We report the background-free observation of atomic antihydrogen, produced by interactions of an antiproton beam with a hydrogen gas jet target in the Fermilab Antiproton Accumulator. We measure the cross section of the reaction pp ! He 2 p for p beam momenta between 5203 and 6232 MeV͞c to be 1.12 6 0.14 6 0.09 pb. [S0031-9007(98)

We propose an experiment to measure the Lamb shift and fine structure ͑the intervals 2s 1/2 Ϫ2 p 1/2 and 2 p 1/2 Ϫ2p 3/2 ) in antihydrogen. A sample of 10 000 antihydrogen atoms at a momentum of 8.85 GeV/c suffices to measure the Lamb shift to 5% and the fine structure to 1%. Atomic collisions excite antihydrogen atoms to states with nϭ2; field ionization in a Lorentz-transformed laboratory magnetic field then prepares a particular nϭ2 state, and is used again to analyze that state after it is allowed to oscillate in a region of zero field. This experiment is feasible at Fermilab.

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Study of the q,(1 ' S o ) state of charmonium formed in pp annihilations and a search for the qL(2 ' S o )T.A. A r m~t r o n g ,~