In the 2019 science fiction movie Ad Astra, Roy McBride is sent on a mission to the Lima project station which antimatter power source is out of control. One reason that makes it impossible to build such a fantastic power source today is that we do not know how to efficiently produce large amounts of antimatter. It is also true that we observe way less antimatter than matter in the Universe. This observation is quite intriguing and has some deep implications. But let us first take a few steps back: what is antimatter? What do we know about it?
The idea behind antimatter is that each charged particle of matter such as electrons or protons is associated with a particle of antimatter which has the exact same properties but an opposite electrical charge. In fact, during particle collisions at high enough energy, particles are created in pairs. For example, an electron and a positron (the antimatter counterpart of an electron) can be generated from vacuum. We also observe that particles of matter and antimatter annihilate together when they come in contact. The fact that we observe a large unbalance of matter over antimatter in the Universe is therefore very puzzling.
Clearly, there must be a mechanism causing the imbalance between matter and antimatter. A promising direction to resolve this mystery is to investigate the properties of neutral antimatter compounds such as antihydrogen (the antimatter counterpart of a hydrogen atom) and positronium (the short-lived bound state of an electron with a positron) and compare them with the matter equivalent. In order to reach high precision in the measurement of the properties of these species, one needs to bring them to as low temperature as possible. The present project proposes to laser-cool positronium atoms using the Doppler cooling technique where fast atoms travelling towards the laser source can be selectively slowed down by absorbing and re-emitting photons several times.
In 2020, we reported on a background-free technique to measure the velocity distribution of a positronium ensemble of atoms. We also demonstrated morphologically tuned nanochannel silicon targets to produce thermalized positronium at room temperature with high yield.
In 2021, we published the first realistic simulation of positronium laser cooling in a magnetic field. In parallel, the laser system developed for this purpose has been commissioned.
Despite being the most tested and successful theory so far, the Standard Model cannot explain the baryonic asymmetry (the excess of matter over antimatter in the Universe). Positronium (Ps, the bound state of an electron and its antimatter counterpart, a positron) is a critical system in antimater research as it can be used itself for precise investigations but it is also used in antihydrogen experiments. The precision of gravity and spectroscopic measurements is strongly limited by the temperature of the antimatter system under investigation. Laser cooling of positronium (Ps) has never been performed so far and is a decisive achievement to boost research on antimatter.
We propose to develop and implement the laser systems and detectors necessary to efficiently cool a cloud of ortho-Ps produced by implantation of positrons into a nanochannel convertor (in AEgIS, at CERN) and measure its Doppler profile. To reach lower temperatures, we will develop (partially in Oslo) and implement a cooling membrane featuring musket-like through-holes resulting in adiabatic cooling inside the holes prior to laser cooling. We will perform precise spectroscopy of the 1S-Rydberg transition in Ps to prepare for a Rydberg deceleration of Ps.
Critical R&D challenges are found in the development of the cooling laser system. In particular, the energy (2mJ), the spectral bandwidth (250Ghz) and the pulse duration (200 ns) are critical and might be difficult to reach. For Doppler velocimetry of Ps at 205 nm, the existing laser will be modified to increase the bandwidth with the exciting and ambitious perspective to approach a single shot diagnostics. The development of cooling membranes for production of cold Ps stands at the limit of current state of the art.
A cold source of Ps will allow to boost the production of antihydrogen, test the Weak Equivalence Principle on Ps, perform precise test of QED, open the road to Ps Bose-Einstein Condensation, a Ps microscope and a lepton nano-collider.