Dr. Andreas Josef Dax

Andreas Dax is member of the SwissFel gun laser group at PSI. He got his degrees in Physics (Diploma and PhD) from Bonn University, Germany. He was working in research institutes like NIST in Boulder, Max-Planck-Institute for radiation chemistry in Muehlheim, GSI in Darmstadt, CERN and Paul Scherrer Institute.

Member of the gun laser group for SwissFel. Make sure that all of our lasers and related components are running 24/7 including Pikett service. Further technical developments to improve the existing system and to allow new modifications of the electron beam in space and in time. Member of the HERO group. Preparation of Echo-Enabled Harmonic Generation a laser assisted electron beam manipulation scheme to seed the soft x-ray beam line of SwissFel.

Currently Andreas Dax focuses on getting the free electron laser SwissFel as stable and versatile as possible as far as the photo cathode laser is concerned. A refinement  of our laser arrival monitor is about to be finished. With the HERO project, we will have the opportunity to make out of an x-ray flash lamp a real laser, which then will open the way to many more exciting precision experiments.

For an extensive overview we kindly refer you to our publication repository DORA 

Laser spectroscopy of pionic helium atoms

M.Hori, H.Aghai-Khozani, A.Soter, A.Dax, D.Barna

Nature 581, 37-41 (2020)

Pions are mesons made of two quarks predicted by Yukawa in 1935 and found experimentally in 1947 the first time. They mediate the strong force between protons and neutrons, which keeps the nuclei together. In our experiment, it was possible for the first time to show in a direct way that pions can be captured in Helium atoms. Here a pion replaces an electron and survives in a metastable state for several nanoseconds. This way it can be studied by laser spectroscopy, which will be the purpose of forthcoming experiments.

The size of the proton

Nature, 466, 213-216 (2010)

R. Pohl, A. Antognini, F. Nez F., F.D. Amaro, F. Biraben, J.M.R. Cardoso, D.S. Covita, A. Dax  … F. Kottmann

For many years there were two independent methods aiming to measure the charge radius of the proton: electron scattering and precision hydrogen spectroscopy where the proton charge radius contributes to transition frequencies. The results of both methods converged to a value of about 0.877 fm. Our experiment came up with a third method which was spectroscopy on muonic hydrogen where the electron is replaced by a muon being about 200 times closer to the nucleus. We measured a radius of 0.841 fm about 4% smaller than the other result which amounts to a  5-σ discrepancy. This result became known as the proton radius puzzle. After this experiment we performed several others like on muonic deuterium and muonic helium published in Nature, Science, Annalen der Physik and others able to reject or confirm several theories of how to solve the proton radius puzzle.

Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio

M Hori, A Sótér, D Barna, A Dax, R Hayano, S Friedreich, B Juhász, ...

Nature 475 (2011), 484-488

It is well assumed that right after the big bang there was as much matter produced as antimatter. However, cosmologists cannot find antimatter in the universe. Therefore, it is reasonable to think of that there must be a small difference in the interaction properties between matter and antimatter. In this experiment performed at CERN, we replaced an electron in a helium atom by an antiproton, which can survive in high lying Rydberg states for several microseconds. This is plenty of time to study its properties. Via two-color two-photon spectroscopy in a cryogenic helium gas target, we succeeded to measure several transitions. The transition frequencies we referenced to a frequency comb. Since then we know that the mass of the proton and that of the antiproton agree with each other on a 10^-9 level.

High precision hyperfine measurements in Bismuth challenge bound-state strong field QED

J.Ullmann, Z.Andelkovic, C.Brandau, A.Dax, ….W.Noertershaeuser

Nature Communications 8, 15484 (2017)

Quantum Electrodynamics (QED) is one of the most successful theories in physics. But if you test it in electric fields like 10¹⁶Volts/cm and magnetic fields like 20000 Tesla is it then still valid? At GSI in Darmstadt we measured the hyperfine splitting (HFS) in the ground state of  highly charged ions, where all electrons but a few are stripped off, in a storage ring. Since the splitting goes with Z³ the transition wavelength is not in the radiofrequency region like in hydrogen anymore but in the optical region where one can do precision laser spectroscopy. In order to cancel nuclear effects on the transition wavelength we measured the HFS in hydrogen like Bismuth and lithium like Bismuth during the same run and took the so called specific difference. Our result is the most decisive tests of QED in strong fields and we found a 7-σ discrepancy with theory, which is now called the hyperfine puzzle.

Isotope-shift measurements of stable and short-lived lithium isotopes for nuclear-charge-radii determination

W.Noertershaeuser, R.Sanchez, G.Ewald, A.Dax, …C.Zimmermann

Physical Review A83, 012516 (2011)

If one goes to the drip lines of the nuclear chart i.e. from the valley of stable isotopes to the proton rich or neutron rich isotopes respectively one can find halo nuclei, which have a core nucleus surrounded by a halo of orbiting protons or neutrons. One of these is ¹¹Li having a halo made out of two neutrons. In order to understand this effect in more detail we wanted to find out how much the two outer neutrons in ¹¹Li dilute the charge radius of the nuclear core. At GSI and Triumph we succeeded in doing precision isotope shift measurements via a combination of two photon spectroscopy and laser resonance ionization spectroscopy on all Lithium isotopes (⁶Li, ⁷Li, ⁸Li, ⁹Li and ¹¹Li) from which, together with a refined theory, the difference of the mean square charge radii could be extracted. Finally, we could conclude that the charge radius decreases progressively from ⁶Li to ⁹Li and then it jumps back to a value even larger than that of ⁷Li. This finding ruled out many nuclear models.