First Method to Produce Slow Antihydrogen Atoms:
During Positron Cooling of Antiprotons in a Nested Penning Trap

The first of two methods to produce cold antihydrogen worked much as we initially imagined when we proposed that three-body recombination would form antihydrogen atoms from cold positrons and antiprotons as they interacted in a nested Penning trap. The nested Penning trap is required to make particles of opposite sign to interact.

We went though several steps on the way to antihydrogen production. First we demonstrated the first electron cooling of protons in a nested trap. Second we managed to load positrons and antiprotons into a nested Penning trap for the first time, in the week before LEAR closed down. Third, at the new AD facility that replaced LEAR at CERN, we demonstrated the first positron cooling of antiprotons in a nested Penning trap.

The method was ready and demonstrated. It remained to demonstrate that antihydrogen atoms were actually being produced during the positron cooling of antiprotons in a nested Penning trap. In 2002 we provided such a demonstration in two ATRAP reports [1, 2] as did the ATHENA Collaboration*. At ATRAP we used a field ionization method for detection. ATHENA detected the annihilations of positrons and antiprotons (as we had proposed in the 1987 long term goals quoted above).

A lot of publicity accompanied these first observations of cold antihydrogen, leading the American Institute of Physics to label these observations of slow antihydrogen as the AIP physics story of the year.

Our field ionization method to detect antihydrogen has the advantage that it reveals directly how tightly bound are the detected antihydrogen atoms. It confirms that we are producing "guiding center" atoms as was expected.** Our calculation provides the calibration. The antihydrogen spectrum that we measure and the hydrogen speeds that we observe agree quite well with our later theoretical interpretation.

This first method for producing slow antihydrogen atoms has produced most of the antihydrogen atoms that we observed at ATRAP (we also used a second method described next) and all the antihydrogen atoms observed by ATHENA and ALPHA. A review of antihydrogen experiments covers this method.