Dr. Robicheaux

Antimatter

We have performed several calculations that provide data for experiments that have made antihydrogen. The focus of our current calculations is on simulating processes in the ALPHA trap to aid in the interpretation of measurements and suggest ways to improve accuracy. See review articles (F. Robicheaux, "Atomic processes in antihydrogen experiments: a theoretical and computational perspective," J. Phys. B 41, 192001 (2008). PDF (369 kB) and C.O. Rasmussen, N. Madsen, and F. Robicheaux, "Aspects of 1S-2S spectroscopy of trapped antihydrogen atoms," J. Phys. B 50, 184002 (2017). PDF (941 kB) Erratum) about our understanding of atomic processes in these experiments.

We are part of the ALPHA collaboration which includes groups from over 10 countries. The experiment is performed at CERN. The eventual goal is to perform spectroscopy on antihydrogen and compare the frequencies with those in hydrogen. Currently, the 1S-2S frequency is known to 15 significant digits in hydrogen. Our measurements in antihydrogen have achieved 12 significant digits of accuracy with very few atoms (of order 1000). We have also measured the hyperfine splitting of the 1S and 2S states and the net charge of antihydrogen. There would be profound implications for our understanding of fundamental physical laws if there were to be any difference between the spectra of antihydrogen and hydrogen.

Below is a brief description of results in four important publications.

G. B. Andresen, et al (ALPHA collaboration), "Trapped antihydrogen," Nature 468, 673 (2010). PDF (946 kB) (Many online articles)

This was the first report of trapped antihydrogen and gives an idea of what is needed to make and trap antihydrogen.

This image shows a schematic of the trap (upper image) and the electric potentials which trap the antiprotons and positrons before they are combined to form antihydrogen.

The trapped antihydrogen were detected by quenching the magnetic fields from the octupole and mirror coils. The image above shows where (along the trap axis) and when the antihydrogen annihilated on the walls of the trap after the quench. The solid symbols are the experimental data. The dots are from our calculations. The upper plot shows calculated data for trapped antihydrogen. The lower plot shows calculated data for trapped antiprotons. The agreement in the upper plot and disagreement in the lower plot rules out mirror trapped antiprotons and confirms the trapping of antihydrogen.

M. Ahmadi, et al (ALPHA collaboration), "Characterization of the 1S-2S transition in antihydrogen," Nature 557, 71 (2018). PDF (1850 kB) (CERN comment, CERN Courier about previous work cover and pgs 30-34)

This was the second report on the 1S-2S transition but the first that mapped out the transition line. This was one of the biggest of the original goals of the ALPHA experiment. This line was measured with an accuracy of a few Hz in hydrogen. This first measurement achieved an accuracy of a few kHz even though we have several orders of magnitude fewer atoms and our experiments are in strong magnetic fields.

This image shows the energy levels involved in the measurement: 1Sd to 2Sd.

This upper figure shows the measured transition line with the calculation (solid line). The lower figure shows simulations for different amounts of laser power. The comparison between experiment and calculation suggests 1 W was circulating in the optical cavity.

M. Ahmadi, et al (ALPHA collaboration), "Observation of the 1S-2P Lyman-alpha transition in antihydrogen," Nature 561, 211 (2018). PDF (1770 kB)

This was the first report of the Lyman-alpha transition in antihydrogen. This measurement is important because it strongly suggests a method for laser cooling antihydrogen.

The transition is caused by laser pulses that are tuned to the 1S-2P transition in a ~1 T field. This image shows several aspects of the measurement and the comparison with calculations. a) Shows the transition probability as a function of the detuning of the laser. b) Shows the distribution of times after a laser pulse when an antihydrogen hits the wall of the trap. c) Shows the distribution of positions along the trap axis where the antihydrogen hits the wall after a laser pulse.

C.J. Baker, et al (ALPHA collaboration), “Laser cooling of antihydrogen atoms,” Nature 592, 35 (2021). PDF (3650 kB) CERN press release, Purdue press release, various news reports

This reports the successful exprimental realization of laser cooling of antihydrogen verifying our prediction in 2013 that the restricted geometry would not too adversely affect the cooling. This is an important milestone because it will lead to more accurate determination of the transition frequency and higher signal strength.

The above image shows the measured narrowing of the 1S2S transition frequency when the antihydrogen is cooled.

Five Other Recent Publications

C.J. Baker, et al (ALPHA collaboration), “Sympathetic cooling of positrons to cryogenic temperatures for antihydrogen production,” Nat. Commun. 12, 6139 (2021). PDF (1680 kB)

M. Ahmadi, et al (ALPHA collaboration), "Investigation of the fine structure of antihydrogen," Nature 578, 375 (2020). PDF (1890 kB)

M. Ahmadi, et al (ALPHA collaboration), "Antihydrogen accumulation for fundamental symmetry tests," Nature Comm. 8, 681 (2017). PDF (703 kB)

M. Ahmadi, et al (ALPHA collaboration), "Observation of the hyperfine spectrum of antihydrogen," Nature 548, 66 (2017). PDF (762 kB)

M. Ahmadi, et al (ALPHA collaboration), "An improved limit on the charge of antihydrogen from stochastic acceleration," Nature 529, 373 (2016). PDF (641 kB)

Francis Image

robichf[at]purdue.edu
284 Physics Building (Office)
+1 765 494 3029 (Office)
+1 765 494 0706 (FAX)

Mail:
Department of Physics
525 Northwestern Avenue
West Lafayette, IN 47907

Links:

Phyics Department

Purdue University

ALPHA Project

Prof. Francis Robicheaux

Antimatter