Our small tabletop-like experiments synthesize some exotic atoms and molecules containing antiprotons which are the antimatter counterparts of protons, and pions which are the lightest particles composed of a quark and antiquark. We then test the validity of the theory of quantum electrodynamics (QED) in such exotic hadronic atoms by precisely measuring the atomic energies using laser spectroscopy. QED remains the most accurately understood variant of relativistic quantum field theory that forms the basis of the Standard Model.

In our past experiments, the ratio of the antiproton mass and the electron mass was determined as 1836.1526734(15). The fact that this result agreed so well with the proton-to-electron mass ratio constitutes a consistency test of the charge conjugation, parity and time reversal (CPT) symmetry of nature. According to this theorem the “antiworld” – constructed by replacing all the matter particles in the universe with antimatter, inverting their spatial configuration, and reversing the flow of time - would be indistinguishable from our real matter world. In the future, we hope to determine the masses of antiprotons and pions with factor >100 higher precision than before. Limits will be set on possible physics beyond the Standard Model, including fifth forces that may arise between the particles in the atom at angstrom length scales, and their possible interactions with dark matter.

  • ICL leads a laser spectroscopy experiment of antiprotonic helium atoms carried out at the Extra Low ENergy Antiproton (ELENA) storage ring facility of CERN. This is a three-body atom composed of a helium nucleus, an electron, and an orbital antiproton, and constitutes the matter-antimatter bound system having the longest known lifetime of a few microseconds. In the experiment, slow antiprotons are allowed to come to rest in cryogenic helium, thereby synthesizing cold samples of antiprotonic helium. The atoms are irradiated by five laser beams which deexcite the antiproton orbiting the helium nucleus via nonlinear sub-Doppler two-photon transitions. In this way, sharp spectral lines are observed, and the atomic energies measured. This should ultimately allow the antiproton-to-electron mass ratio to be determined with a precision of 10-12, though this will take many years. Quantum metrology techniques such as optical frequency combs and frequency reference standards are used.
  • We also carry out laser spectroscopy of pionic helium atoms at the 590 MeV ring cyclotron facility of Paul Scherrer Institute near Zurich. The protons and neutrons that make up the atomic nuclei in normal matter are each made of three quarks. Quarks have an interesting characteristic: they cannot exist alone in isolation, but three quarks together can coexist very stably in the form of a proton. Mesons are classes of particles made of a quark and an antiquark - the antimatter counterpart of the quark. They could not be previously studied using lasers owing to their extremely short lifetimes. Among all the mesons, charged pions are the longest-lived variety with a lifetime of 26 nanoseconds. Pionic helium is a three-body atom composed of a helium nucleus, an electron, and an orbital pion; it has a lifetime >1000 times longer than any other pionic atom that can be experimentally synthesized in a practical way. We have recently achieved laser spectroscopy of pionic helium, which constituted the first laser excitation of an atom containing a meson. The goal within the next few years is to improve the resolution of the experiment using high-precision lasers so that the pion mass may be determined with a fractional precision of 10-7


For more information, please contact Dr. Masaki Hori (m.hori@imperial.ac.uk).