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arXiv:2003.05032 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Two-symmetry Penning-Ioffe trap for antihydrogen cooling and spectroscopy

E. R. Tardiff, Xing Fan, G. Gabrielse, D. Grzonka, Chris Hamley, Eric A. Hessels, Nathan B. Jones, G. Khatri, W. Steven Kolthammer, Daniel Zambrano, Cole Meisenhelder, T Morrison, E. Nottet, Elise Novitski, C. H. Storry

Published Jun 24, 2020• 6 citations
arXivHTMLPDF

Abstract

High-accuracy spectroscopic comparisons of trapped antihydrogen atoms (H¯) and hydrogen atoms (H) promise to stringently test the fundamental CPT symmetry invariance of the standard model of particle physics. ATRAP’s nested Penning-Ioffe trap was developed for such studies. The first of its unique features is that its magnetic Ioffe trap for H¯atoms can be switched between quadrupole and octupole symmetries. The second is that it allows laser and microwave access perpendicular to the central axis of the traps.

DOI
10.1016/j.nima.2020.164279

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High-accuracy spectroscopic comparisons of trapped antihydrogen atoms (H¯) and hydrogen atoms (H) promise to stringently test the fundamental CPT symmetry invariance of the standard model of particle physics. ATRAP’s nested Penning-Ioffe trap was developed for such studies. The first of its unique features is that its magnetic Ioffe trap for H¯atoms can be switched between quadrupole and octupole symmetries. The second is that it allows laser and microwave access perpendicular to the central axis of the traps.
The recent demonstration of trapped atomic antihydrogen for 15 to 1000 seconds is a milestone towards precise spectroscopy for tests of CPT invariance. The confinement of a total of 105±21 atoms in a quadrupole magnetic trap was made possible by several improved methods. Improved accumulation techniques give us the largest numbers of constituent particles yet: up to 10 million antiprotons and several billion positrons. A novel cooling protocol leads to 3.5 K antiprotons, the coldest ever observed. Characterizing and controlling the geometry and density of these confined antimatter plasmas allow for consistency in antihydrogen production. Continued use of these methods along with the larger trap depth of a unique second-generation magnet are expected to yield greater numbers of trapped antihydrogen. The new magnet generates both quadrupole and octupole trap geometries, which should make it possible to reduce charged particle loss and will prove useful for laser cooling and spectroscopy. The ultra-low inductances of the magnet have been shown to vastly reduce turn-off times, which will optimize single-atom detection. Finally, improved detector characterization already makes us sensitive to smaller numbers of trapped antihydrogen atoms than before.
Antihydrogen, the opposite number of hydrogen, is the simplest antimatter, and stable in vacuum, which guarantees a long observation time for high precision spectroscopy. This review discusses a short history of antimatter followed by the motivation of the cold antihydrogen research, and then a couple of latest results, which open a new era of antihydrogen research enabling high precision spectroscopy for the first time. A comparison of the spectroscopic properties of antihydrogen with those of hydrogen constitutes a stringent test of the CPT symmetry, the most fundamental law of physics. One of the important achievements of cold antihydrogen study in the last couple of years is the trapping of antihydrogen atoms in an octupole magnetic bottle. Considering the bottle depth, the trapped antihydrogen atoms were really cold, less than 0.5K in their temperature. High precision laser spectroscopy of 1S-2S transition is foreseen once antihydrogen atoms can be laser-cooled. The other milestone development was a successful synthesis of antihydrogen atoms in a so-called cusp trap, where an anti-Helmholtz coil configuration is adopted. The cusp trap enables extraction of antihydrogen atoms in a field-free region as an intensified spin-polarized beam,which realizes high resolution microwave spectroscopy of ground state hyperfine transitions.
ATRAP’s new experimental apparatus is successfully commissioned and used to synthesize and capture antihydrogen atoms. While the number trapped per trial, 5 ± 2, is about the same as our yield in 2011, the rate of antiproton use is a factor of three lower, and the length of a trial is cut in half. A more robust apparatus, the use of smaller radius plasmas, and plasma imaging all contribute. Simulations I built and ran on Harvard’s Odyssey cluster focus on both Lyman-α laser cooling and 1S-2S spectroscopy of trapped antihydrogen, using the laser systems available at ATRAP. These show how laser cooling can reduce average atomic energies by more than an order of magnitude in tens of minutes. Spectroscopy of the 1S-2S line requires similar amounts of time, and laser cooling preceding spectroscopy narrows the observed spectral width close to that seen in hydrogen beam experiments. The computational work points the way to efficient ATRAP laser experiments. Precise 1S-2S spectroscopy on antihydrogen, an important test of CPT symmetry, is within reach.
The purpose of cold antihydrogen research is briefly reviewed together with the latest developments of manipulating antihydrogen atoms. Two major progresses last year were the trapping of antihydrogen atoms in a magnetic bottle and synthesis of antihydrogen atoms in a cusp trap, where a spin-polarized antihydrogen beam can be extracted as an intensified beam. The magnetic bottle consists of an octupole coil and a pair of mirror coils, which improved the magnetic field uniformity near the axis, and so the stability of trapping antiprotons and positrons. Eventually, antihydrogen atoms were trapped for more than 1000s, which is ready to be testified with high precision laser spectroscopy. The cusp trap consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.