keyword here; to make this comparison with the highest possible precision, it is essential to use the most precise methods and instruments available.

Antimatter is extraordinarily difficult to handle in the laboratory because, upon coming into contact with ordinary matter (even the air molecules in a room), it immediately annihilates, converting into energy and new particles. In 1997, researchers from MPQ in cooperation with European, Japanese and American groups began construction of a facility at CERN called the Antiproton Decelerator. Here, antiprotons produced in high-energy collisions are collected and stored in a vacuum pipe arranged in a 190-m-long racetrack shape. The antiprotons are gradually slowed down before being transported to several experiments. To create and study antiprotonic atoms, the ASACUSA collaboration sends the antiprotons into a helium target.

Normal helium atoms consist of a nucleus with two electrons orbiting around it. In antiprotonic helium, one of these electrons is replaced by an antiproton, which finds itself in an excited orbit some 100 pm from the nucleus. Scientists fire a laser beam onto the atom and carefully tune its frequency until the antiproton makes a quantum jump from one orbit to another. By comparing this frequency with theoretical calculations, the mass of the antiproton can be determined relative to the electron.

An important source of imprecision arises because the antiprotonic atoms jiggle around randomly according to their thermal energy, so that atoms moving toward the laser beam experience a different frequency compared with those moving away.

In their previous measurement in 2006, the same team used one laser beam, and the achievable accuracy was governed by this jiggling effect. This time, they used two beams moving in opposite directions, with the result that the jiggle for the two beams was partly canceled out. The result was a four- to sixfold boost in accuracy.

The first laser caused the antiproton to make a quantum jump to a virtual energy level normally not allowed by quantum mechanics, so that the second laser actually could bring the antiproton up to the closest allowed state. Such a two-photon jump is normally difficult to achieve because the antiproton is heavy, but the MPQ scientists accomplished it by building two ultrasharp lasers and carefully choosing a special combination of laser frequencies. To do this, they used an optical frequency comb.

“We have measured the mass of the antiproton relative to the electron with a precision of 10 digits and have found it exactly the same as the proton value known with a similar precision,” Hori said. “This can be regarded as a confirmation of the CPT theorem. Furthermore, we learned that antiprotons obey the same laws of nonlinear quantum optics like normal particles, and we can use lasers to manipulate them. The two-photon technique would allow much higher precisions to be achieved in the future, so that, ultimately, the antiproton mass may be better known than the proton one.”

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laser spectroscopy
That part of the science involved in the study of the theory and interpretation of spectra that uses the unique characteristics of the laser as an integral part in the development of information for analysis. Raman spectroscopy and emission spectroscopy are two areas where lasers are used.
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