exotic atoms and nuclei



An ordinary hydrogen atom is made up of a (positively charged) proton and a (negatively charged) electron. This is reversed in antihydrogen, made up of a (negatively charged) antiproton and a (positively charged) positron.
Antihydrogen is the first "anti-element" if we were to extend the periodic table to the "negative" side, as shown above.

Positron - The positive electron, or the positron, was predicted by Dirac, and was discovered by Anderson (in cosmic rays) in 1933. We can now obtain positrons relatively easily from positron-emitting radio isotopes such as 22Na. In our antihydrogen experiment at CERN, we use 22.

Antiproton - Antiprotons are practically absent in cosmic rays, and hence an accelerator capable of accelerating protons up to several GeV was necessary for its discovery. In 1955, Chamberlain and Segrè discovered the antiproton by bombarding a metallic target with the protons accelerated by the Bevatron accelerator. Today, the antiprotons we use for antihydrogen studies are produced in the same way, using the 26-GeV protons from the CERN proton synchrotron.


Antihydrogen - After nearly 50 years, in 2002 G. Baur et al., "Production of Antihydrogen", Phys. Lett. B 368, 251 (1996) produced about 10 relativistic antihydrogen atoms by using 2 GeV antiprotons and a xenon gas-jet target. In this experiment, the positron was "produced from the vacuum". When a high-energy antiproton flew near a xenon nucleus, an electron-positron pair could be sometimes produced. The antiproton could then, with some probability, pick up the produced positron, and emerge as an antihydrogen. Due to its high speed, the produced atoms instantly annihilated on the walls of the vacuum chamber, and hence the antihydrogens produced in such methods cannot be used for high-precision spectroscopy. , we succeeded in producing a large number of cold antihydrogen atoms Accomplished by the ATHENA collaboration:M. Amoretti et al, "Production and detection of cold antihydrogen atoms", Nature 419 (2002) 456-459, http://dx.doi.org/10.1038/nature01096 . As shown schematically in the figure to the left, some 100 million positrons were confined in a Penning trap (using a 3-T magnetic field and a set of electrodes) kept at 15K and ultra-high vacuum, and some 10 thousand antiprotons were injected into the positron cloud.
This resulted in the production of a few hundred to a few thousand antihydrogen atoms per mixing. Since the produced antihydrogen are neutral, the Pennig trap cannot confine them; they left the antiproton-positron mixing region, and annihilated on the electrode after a few microseconds.


An antiproton annihilation is accompanied by the emission of several energetic particles (mostly pions), while a positron annihilation emits two 511-keV back-to-back gamma rays. We confirmed the production (and subsequent annihilation of) antihydrogens by requiring that an antiproton and a positron annihilated at the same time at the same point in space.
The picture to the right shows a typical event display. The four yellow lines drawn from the center are the pion tracks reconstructed with double-sided silicon detectors (pink strips), and the two red lines are the 511-keV gamma rays detected with CsI crystals (red bricks).

Towards antihydrogen spectroscopy - Hydrogen has been studied to 14-digit precision The 1s-2s frequency: 2466061413187103±46 Hz, ground-state hyperfine splitting: 1420405751.768±0.001 Hz. . If similar precision can be reached for the corresponding transitions in antihydrogen, this would be the ultimate test of the matter-antimatter (CPT) symmetry. This is the goal of on-going experiments at CERN. We aim at measuring the 1s-2s frequency in ALPHA, and the ground-state hyperfine splitting in ASACUSA Antihydrogen spectroscopy has turned out to be harder than we initially thought. As of today, nobody has yet succeeded to perform spectroscopy measurements on antihydrogen atoms.