Abstract
The Kα X-rays from pp̄ atoms formed in H2 gas at normal temperature and pressure are unambiguously identified by coincidences with L X-rays populating the 2P level. Background due to inner bremsstrahlung is suppressed by selecting events annihilating into neutral final states only. The Kα line is observed with a significance of more than 25 standard deviations at an energy of 8.67 (15) keV. From fits to the Kα line we obtain a strong-interaction shift and width of the 1S level, averaged over the unresolved spin singlet and triplet contributions, of ΔE + iΓ/2 = [−0.70(15) + i0.80(20)] keV.
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