We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (${10}^{-9}\mathrm{mbar}$) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than ${10}^{-13}\mathrm{mbar}$. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than $1.2\times {10}^9$ positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about $20\times {10}^6$ positrons were compressed to a density of $2.6\times {10}^{10}{\mathrm{cm}}^{-3}$. This is a factor of 6 improvement over earlier measurements.

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.

We have applied a combination of laser excitation and electric-field detachment to negative atomic ions for the first time, resulting in an enhancement of the excited-state detection efficiency for spectroscopy by at least 2 orders of magnitude. Applying the new method, a measurement of the bound-bound electric-dipole transition frequency in ${}^{192}\mathrm{Os}^{-}$ was performed using collinear spectroscopy with a narrow-bandwidth cw laser. The transition frequency was found to be 257.831 190(35) THz [wavelength 1162.747 06(16) nm, wave number $8600.3227(12){\mathrm{cm}}^{-1}$], in agreement with the only prior measurement, but with more than 100-fold higher precision.

Cold antihydrogen atoms were produced by mixing cold samples of antiprotons and positrons. The temperature of the positron plasma was increased by controlled radio-frequency (RF) heating, and the antihydrogen production was measured. Formation is observed to decrease with increased temperature but a simple power law scaling is not observed. Significant production is still present at room temperature