Clock-based constraints on antiproton anomalous gravitation and a 4-fold improved antiproton charge-to-mass ratio measurement (05.01.2022)


In a paper published today in Nature, the BASE collaboration reports on the most precise comparison yet between a fundamental property of protons and antiprotons. Analysing a total of about 24000 proton and antiproton cyclotron frequency measurements, taken over the course of 1.5 years, we found that the charge-to-mass ratios of protons and antiprotons are identical to within a record experimental uncertainty of 16 parts per trillion. This confirms that a fundamental symmetry in the Standard Model - the so-called CPT invariance - is valid with 4-fold improved resolution.

In addition, we placed stringent limits on the weak equivalence principle for matter and antimatter clocks. According to this principle, different bodies in the same gravitational field experience the same acceleration. Because our experiment is placed on the surface of the Earth, the proton and antiproton cyclotron-frequency measurements were made in the gravitational field on Earth’s surface. Sampling the varying gravitational field in our laboratory as the planet orbits around the Sun, if matter and antimatter would react differently to gravity, we should observe time dependent signatures in the measured proton/antiproton cyclotron frequency ratios. We did not find such oscillating signatures and our measurements show that the difference in the gravitational force must be smaller than 3 %. This study constitutes the first differential test of the weak equivalence principle for baryonic matter/antimatter clocks, the result indicates, within the achieved experimental resolution, that matter and antimatter behave similar under gravity.

Link to the publication: A 16 parts per trillion comparison of the proton/antiproton charge-to-mass ratios

Link to the BASE collaboration homepage: BASE collaboration homepage

Link to the Fundamental Symmetries Lab at RIKEN: News post about this article

Image Credits: Stefan Ulmer / RIKEN


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Physics World nominates our work on sypmathetic proton cooling in the Top 10 of Physics Breakthroughs 2021

One of the highlights in the annual Physics calendar is the announcement of the “Physics Breakthroughs of the Year”, by the journal Physics World. This year, our paper on "Sympathetic cooling of protons mediated by a superconducting LC-circuit", published in Nature, was selected as one of the top ten achievements made in 2021.  The work was conducted here in our group at the Institute for Physics at the University of Mainz (see press release). It is a key method to improve future proton and antiproton precision measurements in the BASE collaboration.

According to Physics World, the selections must meet the following criteria:

  • Significant advance in knowledge or understanding
  • Importance of work for scientific progress and/or development of real-world applications
  • Of general interest to Physics World readers

Our paper was nominated in combination with work by our antimatter research colleagues from the ALPHA collaboration at CERN, which demonstrated for the first time laser cooling of antihydrogen atoms. Both efforts together are nominated for “the development of innovative particle cooling techniques”. The muon g-2 experiment at Fermilab and their improved measurement of the muon magnetic moment anomaly is also on the list - a work with contributions from Mainz by the group of Martin Fertl. Further nominations are the event horizon telescope collaboration and their first image showing the polarization of light in the region surrounding a supermassive black hole, and the observation of Pauli blocking in ultracold gases of fermionic atoms.

For the full list of finalists follow this link.




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A new cooling technique (25.08.2021)

Laser cooling power transmitted through an LC circuit into another trap

Today, we report in the journal Nature (read the article here) on the first sympathetic cooling of a trapped proton using laser-cooled beryllium ions in a spatially separated trap. Energy exchange between the proton and the laser cooled beryllium ions is mediated by image currents of the ions in the trap electrodes that are transmitted through an LC circuit that connects to the two traps (see Figure below). In our first demonstration of this method, we cool the proton to a temperature about one order of magnitude lower than the cooling limit of the conventional resistive cooling method.

Fig. 1: Schematic of the implementation of the image-current cooling scheme


Cooling trapped particles is essential for any kind of precision measurement, since the remaining energy of the particle causes measurement fluctuations or systematic uncertainties. Laser-cooling has been used to prepare trapped charged particles in the motional ground state, however this technique can only be applied to a fraction of ions which have suitable closed cycle for laser cooling. Heinzen and Wineland therefore proposed a technique to extend the laser cooling from a suitable ion via image currents to a separate trap (see their publication here). The advantage is that this method can be applied to any kind of charged particle, in particular those that are difficult to cool otherwise – such as positrons, antiprotons, highly-charged ions or molecular ions.

Our motivation to implement this technique is to improve the comparison of the proton and antiproton magnetic moment, to improve our insight into the matter-antimatter balance observed in the universe. For these measurements, we require protons and antiprotons at energies of about 100 mK to resolve single spin transitions – an essential prerequisite to perform these nuclear magnetic moment measurements at all. To prepare such cold particles using resistive cooling is quite time-consuming, and our demonstration of the image-current cooling shows that this limitation can be overcome.

One experimental challenge of implementing these technique in our trap system is that the image currents are tiny. Our demonstration uses therefore a superconducting LC circuit that resonantly amplifies the image currents to make the cooling of the proton by energy transfer to the laser cooled ions and the proton possible. This new cooling method constitutes in particular an advance for antimatter physics since it can be directly applied to antiprotons. It is a stepping stone to improve the precision measurements of single trapped protons and antiprotons.


Fig. 2: Matthew Bohman (left) and Christian Smorra (right) during the installation of the trap system


Text credits: C. Smorra (JGU), Stefan Ulmer (RIKEN)

Figure and photo credits: M. Bohman (MPIK/RIKEN), F. Sämmer (JGU)


Other news outlets

JGU Press Releases: Press release (25.08.2021)

CERN Courier: Article (25.08.2021)

CERN homepage: News

RIKEN Press Release: English/Japanese

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