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ALPHA: Testing fundamental symmetries via antihydrogen-hydrogen comparison

Location: CERN, Switzerland
Participating Canadian institutions: Calgary, Simon Fraser, TRIUMF, British Columbia, York
International partners: Brazil, Denmark, Israel, Sweden, Switzerland, UK, USA
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The ultimate goal of antihydrogen studies is to test fundamental symmetries between matter and antimatter with the highest possible precision. Precision tests of CPT and the Weak Equivalence Principle (WEP) will confront the foundations of modern physics – quantum field theory and general relativity. The ALPHA experiment located at CERN has played the leading role in this endeavour over the past 15 years. The current configuration of the apparatus is shown in Fig. 1.

Figure 1: The ALPHA-2 trap for precision laser spectroscopy (left) and the vertical ALPHA-g trap for gravity and microwave studies (right) at the CERN AD facility.

ALPHA is now firmly into the precision measurement stage using the ALPHA-II atom trap (left part of Fig. 1), and over the past five years has delivered a range of firsts: Accumulation of over one thousand antihydrogen atoms for a single run 1; test of charge neutrality of antihydrogen at the \(10^{-9}\) level 2; determination of the hyperfine splitting 3; laser spectroscopy 4 and measurement of the \(1S - 2S\) interval with \(10^{-12}\) level precision 5; observation of the \(1S - 2P\) transitions  6, as well as fine structure and the Lamb shift 7 (shown in Fig. 2). Recently, laser cooling of antihydrogen was demonstrated. These accomplishments open the door to a new chapter in antimatter physics where spectroscopic precisions achievable with antihydrogen will rival those of hydrogen. In terms of sensitive symmetry tests, this implies that likely not only antihydrogen, but also hydrogen spectroscopy has to increase in accuracy. The near term goal is to achieve sub-kHz (i.e., \(10^{-13}\)) precision. ALPHA-Canada, which constitutes more than one third of the collaboration, continues to provide leadership in particle detection, spectroscopy and laser cooling.

Figure 2: The 1S-2P fine-structure spectrum of antihydrogen. Experimental data (filled circles) and fitted lineshapes for doubly spin-polarized antihydrogen samples. Taken from The ALPHA Collaboration, Nature 578, 375 (2020).

For the coming 5 year period, emphasis will be on the new, CFI-funded, ALPHA-g apparatus (right side of Fig. 1, currently being readied for deployment, for the measurement of gravitational free-fall of neutral anti-matter, probing the gravitational interaction of antimatter. This device will also allow 100-fold improved hyperfine spectroscopy. As this device moves online, a next generation effort will be launched in Canada, HAICU (Hydrogen-Antihydrogen Infrastructure at Canadian Universities), which will exploit the amazing progress being made in the field of quantum sensing. In HAICU, the (anti)hydrogen atoms will be cooled to \(\mu\)K temperatures, several orders of magnitude colder than for the presently most precise measurements of the hydrogen spectrum, enabling substantial progress with hydrogen during offline commissioning in Canada, with the introduction of atomic fountain and atom interferometry techniques.

On the 2027-36 horizon HAICU would be deployed at CERN, where atomic fountain based measurement can be performed with antihydrogen and hydrogen in the same apparatus, greatly diminishing the potential for systematic errors. Ultimately, it might be possible to form antimatter molecules and perform a CPT test at the \(10^{-17}\) level.

  1. M. Ahmadi et al., Nat. Comm. 8, 681 (2017).↩︎

  2. M. Ahmadi et al., Nature 529, 373 (2016).↩︎

  3. M. Ahmadi et al., Nature 548, 66 (2017).↩︎

  4. M. Ahmadi et al., Nature 541, 506 (2017).↩︎

  5. M. Ahmadi et al., Nature 557, 71 (2018).↩︎

  6. M. Ahmadi et al., Nature 561, 211 (2018).↩︎

  7. The ALPHA Collaboration, Nature 578, 375 (2020).↩︎