Probing antiprotonic helium with precise lasers

Abstract

Antiprotons are the antimatter counterpart of protons, which are assumed to have exactly the same mass and absolute value of charge (but opposite sign) as protons. This is a consequence of a fundamental symmetry of nature called CPT, which stands for charge conjugation, parity, and time reversal. It postulates that if all matter in the universe were replaced with antimatter, if left and right were inverted (meaning vectorial quantities such as velocity or electric field would reverse direction), and if the flow of time were reversed, this ‘anti-world’ would be indistinguishable from our normal matter world. If we could detect any deviation, however small, it would indicate that this fundamental symmetry is broken. ‘Small’ is the key word here: it is important to compare matter and antimatter with the highest possible precision. Our aim is to determine the antiproton mass in units of the electron mass, and compare it with the corresponding value for the proton to validate CPT symmetry. The proton-to-electron mass ratio is currently known to a fractional precision of 4 10 10. Particle accelerator facilities have produced antiprotons for many years, but they are notoriously difficult to handle because when antiprotons come into contact with ordinary matter, they are absorbed by the atomic nuclei and annihilate within a picosecond. However, nature provides an exception to this rule. Some of the antiprotons stopped in helium gas survive for longer due to the spontaneous formation of an exotic helium atom called antiprotonic helium (chemical symbol pHe p CHe C e , where p is an antiproton, He is a double-ionized helium atom, and e is an electron).1, 2 Within this half-matter, half-antimatter atom, an antiproton replaces one of the two electrons that normally orbit around the helium nucleus: see Figure 1. Since this antiproton orbital has a circular shape with a diameter of about 100pm that has very little overlap with the atomic nucleus, the antiproton can evade annihilation for many microseconds. The remaining electron is retained Figure 1. Schematic drawing of an antiprotonic atom (pHe) excited by two counter-propagating laser beams of optical frequencies !1 and !2. p: Antiproton. He: Double-ionized helium atom. e : Electron.