P. Crivelli (SNSF Ambizione grant PZ00P2_132059 and ETH-Grant ETH-47 12-1)
This project aims to improve the current accuracy in the measurement of the energy interval of positronium (Ps) from its ground state (1S) to the first excited state (2S) by a factor of 5. This will provide a very stringent test of the theory describing atomic systems called bound states quantum electrodynamics (QED). Very recently a serious discrepancy of 5 sigma in the charge radius of the proton extracted from the muonic-hydrogen experiment at PSI compared to other experiments was found and has not yet been explained. This increases the importance of studying hydrogen-like and especially non-baryonic (with no quarks, e.g no protons) systems like positronium or muonium where finite-size effects due to the nucleus are absent. The proposed measurement will also result in the best determination of the positron-electron mass ratio that should be exactly one in order for the CPT symmetry to be conserved. This symmetry is a pillar of quantum field theory which is the base of our current understanding of particles and their interactions.
If the accuracy of the measurement could be further increased by a factor 5, this could provide a model independent test of the effect of gravity on anti-matter. The gravitational redshift predicted by general relativity was one of the great implications of Einstein's theory that was demonstrated experimentally. The pace of two identical clocks placed at different gravitational potentials is different. If gravity would act differently on antimatter, a shift between Ps and a reference clock made of matter (e.g. a cesium clock) should be observed in the different gravitational potential created by the large variation of 5 millions km of the earth's orbit around the sun during the year.