NEWS

10/19/2017 A parts per billion measurement of the antiproton magnetic moment

Today we report on an improved measurement of the magnetic moment of the antiproton. The new measurement outperforms our old record measurement by a factor of 350 in experimental precision. Our updated value μρ̄ = -2.792 847 344 1 (43) μΝ is consistent with the magnetic moment of the proton μρ= 2.792 847 350 (9) μΝ’ and thus supports the combined charge, parity, and time-reversal (CPT) invariance, an important symmetry of the Standard Model of particle physics. Remarkably, this is the first time physicists have carried out a more precise measurement on antiprotons than on protons. Together with the exciting new antihydrogen results, this milestone achievement is a demonstration of the immense progress made at CERN’s antiproton decelerator facility. This extraordinary improvement in experimental accuracy was made possible by the invention of a novel two-particle multi-Penning-trap measurement method, developed at Ulmer FSL, which combines the non-destructive detection of the antiproton’s spin quantum state with particle-based high-resolution magnetic field measurements.

The determination of the magnetic moment of a single trapped particle is based on the measurement of two characteristic frequencies. The first is the cyclotron frequency νc, which describes the particle’s revolutions per second in the magnetic field of the Penning trap, and the second, the precession frequency νLof the particle’s spin. Together, these allow us to access the particle’s magnetic moment through the ratio

whereμν is the nuclear magneton. Previous antiproton measurements, such as those performed by the ATRAP collaboration in 2013 and later by BASE, used a single Penning trap with a superimposed magnetic bottle. This strong inhomogeneity in the magnetic field allows for non-destructive detection of the particle’s spin-quantum-state, a precursor to any determination of the Larmor frequencyνL. However, such a bottle broadens the particle’s resonance lines and limits the precision of the measurement, typically to the parts per million level.

To overcome this limitation, experimentalists apply a two trap method which separates the high-precision frequency measurements to a homogeneous precision trap and the spin state analysis to a trap with the superimposed magnetic inhomogeneity. While an elegant technique, this double trap method is very challenging to implement. It took seven years of research and development work until we were able to demonstrate this double-trap method with a single trapped proton, and later applied it in a measurement of the proton magnetic moment to nine significant figures.

In the measurement reported today, we have extended the double-trap technique to a three trap / two particle scheme, in which we use a “hot” particle with an effective temperature of 300K for magnetic field measurements and a cold particle at 0.12K for spin transition spectroscopy. By alternatingly shuttling the two particles to the precision trap we were able to quasi-simultaneously sample μL and νc by a fast adiabatic particle exchange in the same ultra-homogeneous magnetic field. However, unlike the double trap method, the two particle technique avoids time consuming resistive cooling cycles to sub-thermal temperatures, and thus, enables measurements at drastically improved frequency sampling rate, which was the major breakthrough to accomplish the goal of measuring the antiproton magnetic moment with parts per billion precision.

By combining the new 350-fold improved antiproton result with our previously measured proton result we obtain

This constitutes one of the most precise tests of CPT invariance in the baryon sector, and enables us to set drastically improved constraints on CPT-violating extensions of the Standard Model.

For details see

  • Link to publication
  • 07/18/2017 Improved Measurement of the Proton Mass in Natural Units

    Today a paper was published in Phys. Rev. Lett.in which a collaboration of researchers from Max Planck Institute for Nuclear Physics, Heidelberg, Germany and Members of Ulmer Fundamental Symmetries Laboratory report on a three-fold improved measurement of the proton mass in natural units. The resulting value 1.007276466583(15)(29) u is three times more precise than the presently recommended value, however it is significantly smaller than the current CODATA standard value. Measurements by other authors yielded discrepancies with respect to the mass of the tritium atom, the heaviest hydrogen isotope (T = 3H), and the mass of light helium (3He) compared to the “semiheavy” hydrogen molecule HD (D = 2H, deuterium, heavy hydrogen). Our result contributes to solving this puzzle, since it corrects the proton’s mass in the proper direction. The experiment has been developed by a group of young physicists around Sven Sturm, and Klaus Blaum from MPI-K, Andreas Mooser and Stefan Ulmer from RIKEN FSL developed parts for the ultra-sensitive single particle detectors which are used in this experiment.

    For details see

  • Link to publication
  • 03/21/2017 Observation of single spin transitions of a single trapped antiproton

    Today we have published a paper in Phys. Lett. B in which we report on the detection of individual spin quantum transitions of a single trapped antiproton in a Penning trap. The spin state determination is based on the unambiguous detection of axial frequency shifts which are induced by the spin transition in presence of a magnetic bottle. We have achieved a detection fidelity of 92.6 % and demonstrated spin state initialization with 99.9% fideltiy.

    For details see

  • Link to publication
  • 01/18/2017 Most precise measurement of the antiproton magnetic moment

    Today our article on an improved measurement of the magnetic moment of the antiproton, with a fractional precision of 0.8 parts in a million, was published in Nature Communications (link is external). This is, so far, the culmination point of 10 years of dedicated work on proton and antiproton magnetic moment measurements. Previously, the magnetic moment of the antiproton has been extracted from exotic atom spectroscopy, with a fractional resolution on the 0.001-level. In 2013 the ATRAP collaboration published a 680-fold improved measurement using a single antiproton in a Penning trap. BASE was approved by CERN's research board in 2013 to contribute to this search. The result which is reported here follows the high-precision comparison of the antiproton-to-proton charge-to-mass ratio, and is the second fundamental physics measurement produced by BASE. By using the continuous Stern Gerlach effect in Penning traps, we performed non-destructive single-antiproton spin-transition spectroscopy to measure the particle's Larmor frequency. Combining this with measurements of the cyclotron frequency of the single trapped antiproton, the magnetic moment of the particle was obtained in units of the nuclear magneton, this ratio is also called "g-factor". Within the experimental uncertainties, our result (g/2)pbar=2.7928465(23) is consistent with our recent proton g-factor measurement (g/2)p=2.792847350(9), which supports the CPT invariance of the Standard Model. The result also improves constraints on coefficients of the prominent Standard Model extension by up to a factor of 22. The much more precise measurement of the proton magnetic moment was carried-out by using the double Penning trap technique. A logical next step is the application of this much more challenging technique as well to measure the antiproton magnetic moment. Together with very recent measurements carried out by ASACUSA and ALPHA, we are making rapid progress in our understanding of antimatter.

    For details see

  • Link to publication
  • 11/11/2016 Happy Birthday Antiprotons !!!

    A crucuial device in the BASE multi-Penning trap system is a reservoir trap for antiprotons. This trap is loaded with a cloud of antiprotons provided by CERN's antiproton decelerator and methods were developed to extract single particles from this reservoir and to supply them to our high precision measurement traps. This allows BASE to continuously perform experiments with antiprotons, independent from accelerator maintenance and shutdown cycles. We have loaded the last bunch of antiprotons on the 12th of November 2015, still performing experiments with these particles

    For details see
  • Link to CERN article
  • 09/20/2015 High-precision comparison of the antiproton-to-proton charge-to-mass ratio

    In a paper published in Nature we report the high-precision comparison of the antiproton-to-proton charge to mass ratio. In our measurements we compared the cyclotron frequencies of antiprotons to that of negatively charged hydrogen ions, which are used as a proxy for the proton. We achieved a fractional precision of 69 parts in a trillion, which corresponds in our magnetic field of 1.95 Tesla to an absolute energy resolution of 2mHz. Our result is consistent with CPT invariance. We profited from ideas developed by the TRAP collaboration, which applied a comparable scheme already in the late 1990ies. In BASE, however, we established techniques which allow us to perform a single ratio comparison within only 4 minutes, which is much faster than in previous experiments. This enabled us to perform about 6500 ratio comparisons. The high sampling rate allowed us to study diurnal ratio-variations with high temporal resolution. The Standard Model of particle physics – the theory that best describes particles and their fundamental interactions – is known to be incomplete, inspiring various searches for “new physics” that goes beyond the model. These include tests that compare the basic characteristics of matter particles with those of their antimatter counterparts. While matter and antimatter particles can differ, for example, in the way they decay (a difference often referred to as violation of CP symmetry), other fundamental properties, such as the absolute value of their electric charges and masses, are predicted to be exactly equal. Any difference – however small — between the charge-to-mass ratio of protons and antiprotons would break a fundamental law known as CPT symmetry. This symmetry reflects well-established properties of space and time and of quantum mechanics, so such a difference would constitute a dramatic challenge not only to the Standard Model, but also to the basic theoretical framework of particle physics.

    For details see
  • Link to publication
  • 08/18/2014 Direct high-precision measurement of the proton magnetic moment

    In a paper just published in Nature we report on the first direct high precision measurement of the proton magnetic moment. By application of the elegant double Penning trap technique we achieved a fractional precision of 3.3 parts per billion. Our value is consistent with the currently accepted CODATA value, but 2.5 times more precise.

    The currently most precise value of the proton magnetic moment is based on spectroscopy of the ground state hyperfine splitting of atomic hydrogen in a magnetic field using a MASER. From these experiments, carried out about 42 years ago, the proton magnetic moment is extracted using input of two independent experiments and bound state corrections at the level of 17.7 ppm.  In contrast, we measured the magnetic moment of a single trapped proton directly, which is about 760 times more precise than any direct measurement performed so far.

    The double trap measuring-scheme can be directly applied to measure the magnetic moment of a single trapped antiproton, which is planned at the BASE-CERN experiment, currently being setup for the AD antiproton run 2014. The magnetic moment of the antiproton has recently been measured by the ATRAP collaboration with a fractional precision of 4.4ppm, which is 680 times more precise than values extracted from antiprotonic helium spectroscopy performed by the ASACUSA collaboration. In BASE we plan to apply the double trap scheme to improve the 4.4ppm value by more than a factor of 1000. This will constitute a stringent test of CPT invariance with baryons.

    Andreas Mooser, first author of the paper, just joined the BASE-team at CERN. His post-doctoral research will be funded by one of the highly competitive RIKEN foreign post-doctoral researchers fellowships.

    For details see
  • Link to publication