Abstract
At CERN‘s Antiproton Decelerator (AD) facility, the Atomic Spectroscopyand Collisions Using Slow Antiprotons (ASACUSA) collaboration is carrying out precise laser spectroscopy experiments on antiprotonic helium (p̅He+ ≡ p̅+He2++e−) atoms. By employing buffer-gas cooling techniquesin a cryogenic gas target, samples of atoms were cooled to temperatureT = 1.5–1.7 K, thereby reducing the Doppler width in the single-photon resonance lines. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as Mp̅/me = 1836.1526734(15). This agreed with the known proton-to-electron mass ratio with a precision of 8 . 1010. Further improvements in the experimental precision are currently being attempted. The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should further increase the experimental precision.
Highlights
The metastable antiprotonic helium atom is an exotic long-lived system made 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 [1,2,3]
By comparing the values with the results of three-body quantum electrodynamics (QED) calculations, the antiproton-to-electron mass ratio Mp/me can in principle be determined with a relative precision of ⇠ 10−11 [4,5,6]
This corresponds to the best determinations of the proton-to-electron mass ratio Mp/me obtained from Penning trap experiments [7,8,9,10,11], or laser spectroscopy of HD+ molecular ions [12,13,14]
Summary
The metastable antiprotonic helium (pHe+ ⌘ p + He2+ + e−) atom is an exotic long-lived system made 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 [1,2,3]. The unusual longevity of the atom allows us to measure its transition frequencies by laser spectroscopy. By comparing the values with the results of three-body quantum electrodynamics (QED) calculations, the antiproton-to-electron mass ratio Mp/me can in principle be determined with a relative precision of ⇠ 10−11 [4,5,6].
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