Speaker
Description
Positronium (Ps), the bound state of an electron and its antiparticle, the positron, is one of the simplest atomic systems suitable for rigorous tests of fundamental physics. Specifically, more precise experimental measurements and theoretical calculations of the 1S–2S interval of ortho-Ps hold the potential to discover indications of new physics beyond the Standard Model. Currently, a calculation of 0.6 ppb[1, 2] and a measurement of 3.2 ppb[3] have been reported for this transition, showing a discrepancy of almost 2 sigma. An order-of-magnitude improvement in experimental precision is crucial to examine this discrepancy. Furthermore, achieving a 0.1-ppb precision may enable the observation of an annual shift in the transition frequency due to the gravitational effect of the sun on antimatter[4]. Recently, significant experimental efforts have been devoted to pursuing better precision and accuracy in this transition, utilizing sophisticated continuous-wave (CW) and pulsed laser technologies, alongside the proposal and demonstration of novel methods such as Doppler-free Ramsey spectroscopy and detection via Rydberg states [5, 6, 7]. These highlight the urgent need to overcome existing experimental limitations.
Deceleration of Ps atoms is the key to improving the precision of 1S–2S spectroscopy, as the thermal motion of this ultralight atom (with a mass of only two electrons) has been the primary limitation for precision. To overcome this constraint, our group has developed a laser cooling technique using a chirped pulse train, where the instantaneous frequency is linearly swept to follow the deceleration of Ps. We have recently reported[8] the world's first successful laser cooling of Ps using this method. In this presentation, we discuss how laser cooling reduces key systematic uncertainties in 1S–2S spectroscopy, specifically: (1) the second-order Doppler shift, (2) transit-time broadening, and (3) the AC Stark shift. We also present the specific design of the light source for two-photon Doppler-free spectroscopy on cooled Ps and report on the current status of the experiment.
[1] K. Pachucki and S. G. Karshenboim, Phys. Rev. A 60, 2792 (1999).
[2] K. Melnikov and A. Yelkhovsky, Phys. Lett. B 458, 143 (1999).
[3] M. S. Fee et al., Phys. Rev. Lett. 70, 1397 (1993).
[4] P. Crivelli, D. A. Cooke, and S. Friedreich, Int. J. Mod. Phys. Conf. Ser. 30, 1460257 (2014).
[5] L. de Sousa Borges et al., arXiv:2512.16018 (2025).
[6] M. W. Heiss et al., Phys. Rev. A 111, 012810 (2025).
[7] E. Javary et al., Eur. Phys. J. D 79, 15 (2025).
[8] K. Shu et al., Nature 633, 793 (2024).