Particle physics today faces the challenge of explaining the mystery of dark matter, the origin
of matter over anti-matter in the Universe, the origin of the neutrino masses, the apparent …

Positronium (Ps) is an exotic hydrogenic atom composed of an electron bound to a positron
via the Coulomb force. Being composed of two low-mass leptons, positronium is, for all …

Einstein's general theory of relativity from 1915 1 remains the most successful description of
gravitation. From the 1919 solar eclipse 2 to the observation of gravitational waves 3, the …

Charged lepton system symmetry under combined charge, parity, and time-reversal
transformation (CPT) remains scarcely tested. Despite stringent quantum-electrodynamic …

The photon—the quantum excitation of the electromagnetic field—is massless but carries
momentum. A photon can therefore exert a force on an object upon collision. Slowing the …

Abstract At the historic Shelter Island Conference on the Foundations of Quantum
Mechanics in 1947, Willis Lamb reported an unexpected feature in the fine structure of …

When atoms are placed into liquids, their optical spectral lines corresponding to the
electronic transitions are greatly broadened compared to those of single, isolated atoms …

The positron, the antiparticle of the electron, predicted by Dirac in 1931 and discovered by
Anderson in 1933, plays a key role in many scientific and everyday endeavours. Notably, the …

Abstract In 1906, Theodore Lyman discovered his eponymous series of transitions in the
extreme-ultraviolet region of the atomic hydrogen spectrum,. The patterns in the hydrogen …

We report the results of a new measurement of the positronium 2 S 3 1→ 2 P 3 2 (ν 2)
interval. Using a modified experimental arrangement we have significantly reduced the …