Excited Positronium Hyperfine Structure

As a bound state of two spin-1/2 fermions, positronium has spin singlet (parapositronium, pPs) and triplet (orthopositronium, oPs) states. The energy splitting between ortho-positronium and para-positronium is sensitive to the higher order correction of the quantum electrodynamics (QED) prediction and to the new physics beyond the Standard Model via the quantum oscillation of ortho-positronium into a virtual photon. This hyperfine splitting (Ps- HFS) corresponds to a transition frequency of about 203 GHz in the ground state and has been very precisely measured. The QED prediction disagrees with those measurements by more than three sigma. The goal of the proposed experiment is to perform the first measurement of the hyperfine splitting in the first excited state of positronium, the transition from the triplet state to the singlet state. It is building on the present work to measure the laser transition from the ground state 1S to the first excited state 2S of positronium which is currently being set up at the institute of particle physics at the ETH in Zürich in Switzerland under the lead of Dr. Crivelli. This experiment would provide the first direct measurement of the hyperfine splitting of the 2S state. It would further be a very important cross-check for hyperfine transition measurements in the positronium ground state since there still exists a serious discrepancy of 3.5 sigma between between the measured values and the QED prediction. In order to perform the microwave spectroscopic hyperfine measurement of the triplet to singlet 2S transition, the ground state positronium needs to be enhanced to the first excited state. Once the positronium atoms are excited via laser stimulation, they will enter a microwave resonator where an appropriately chosen magnetic field at the theoretical transition frequency of 25.43 GHz will stimulate de-excitation of the positronium atoms to the singlet level of the same state. Due to the short life time of the singlet 2S state of about 1 ns, this will result in a sudden annihilation in two photons which will be detected using a gamma ray detector, mounted around the region where the resonator is placed. The immediate goal is to observe this hyperfine transition for the first time. As a second step, detailed numerical simulations of the transition process will have to be performed, which will help to optimize the measurement parameters and thus increase the precision of the measurement to the one achieved with previous hyperfine measurements in the ground state of positronium as well as with theory. This is fundamental in order to allow a comparison to existing experimental data to help understand the origin of the discrepancy between theory and experiment.

We were carrying out a measurement on the hyperfine structure of positronium - the bound state of the positron and the electron - in an excited state. The aim of this experiment was to reach a precision of 0.5 parts per billion for the measured 1S-2S hyperfine transition frequency in order to cross-check the underlying quantum electrodynamic theory. After reviewing the currently available sources of positronium, we considered laser cooling as a route to push the precision in the measurement down to 0.1 parts per billion. If such an uncertainty could be achieved, this would be sensitive to the gravitational redshift and therefore be able to assess the sign of gravity for antimatter, i.e. if gravity affects antimatter differently than matter.The first observation of the annihilation of positronium from the 2S state was achieved. Positronium was excited with a two-photon transition from the 1S to the 2S state where its lifetime was increased by a factor of eight compared to the ground state due to the decrease in the overlap of the positron-electron wave function. The yield of delayed annihilation photons detected as a function of laser frequency was used as a new method of detecting laser-excited positronium in the 2S state. This can be considered the first step towards a new high precision measurement of the 1S2S positronium line. This experiment could shed some light on the apparent matter-antimatter asymmetry in the universe and perhaps on the nature of dark matter.