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News Archive 2022

15.06.22 | Precision measurements
First direct g-factor difference measurement of two co-trapped isotopes

Quantum electrodynamics (QED), the fundamental theory of light-matter interaction, has been shown to be in excellent agreement with experimental results (see e.g. our news of 08.07.11 and 20.02.14). Modern advanced high precision Penning-trap measurements allow for tests of state-of-the-art QED calculations. The magnetic moment (or g factor) of hydrogen-like ions, that provide a simple bound-state system, can be accessed experimentally and is also predicted very precisely by theory. Thus, accurate measurements of the bound-electron g factor of highly charged ions (HCI) in Penning traps provide stringent tests of the Standard Model in the strongest electromagnetic fields (see e.g. our news of 25.02.19).
The g factor of the bound electron in hydrogen-like ions results from the g factor of a free electron modified mainly by the additional electric field. Since many other contributing nuclear effects (e.g. nuclear mass, charge radius) are smaller than the QED contributions and their uncertainties, it is difficult to study them. This theoretical limitation can be overcome by studying the g-factor difference of two highly charged isotopes. In this case, the common identical QED contributions and their uncertainties do not have to be considered, emphasizing the isotopic differences due to the nucleus. This e.g. enables to resolve and test the nuclear recoil contribution to the g factor, that considers the nuclear recoil in elaborate QED calculations.
In order to perform high-precision tests of these accurate QED calculations, the experimental limitations due to the precision of the ion masses and the achievable magnetic field stability (see our news of 18.01.16) have to be overcome. By co-trapping two HCIs in a Penning trap and measuring the difference of their g factors directly, the influence of the magnetic field fluctuations could be considerably reduced. The drastically improved measurement accuracy would allow to test the corresponding QED calculations with very high precision.

In a recent article published in "Nature", our division members Dr. T. Sailer et al. report on the application of a newly developed technique to measure the difference of g factors directly. The high-precision measurements were performed in the ALPHATRAP Penning-trap experiment at MPIK in Heidelberg, which can manipulate and spatially confine ions at low energies by a superposition of static electric and magnetic fields. The setup consists of a cryogenic double-trap system in a superconducting 4-T magnet, with a precision trap (PT) and an analysis trap (AT). The PT with a highly homogeneous magnetic field is used for high-precision spectroscopy, whereas in the AT with a strong magnetic bottle, the Larmor frequency νL is determined by using the continuous Stern-Gerlach effect to detect millimetre-wave (MW) induced spin flips around νL.

The determination of the g-factor of the stored ions is based on the measurement of the frequency ratio νL/νc of the Larmor frequency (νL) and the cyclotron frequency (νc). νc can be precisely determined with the PT from the three independent harmonic oscillations of the ion motion νz (axial frequency), ν+ (modified cyclotron frequency) and ν- (magnetron frequency) via the invariance theorem: νc2=ν+2+νz2+ν-2.
In order to directly measure the g-factor difference of two HCI for stringent tests of accurate bound-state QED calculations, the neon isotopes 20Ne9+ and 22Ne9+ were stored simultaneously in the Penning trap system. For this purpose, the scientists developed a novel measurement technique based on the principle of the "Two-Ion-Balance", developed at MIT (see Phys. Rev. A 45, 3049 (1992) external Link and Science 303, 334–338 (2004) external Link).

Here, the 20Ne9+ and 22Ne9+ ions were coupled on a common magnetron orbit separated by only about 400 μm in the PT. This allowed for a coherent measurement of their Larmor frequency difference by performing a Ramsey-type measurement (i.e. irradiation of two short MW π/2-pulses, with a variable time between them). From the extracted difference of the Larmor frequencies ΔνL, the g-factor difference Δg of 20Ne9+ and 22Ne9+ was derived. The achieved precision of 5.6 × 10−13 (0.56 p.p.t.) relative to the absolute g factors is more than two orders of magnitude better than all previous such comparisons.

This improved accuracy allows resolving and testing the QED contribution to the nuclear recoil for the very first time. A. V. Volotka and members of the theory division of Prof. C. H. Keitel performed accurate QED calculations for the comparison of the experimental result with theoretical prediction. The experimental value is in perfect agreement with the calculated value of Δg and 10 times more accurate. Alternatively, the theory can therefore be taken as input and to improve the uncertainty of the charge radius difference of the two isotopes by about one order of magnitude.
Furthermore, the excellent agreement between theory and experiment allows setting constraints on the search for new physics beyond the Standard Model of particle physics.

Please read more in the "Nature" article external Link and the associated article in News & Views external Link

Further information also in the press release of the MPIK external Link (idw external Link).

Further press releases:

08.06.22 | Precision measurements
First direct measurement of the helium-3 nuclear magnetic moment

The Helium isotope 3He plays an important role for modern physics, particularly fundamental physics (see e.g. our news of 03.09.20), e.g. sensitive tests of the bound state QED theory and muon g-2 experiments as well as for chemistry, medicine and other scientific fields.
In magnetometry, 3He nuclear magnetic resonance (NMR) probes allow for measurements of the absolute magnetic field with higher accuracy and serve as a new standard for ultra-sensitive absolute magnetometry.
However, this requires a high-accuracy and independent determination of the 3He nuclear magnetic moment. Even so, up to now, the 3He nuclear magnetic moment has been measured only indirectly by comparison to water based NMR probes with a relative precision of 12 parts per billion (p.p.b.), only.
The so-called g-factor is a proportionality constant relating the magnetic moment of a charged particle to its spin. Thus, a direct high-precision determination of the nuclear g-factor of 3He will provide an independent calibration for 3He NMR probes for accurate magnetometry.

In a recent article published in "Nature", researchers of our "Stored and Cooled Ions" division, University Mainz and RIKEN report the first direct measurements of the bound electron g-factor and nuclear g-factor of 3He+ with a relative precision of 10–10. The accuracy of the 3He+ zero-field ground-state hyperfine splitting value (magnetic interaction of electron and nucleus) could be improved by two orders of magnitude.
The precision measurements were performed in a novel Penning-trap system. The system is placed in the homogeneous field of a 5.7 T superconducting magnet and consists of a precision trap (PT) and an analysis trap (AT) with a spatially separated strong magnetic inhomogeneity.

In a Penning trap, the motion of a single stored charged particle - in this case the 3He+ ion - consists of three independent harmonic oscillations: the axial frequency νz, the modified cyclotron frequency ν+ and the magnetron frequency ν-. These eigenfrequencies can be detected with high precision in the PT. The free cyclotron frequency νc can be determined from the three eigenfrequencies using the so-called invariance theorem νc2=ν+2+νz2+ν-2.
The electron spin and the nuclear spin of 3He+ are aligned parallel or antiparallel to an external magnetic field. This leads to a hyperfine splitting in 3He+ with four different hyperfine states. In the PT, two electronic and two nuclear spin transition frequencies between these hyperfine states were determined by measuring the cyclotron frequency while a microwave excitation was driving one of the four hyperfine transitions.
The electronic transitions and the nuclear transitions correspond to an electronic or nuclear spin-flip. The AT with the strong magnetic inhomogeneity is used for spin-state detection by applying the continuous Stern-Gerlach effect. A spin-flip changes the magnetic moment of the stored 3He+. Due to the coupling of the magnetic moment to the axial frequency of 3He+ this change results in a shift of the axial frequency. Thus, a spin-flip can be detected by measuring the axial frequency in the AT before and after resonantly driving the hyperfine transition.
From the resonance curves for each of the four hyperfine transitions, the nuclear g-factor of 3He+, the bound electron g-factor and the zero-field hyperfine splitting were extracted. The reached relative precision of 10–10 improved previous results by a factor of 10.

In order to compare the experimental results with modern theoretical predictions, members of the MPIK theory group around Zoltán Harman in the division of Christoph H. Keitel performed highly accurate QED calcuations. The theoretical value of the 3He+ bound electron g-factor is consistent with the experimental result. The difference between the experimental 3He+ zero-field ground-state hyperfine splitting value and the calculated value is 6 parts per million (p.p.m.).
Further, from the measured shielded nuclear g-factor of 3He+ and an accurate QED calculation of the diamagnetic shielding constant, the g-factor of the "bare" (i.e. unshielded) 3He nucleus was derived.

This first direct calibration for 3He NMR probes improves previous indirect results by one order of magnitude (1 p.p.b. instead of 12 p.p.b.). It is planned to further improve the Penning trap measurements by reducing the magnetic inhomogeneity of the PT. Additionally, applying fast phase-sensitive detection methods - e.g. the "Pulse and Amplify" method - will allow for more precise magnetic field measurements. The new measurement method used for helium-3 can also be applied to determine the nuclear magnetic moment of heavier hydrogen-like ions. A direct Penning trap measurement of the magnetic moment of the bare 3He nucleus is the next step. By implementing sympathetic laser cooling, a relative precision on the order of 1 p.p.b. or better will be achieved.

Please read more in the "Nature" article ... >

Further information also in the press releases of the MPIK external Link (idw external Link) and the MPG external Link.

Further press releases:

14.01.22 | Precision measurements
Precision laser spectroscopy tests modern nuclear theories in the nickel region

Modern nuclear theories beyond the simple shell-model aim to consistently describe atomic nuclei across the entire nuclear chart. In recent years, there has been significant theoretical progress in nuclear many-body methods as well as in the development of ab initio methods based on chiral effective field theory (EFT) interactions. Today, nuclear charge radii can be measured with high accuracy and thus serve as robust benchmarks for ab initio calculations and well-calibrated energy density functionals, such as the Fayans functional (see e.g. our news of 08.02.16, 04.09.18 and 16.05.19). For the development of a coherent theoretical nuclear framework, ab initio models have to be connected to nuclear density functional theory (DFT).

In a recent article published in "Physical Review Letters", S. Malbrunot-Ettenauer et al. report the determined nuclear charge radii of short-lived nickel isotopes 58-68,70Ni (Z=28). In terms of nuclear charge radii, these nickel isotopes constitute the last unexplored "magic" isotopic chain in this mass region. The precision experiments were conducted with the collinear laser spectroscopy apparatus COLLAPS external Link at the radioactive ion beam facility ISOLDE external Link/CERN external Link, Geneva. Details on the COLLAPS setup can be found in R. Neugart et al. (2017) external Link (see our news of 24.04.17).
In the presented laser spectroscopy experiment the charge radii of 59,63,65-67,70Ni have been determined. The charge radius of 68Ni has already been reported in the article of S. Kaufmann et al. (2020) external Link which also gives a detailed description of the experiment at ISOLDE/CERN.
After neutralization of the Ni ions in a charge exchange cell, collinear laser spectroscopy on the neutral Ni atoms was performed. The 3d9 4s 3D3 -> 3d9 4p 3P2 transition at 352.45 nm was excited by a frequency-doubled single-mode cw titanium-sapphire laser. All nickel isotopes were measured alternating with the reference isotope 60Ni to compensate for remaining long-term drifts in ion velocity or laser frequency. The frequency-time spectrum of the Ni resonances was recorded with the new data acquisition system "TILDA".
The isotope shifts δν60,A = νA - ν60 were calculated from the center frequency νA of a Ni isotope with respect to the center frequency ν60 of the reference isotope 60Ni.
From the isotope shifts, the changes in mean-square nuclear charge radii δ<rc2>60,A of the measured nickel isotopes with respect to the reference isotope 60Ni were extracted. The absolute charge radii Rc were determined from δ<rc2>60,A utilising Rc (60Ni).

The experimental results for Rc and δ<rc2> were compared with two DFT approaches (the Skyrme functional SV-min and the Fayans functional Fy(Δr, HFB)) as well as three independent ab initio methods based on chiral EFT interactions.
Charge radii Rc allow a comparison between theory and experiment on the absolute scale. Since various theoretical uncertainties cancel in the differential charge radii δ<rc2>, δ<rc2> probe local variations in the nuclear charge distribution more closely.

When the same chiral EFT based nuclear potential NNLOsat was utilized in all ab initio calculations, their results for Rc and δ<rc2> showed excellent consistency and they agreed well with the experiment. Ab initio calculations exploiting other nuclear interactions deviated notably from experiment in the absolute charge radii Rc and only performed well for δ<rc2>.

Unusually-large odd-even staggering or kinks in Rc at shell closures have been successfully described by Fayans-based functionals (see e.g. our news of 08.02.16, 04.09.18 and 16.05.19). However, in the absence of such features, as is the case with the investigated "magic" nickel isotopic chain, interestingly, Skyrme-based DFT yields results closer to experiment than the Fayans functional.

The reported comparative work combining experiment, density functional theory and ab initio calculations established a theoretical accuracy of about 1% for the description of nuclear charge radii in the Ni region.

Please read more in the article ... >

05.01.22 | Precision measurements
BASE improves the experimental constraints on the violation of CPT invariance and the Weak Equivalence Principle

The Standard Model (SM) is the most successful theory of particle physics. It describes three of the four known fundamental forces (the electromagnetic, weak, and strong interaction) and classifies all known elementary particles. A great success of the SM was e.g. the prediction of the Higgs Boson in 1964, which was finally discovered in 2012 at CERN, Geneva.
However, there is also strong observational evidence that the Standard Model of particle physics is incomplete. As a local, unitary and Lorentz-invariant quantum field theory, the SM is Charge, Parity, and Time Reversal (CPT) invariant. Thus, according to the SM the fundamental properties (e.g. mass, magnetic moment, lifetime) of a particle and its antiparticle are exactly equal. Therefore, particles and antiparticles should have been created in equal amounts in the early universe. This means that the SM is unable to explain the striking imbalance of matter and antimatter in the observable universe. This problem inspires tests of the CPT symmetry by comparing the fundamental properties of matter/antimatter conjugates with high precision. Together with new theoretical models that induce CPT-violation, experiments with further improved measurement precision might lead to Physics Beyond the SM.

Another hot topic in physics is whether antimatter obeys the weak equivalence principle (WEP), which is a cornerstone of General Relativity and other gravity theories. The WEP states that the gravitational mass and inertial mass are equivalent. It has been tested for matter to high accuracy by various experiments. An experimental test of the WEP with antimatter has not been available up to now. In particular, cyclotron-clock-studies with matter/antimatter conjugates allow to test the "cyclotron clock weak equivalence principle" (WEPcc).

In a recent article published in "Nature", the BASE collaboration external Link addresses both mentioned fundamental questions. The researchers performed high-precision tests of the CPT symmetry and the WEPcc. For this purpose, they used the cryogenic BASE multi-Penning trap system consisting of a measurement trap and a reservoir trap at the Antiproton Decelerator external Link (AD) facility of CERN, Geneva.

To test the CPT invariance with baryons, the BASE-team compared the proton/antiproton charge-to-mass ratios by measuring the free cyclotron frequencies νc=(q·B)/(2π·m) of single trapped antiprotons and negatively charged hydrogen ions H- in their advanced Penning-trap system. H- was used as an excellent negatively charged proxy for the proton, since comparing particles of the same charge sign avoids inversion of the trapping voltages, and greatly reduces systematic frequency-ratio shifts.
The Brown-Gabrielse invariance theorem νc2=ν+2+νz2+ν-2 relates the modified cyclotron frequency ν+, the axial frequency νz, and the magnetron frequency ν- to νc. The comparison of νc of antiprotons and H- ions in the same magnetic field B0 gives access to the ratio of their charge-to-mass ratios.
To improve the accuracy of the previous best proton/antiproton charge-to-mass ratio comparison of BASE (see our news of 12.08.15, [Nature 524, 196-199 (2015) external Link]), numerous experimental upgrades have been implemented: a rigorous re-design of the cryogenic experiment stage (CERN Document 2702758 external Link), the development of an advanced multi-layer magnetic shielding system (Phys. Rev. Applied 12, 044012 (2019) external Link) and of a frequency adjustable image-current detector (see our news of 18.07.17, [Phys. Rev. Lett. 119, 033001 (2017) external Link]).
The axial frequency νz was determined from a dip spectrum. The modified cyclotron frequency ν+ (and similarly the magnetron frequency ν-) was measured with the well-established sideband-technique observing a "double-dip" spectrum. Additionally, a "peak-technique" with enhanced accuracy was used.
The new value 1.000000000003(16) of the ratio of the proton/antiproton charge-to-mass ratios with a fractional uncertainty of 16 parts-per-trillion (ppt, 10-12) is consistent with CPT invariance. The final result is based on the combination of four independent long term studies, recorded in a total time span of 1.5 years (December 2017 - May 2019). It improves the precision of the previous best measurement of the BASE collaboration by a factor of 4.3 (see our news of 12.08.15) and constrains the CPT violating effects to an energy scale of about 2·10−27 GeV.

WEPcc violating gravitational anomalies for antimatter would result in a frequency difference between a proton cyclotron-clock at νc,p and its CPT conjugate antiproton clock at νc,anti-p. The four measurement campaigns between December 2017 and May 2019 allowed to study the cyclotron frequencies νc of the proton and the antiproton at different gravitational potentials in the BASE laboratory due to the elliptical orbit of the earth around the sun. This enabled the first differential test of the WEPcc using antiprotons, which allowed to circumvent the problem of a potential modification of the absolute value of the gravitational potential by a WEP-violating force. The researchers derived a differential constraint |αg,D−1|<0.030 of the WEPcc violation.

The BASE collaboration plans to reach even higher sensitivity in future experiments by improving magnetic field stability and homogeneity, and by the development of transportable antiproton traps, named BASE-STEP external Link (STEP: Symmetry Tests in Experiments with Portable Antiprotons) to move the antiprotons from noisy accelerator environments to calm precision laboratory space.

Please read more in the "Nature" article ... >

Further information also in the press releases of the MPIK external Link (idw external Link), the Physikalisch-Technische Bundesanstalt (PTB) external Link, the Leibniz University Hannover external Link, the Johannes Gutenberg University Mainz (STEP) external Link, and CERN external Link.

Further press releases (selection):