Single-photon laser spectroscopy of cold antiprotonic helium

Abstract

Some laser spectroscopy experiments carried out by the Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration to measure the single-photon transition frequencies of antiprotonic helium (pHe + ≡ p +He 2+ +e − ) atoms are reviewed. The pHe + were cooled to temperature T = 1.5-1.7 K by buffer-gas cooling in a cryogenic gas target, thus reducing the thermal Doppler width in the single-photon resonance lines. The antiproton-to-electron mass ratio was determined as M p /m e = 1836.1526734(15) by comparisons with the results of three-body quantum electrodynamics calculations. This agreed with the known proton-to-electron mass ratio. IntroductionMetastable antiprotonic helium (pHe + ≡ p + He 2+ + e − ) is a three-body atom composed of a helium nucleus, an electron in the ground state, and an antiproton occupying a Rydberg state of principal and orbital angular momentum quantum numbers n ∼ − 1 ∼ 38 [1-3]. By measuring its transition frequencies by laser spectroscopy [4][5][6][7], and comparing the values with the results of three-body quantum electrodynamics (QED) calculations, the antiproton-to-electron mass ratio M p /m e can in principle be determined with a relative precision of < 10 −11 . This corresponds to the best determinations of the proton-to-electron mass ratio M p /m e from Penning trap experiments [8][9][10][11][12], or laser spectroscopy of HD + molecular ions [13][14][15]. Spectroscopy of pHe + also provides a consistency test of CPT symmetry, which may be complementary to the experiments on antihydrogen atoms [16][17][18].