High-resolution laser resonances of antiprotonic helium in superfluid 4He

Abstract

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. This linewidth increase can often reach a factor of more than a million, obscuring spectroscopic structures and preventing high-resolution spectroscopy, even when superfluid helium, which is the most transparent, cold and chemically inert liquid, is used as the host material1–6. Here we show that when an exotic helium atom with a constituent antiproton7–9 is embedded into superfluid helium, its visible-wavelength spectral line retains a sub-gigahertz linewidth. An abrupt reduction in the linewidth of the antiprotonic laser resonance was observed when the liquid surrounding the atom transitioned into the superfluid phase. This resolved the hyperfine structure arising from the spin–spin interaction between the electron and antiproton with a relative spectral resolution of two parts in 106, even though the antiprotonic helium resided in a dense matrix of normal matter atoms. The electron shell of the antiprotonic atom retains a small radius of approximately 40 picometres during the laser excitation7. This implies that other helium atoms containing antinuclei, as well as negatively charged mesons and hyperons that include strange quarks formed in superfluid helium, may be studied by laser spectroscopy with a high spectral resolution, enabling the determination of the particle masses9. The sharp spectral lines may enable the detection of cosmic-ray antiprotons10,11 or searches for antideuterons12 that come to rest in liquid helium targets.

Laser spectroscopy of long-lived pionic and antiprotonic helium in superfluid helium

“…The experimental frequency 𝜈 exp is larger than the theoretical value 𝜈 th = (183681.8 ± 0.5) GHz obtained from QED calculations [4]. The impact approximation of the binary collision theory of spectral lineshapes [2] predicts a frequency shift of between Δ𝜈 = 96 and 142 GHz due to atomic collisions in the experimental target [1,2,20,[22][23][24]. This shift is in rough agreement with the experimental data.…”

“…The ASACUSA collaboration has carried out laser spectroscopy of antiprotonic helium atoms (𝑝He + ≡ 𝑝 +He 2+ +𝑒 − ) using the Antiproton Decelerator (AD) of CERN [23,24,[26][27][28][29][30][31][32][33][34][35][36][37]. This atom contains an antiproton in a Rydberg state with quantum numbers of 𝑛 ≈ ℓ + 1 ≈ 38 [21,34,38].…”

Kaonic atoms at the DAΦNE collider with the SIDDHARTA-2 experiment

“…At Paul Scherrer Institute (PSI) the properties of pionic atoms were measured and found to be in good agreement with scattering experiments, confirming a consistent picture even at low-energies [7]. Moreover, laser spectroscopy of antiprotonic and pionic helium was recently achieved [8,9], aiming at a precise determination of the particle's masses.…”

The upgrade of the ASACUSA scintillating bar detector for antiproton annihilation measurements

“…The ASACUSA Collaboration (Atomic Spectroscopy And Collisions Using Slow Antiprotons) has been operating at the Antiproton Decelerator facility (AD) at CERN for more than 20 years. The research program aims to test fundamental symmetries and the unbalanced matter antimatter ratio in the Universe by the formation and study of antihydrogen [1][2][3][4] as well as other hybrid atoms like antiprotonic helium [5][6][7]. The antiproton collisions and annihilations on different nuclei is also studied.…”