Precision Penning Trap

Mass of helium-4

A measurement of the g-factor of ⁴He⁺ combined with precise QED theory will result in a value for m_e/M[⁴He]. The atomic mass of ⁴He is hence needed to convert this ratio into the atomic mass of the electron. Currently, there is a large discrepancy between a recent and an earlier precise value for M[⁴He] .

Synthesis of the molecular anti-hydrogen ion

Using single-ion trapping techniques it will be possible to carry out measurements of g-factors, hyperfine structure and ro-vibrational transitions on the diatomic hydrogen ion H₂⁺ and its antimatter counterpart anti-H₂⁺ (two antiprotons, bound by a positron). In particular, measurements of the frequencies of ro-vibrational transitions could be carried out with atomic-clock precision. Such measurements would result in a test of matter-antimatter symmetry, and hence the CPT theorem, with several orders of magnitude higher sensitivity than is achievable using hydrogen and antihydrogen. We are exploring the synthesis of anti-H₂⁺ in an ion trap using simulations and laboratory experiments with ordinary matter.

Development of an apparatus to measure g-factors in light ions

We are currently developing an apparatus for measuring g-factors in light hydrogen-like ions for determining the electron atomic mass, and for measuring g-factors and hyperfine structure in diatomic molecular hydrogen ions to test ab initio QED theory. This project will also serve as a proof-of-principle for a detection method proposed for spectroscopy on the anti-hydrogen molecular ion.

Mass difference between tritium and helium-3

Using our alternating technique we have remeasured the ratios HD⁺/³He⁺ and HD⁺/T⁺, and also, for the first time, measured T⁺/³He⁺. We achieved a factor of 3 improvement in precision compared to our 2015 result, obtaining a Tritium-beta-decay Q-value of 18,592.071(22) eV. This was motivated by the ongoing KATRIN and developing Project-8 neutrino mass experiments.

Deuteron-proton mass ratio from H₂⁺/D⁺ using alternating and simultaneous measurement of cyclotron frequencies

The deuteron to proton mass ratio, m_d/m_p, is an important fundamental constant. Using H₂⁺/D⁺ to obtain m_d/m_p has advantages compared to a direct measurement of D⁺/H⁺ because H₂⁺/D⁺ is a mass doublet, which reduces many systematic errors. However, H₂⁺ can be formed in any one of 20 vibrational levels, each with many rotational levels. All excited levels have lifetimes against radiative decay of a week or longer, and the associated mass-energy must be allowed for. We did this in two ways. In the first, using our alternating cyclotron radius technique , we used Stark-quenching due to the motional electric field experienced by the ions in a 2 mm radius cyclotron orbit to speed up the decay of the H₂⁺ to the vibrational groundstate. In the second, we used the method of simultaneous measurement of cyclotron frequencies in coupled magnetron orbits. This allowed a large number of measurements in a given ro-vibrational level, and enabled us to follow the decay of three H₂⁺ ions to the ground vibrational level; in one case from v=9 to v=0 over a period of several months. From an analysis of the plateaus of cyclotron-frequency-ratios we were able to probabilistically assign some of the plateaus to specific ro-vibrational states, and hence remove uncertainty due to both vibrational and rotational energy. This second measurement resulted in m_d/m_p at 5 x 10^-12, and a new value for the proton mass at 1 x 10^-11. Recently m_d/m_p has gained extra importance because, combined with precise theory and laser spectroscopy of ro-vibrational transitions in HD⁺ , it can be used to obtain a new value for the proton-electron mass ratio m_p/m_e. Alternatively, with m_p/m_e and m_d/m_p from Penning trap measurements, the comparison between theory and measurement for ro-vibrational transitions in HD⁺ can be used to search for an Angstrom-range “fifth-force” between nucleons and other non-standard model physics.

Mass differences of hydrogen and helium isotopes, the "helium-3 puzzle", and rotational energy as mass

As a byproduct of our measurement of the tritium helium-3 mass difference (see below), our measurement of the ³He⁺/HD⁺ mass ratio showed a 4-standard deviation discrepancy with respect to measurements of hydrogen, deuterium and helium-3 by other groups. This is sometimes known as the "helium-3 puzzle", but it may also indicate underestimated errors for the proton and deuteron masses. After upgrading the Penning trap we therefore remeasured the ³He⁺/HD⁺ mass ratio, and also the ratios ³He⁺/H₃⁺ and HD⁺/H₃⁺. Our new results for ³He⁺/HD⁺ from both the direct measurement, and the ratio of the ratios involving H₃⁺, agreed with our previous measurement. We were also able to resolve different highly excited rotational levels of H₃⁺ from the differences in mass-energy and confirm that some of these levels have extreme metastability. This is the first observation of the change in mass of a molecular ion due to different rotational energy.

³T/³He Mass Difference for KATRIN

We have measured the mass ratios T⁺/HD⁺ and ³He⁺/HD⁺ to give a precise value for T⁺/³He⁺ and hence the mass difference between atoms of tritium and helium-3. The result was 18592.01(7) eV, which has an uncertainty more han a factor of 10 smaller than previous measurements. This mass difference, the Q-value, is related to the total kinetic energy release in tritium beta-decay. This Q-value is important for testing models of energy loss processes in the large-scale tritium beta-decay spectrometer KATRIN, which aims to reduce the upper limit on the mass of the electron neutrino by a factor of 10 to 0.2 eV/c².

Masses of the Alkali metals and Isotopes of Yb and Sr

These masses are essential for converting precision atom-recoil measurements of h/m_atom into precision values for the finestructure constant that are insensitive to Quantum Electrodynamic (QED) theory. Such values for the finestructure constant then enable precision measurements of the magnetic moment of the electron to be used to test QED and to search for physics beyond the standard model. Atomic masses of chains of isotopes are also needed to enable searches for new-physics from King-plot analyses of isotope shifts.

Polarizability shifts and dipole moments of HCO⁺ and NH⁺

HCO⁺ is astro-chemically important. Its dipole is needed to interpret the strengths of radio-astronomical signals in terms of concentration in interstellar gas clouds. NH⁺ has an unusual level structure.

Q-Values for Searches for Neutrinoless Double-Beta Decay

Neutrinoless double-beta-decay, if detected, would prove that neutrinos are their own anti-particles. It would also give information on neutrino mass. We have now produced the most precise measurements of the Q-values needed by the leading large-scale experiments that carry out searches for neutrinoless double-beta decay, in particular for the isotope pairs ¹³⁶Xe-¹³⁶Ba, ¹³⁰Te-¹³⁰Xe and ⁷⁶Ge-⁷⁶Se. We have also measured the Q-value for the double-electron capture process ⁷⁴Se-⁷⁴Ge, and for the single beta-decay ¹¹⁵In-¹¹⁵Se(3/2⁺), which is the beta-decay with the lowest known energy.

Atomic masses of ¹⁷O and ¹⁸O

These were required for analysis of spectroscopic data for isotopic variants of the CO molecule.

Mass of ²⁸Si to 2 x 10^-11.

This was required for g-factor (magnetic moment) measurements of hydrogen-like and lithium-like silicon ions.