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Electron cooling of trapped antiprotons allows their storage at energies more than 6x 10 7 times lower than is available in any antiproton storage ring. More than 60000 antiprotons with energies from 0 to 3000 eV are stored in an ion trap from a single pulse of 5.9-MeV antiprotons from LEAR. Trapped antiprotons maintain their initial energy distribution over days unless allowed to collide with a cold buffer gas of trapped electrons, whereupon they slow and cool below 100 meV in 10 s. The antiprotons are cooled in a harmonic potential well suited for precision measurements and have remained more than 2 days without detectable particle loss. Energy widths as narrow as 9 meV are directly observed. PACS numbers: 36.10.-k, 14.20.Dh, 29.25.FbInteresting experiments await the availability of verylow-energy antiprotons. For example, a much more accurate measurement of the inertial mass of the antiproton becomes feasible with antiproton energies below 1 meV. l This would be one of only a few precise tests of CPT invariance, the only such test with baryons. 2 Measuring the gravitational force on sub-meV antiprotons has also been proposed. 3 It may even become possible to produce and study cold antihydrogen, 4 perhaps allowing a measurement of the gravitational force without the severe competition of electrical forces. 5 Although initial slowing and cooling from the GeV energies at which antiprotons are produced and collected is now routinely done in a series of storage rings at CERN, the lowestenergy antiprotons generally available for experiments still have a kinetic energy of 5.9 MeV. These antiprotons are stochastically cooled, stored, and then ejected from the Low Energy Antiproton Ring (LEAR) which was built for this purpose. Lower storage energies ( < 3 keV) have been achieved in only one experiment, when several hundred antiprotons were briefly stored in an ion trap. 6 In this Letter, we report the first observation of electron cooling within a particle trap, whereby antiprotons cool via repeated collisions with a buffer gas of cold-trapped electrons. 7 (A neutral buffer gas as used for cooling many trapped ions species would cause the antiprotons to annihilate.) As anticipated, 8 electron cooling is extremely effective compared to adiabatic or resistive cooling, even when the cooling rate for the latter is enhanced using electronic feedback techniques. 910 The observed cooling is similar in some respects to the cooling of the hotter species in a two-component plasma, to the cooling of energetic particle beams using a collinear electron beam matched in velocity, 12 and to the sympathetic cooling of one ion species by another in an ion trap. 13 Pulses of 5.9-MeV antiprotons, typically 250 ns in duration and containing up to 3xl0 8 antiprotons, leave our LEAR beam line directed upwards through a Ti window. They pass through another Ti window into a completely sealed vacuum enclosure which is cooled to 4.2 K and located in a 6-T magnetic field. The ion trap inside [ Fig. 1 (a)] consists of an aluminum plate at the bo...

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