HyperfineInteractions 76 (1993) 81-93 81 Extremely cold antiprotonsfor antihydrogenproduction G. Gabrielse, W. Jhe, D. Phillips, W. Quint, C. Tseng, L. Haarsma, K. Abdullah Department of Physics, Harvard University, Cambridge, MA 02138, USA J. Gr6bner and H. Kalinowsky InstitutJ~r Physik, UniversitdtMainz, 6500 Mainz, Germany The possibility to produce, trap and study antihydrogen atoms rests upon the recent avail- ability of extremely cold antiprotons in a Penning trap. Over the last five years, our TRAP Collaboration has slowed, cooled and stored antiprotons at energies l0 l° lower than was pre- viously possible. The storage time exceeds3.4 months despite the extremely low energy, which corresponds to 4.2 K in temperature units. The first example of measurements which become possible with extremely cold antiprotous is a comparison of the antiproton inertial masses which shows they are the same to a fractional accuracy of 4 × l0 -8. (This is 1000 times more accurate than previous comparisons and large additional increases in accuracy are antici- pated.) To increase the number of trapped antiprotons available for antihydrogen production, we have demonstrated that we can accumulate or "stack" antiprotons cooled from successive pulsed injections into our trap. 1. Introduction Until several years ago, antiprotons could only be stored in giant storage rings within which they travelled at nearly the speed of light. Experiments were done by either colliding these anfiprotons with counter propagating protons in the same sto- rage ring or by directing the antiprotons into matter and observing the way the anti- proton stopped and annihilated. Over the last five years, a series of experiments done by our TRAP Collaboration [1-4] has dramatically changed the way antipro- tons can be studied. Antiprotons can now be slowed, cooled and then stored at ener- gies below 1 meV= l0 -3 eV, in thermal equilibrium at 4.2 K. This is more than l0 l° times lower than the energy with which antiprotons are stored in the lowest en- ergy antiproton storage ring. As discussed in section 2, antiprotons from the LEAR Storage Ring at CERN are slowed in matter [2], captured in an ion trap [1,3] and are then cooled via collisions with cold electrons [3]. The extremely cold anti- protons can be held for a very long time within the small central volume ( < 1 mm 3) of specially designed Penning trap [5]. In one case, antiprotons were held for two months [4]. During this time we observed no loss of antiprotons from the trap and © J.C. Baltzer AG, Science Publishers