Laser Spectroscopy Refines Antiparticle Measurement
The existence of the antiproton was predicted in 1930 and confirmed experimentally in 1955. These particles, however, can be produced only in high-energy accelerators, and their short lifetime prevents a detailed examination. By studying how antiprotons behave in more stable atomic systems, scientists are investigating whether they really are perfect inverses of protons.
In the summer of 2000, the antiproton decelerator at CERN in Geneva began producing low-energy antiprotons. To create antiparticle systems that exist long enough to study thoroughly, 250-ns pulses of the antiprotons are fired at a cryogenic helium target, once every two minutes. Most are annihilated when they contact the target, but 3 percent of them form antiprotonic helium, an antiproton and an electron bound to a helium nucleus that contains two protons. Each impact creates approximately 500,000 antiprotonic helium atoms.
Over the course of 3 to 4 µs, the antiprotons move from their initial outer orbital to ones closer to the nucleus, finally annihilating with the nuclear protons. This annihilation produces a shower of charged particles that travel through UV-transparent Lucite sheets and release some of their energy as Cerenkov radiation. A Hamamatsu Photonics KK photomultiplier detects the Cerenkov radiation, which is proportional to the annihilation rate. Without additional intervention, the annihilation is a smoothly decaying curve.
The researchers, however, intervene by irradiating the antiprotonic helium with laser light. The second or third harmonic of a Coherent Inc. Infinity Q-switched Nd:YAG laser pumps a Lambda Physik Inc. Scanmate 2E dye laser to produce 4- to 6-ns pulses at wavelengths of 529 to 745 nm. To produce 264- to 372-nm pulses, the dye laser output is doubled in a BBO crystal.
When the antiprotonic helium is illuminated with an energy corresponding to the difference between energy levels, resonances in the annihilation rate emerge. By varying the wavelength of the laser pulses, the researchers create a detailed spectrum of antiprotonic helium. Already, more than 20 resonances have been identified and compared with theoretical predictions.
When combined with previous measurements of the antiproton charge-to-mass ratio, the results from laser spectroscopy confirm that the mass of the antiproton is identical to that of the proton, within 60 parts per billion. John Eades, a scientist with the team, explained that this accuracy is equivalent to measuring two mountains as large as Mount Everest and discovering that their heights are within 0.5 mm of each other.
Repeating classic experiments
The researchers essentially are repeating "many of the classic experiments in the history of the hydrogen atom, which took a century or more," Eades said. But they are doing so over only a few years. The experimental results are already significant, but the researchers are upgrading the system to include more precise lasers.
"We're aiming at high-precision measurements of as many of the antiproton's properties -- mass, charge, magnetic properties, et cetera – as we can get at," Eades said. "The aim is ever-more-precise comparisons of the antiproton with the proton, because even a minute difference would be tremendously important to our picture of, and interpretation of, the world."
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