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The ASACUSA collaboration carries out laser spectroscopy of metastable antiprotonic helium atoms ($\bar{p}He^+=\bar{p}+He^{2+}+e^-$) at CERN’s Antiproton Decelerator facility [1-5]. This is a three-body Coulomb system composed of a helium nucleus, an electron, and orbital antiproton. CERN has recently commissioned the ELENA ring, which produced cooled beams of antiprotons with >100 times higher emittance than before. We propose to utilize the unprecedented high-quality beam of this new facility and the latest laser metrology techniques to carry out sub-Doppler two-photon laser spectroscopy of narrow resonances of $\bar{p}\text{He}^+$ with a far higher precision than before. These experiments allow the antiproton-to-electron mass ratio to be determined [4]. Limits may be established on exotic forces that may arise between the constituent particles. A new antiproton beamline with several special features was constructed and commissioned; a sophisticated laser system is now being developed.
[1] A. Sótér, H. Aghai-Khozani, D. Barna, A. Dax, L. Venturelli, M. Hori, “High-resolution laser resonances of antiprotonic helium in superfluid 4He”, Nature 603, 411 (2022).
[2] K. Nordlund, M. Hori, D. Sundholm, “Large nuclear scattering effects in antiproton transmission through polymer and metal-coated foils”, Phys. Rev. A 106, 012803 (2022). [3] M. Hori, H. Aghai-Khozani, A. Sótér, A. Dax, D. Barna, “Laser spectroscopy of pionic helium atoms”, Nature 581, 37 (2020).
[4] M. Hori et al., “Buffer-gas cooling of antiprotonic helium to 1.5 to 1.7 K, and antiproton-to-electron mass ratio”, Science 354, 610 (2016).
[5] V.I. Korobov, L. Hilico, J.-P. Philippe, “Theoretical transition frequencies beyond 0.1 ppb accuracy in H2+, HD+, and antiprotonic helium”, Phys. Rev. A 89, 032511 (2014).
