Quantum State Distributions in Antihydrogen Beam Measured
The ASACUSA collaboration, including AVA fellow Amit Nanda, reports on the measurement of the principal quantum number distribution of the antihydrogen beam produced by the ASACUSA Cusp trap. Another AVA fellow, Markus Wiesinger, was also part of the ASACUSA collaboration working on this project prior to joining the AVA network.
The new study, published in The European Physical Journal D, characterises the quantum number distribution of the antiatoms in the antihydrogen beam by a method called field ionisation. Here, strong electric fields are applied which allow the positron in the antihydrogen atom to tunnel out of the potential well formed by the antiproton. However, only atoms with high principal quantum numbers are ionised, such that scanning the electric field selects the quantum number of the surviving atoms. The number of surviving antihydrogen atoms is then measured by detecting the annihilation signal of antihydrogen atoms at the end of the beamline.
To produce the antihydrogen beam, antiprotons from the Antiproton Decelerator at CERN are caught and stored in the MUSASHI trap while positrons are obtained from a sodium-22 source and a neon moderator then stored in the positron accumulator. They are merged together to form antihydrogen atoms in a double-Cusp trap (see figure 1), which consists of a multi-ringed electrode trap housed within a magnetic field produced by a pair of superconducting coils in an anti-Helmholtz configuration. Positrons and antiprotons are mixed in a region of strong magnetic field within a nested Penning trap that lies before the first of two cusps. The cusped field helps to focus and polarise cold ground state antihydrogen atoms, thus leading to a polarised antihydrogen beam.
Figure 2: Principal quantum number distribution of antihydrogen atoms in the ASACUSA antihydrogen beam
“Knowing the exact state the antihydrogen atoms are in is an essential prerequisite for using the antihydrogen beam for spectroscopy experiments“, says CERN physicist Bernadette Kolbinger, the first author of the study. In the future, the ASCACUSA collaboration is planning to use the beam of antihydrogen atoms for antihydrogen hyperfine splitting measurements and to compare these with measurements on hydrogen in order to test the CPT symmetry. For these measurements, the antihydrogen atoms need to be in the ground state. The results (see figure 2) show that antihydrogen atoms with a wide range of principal quantum number are available, so de-excitation to the ground state will be necessary.
Currently, upgrades are being performed at CERN’s Antiproton Decelerator in order to increase the number of antiprotons available to experiments for antihydrogen formation. At the same time, ASACUSA is using the matter counterpart of antihydrogen atoms - hydrogen atoms - to study and improve the antihydrogen production and beam formation process, including ways to de-excite Rydberg atoms by plasma collisions or using laser light. In addition, ASACUSA is improving the efficiency of the experiment by upgrading the antihydrogen detector: faster timing and higher position resolution will improve the reconstruction of antihydrogen annihilation events and thus overall detection efficiency.
Kolbinger, B., Amsler, C., Cuendis, S.A. et al. Measurement of the principal quantum number distribution in a beam of antihydrogen atoms. Eur. Phys. J. D 75, 91 (2021).