Sympathetic cooling of positrons to cryogenic temperatures for antihydrogen production

Temperature relaxation rates in strongly magnetized plasmas

Strongly magnetized plasmas, characterized by having a gyrofrequency larger than the plasma frequency (β=ω_{c}/ω_{p}≫1), are known to exhibit novel transport properties. Previous works studying pure electron plasmas have shown that strong magnetization significantly inhibits energy exchange between parallel and perpendicular directions, leading to a prolonged time for relaxation of a temperature anisotropy. Recent work studying repulsive electron-ion interactions (e^{-}-i^{-} or e^{+}-i^{+}) showed that strong magnetization increases both the parallel and perpendicular temperature relaxation rates of ions, but in differing magnitudes, resulting in the formation of temperature anisotropy during equilibration. This previous study treated electrons as a heat bath and assumed weak magnetization of ions. Here, we broaden this analysis and compute the full temperature and temperature anisotropy evolution over a broad range of magnetic field strengths. It is found that when electrons are strongly magnetized (β_{e}≫1) and ions are weakly magnetized (β_{i}≪1), the magnetic field strongly suppresses the perpendicular energy exchange rate of electrons, whereas the parallel exchange rate slightly increases in magnitude compared to the value at weak magnetization. In contrast, the ion perpendicular and parallel energy exchange rates both increase in magnitude compared to the values at weak magnetization. Consequently, equilibration causes the electron parallel temperature to rapidly align with the ion temperature, while the electron perpendicular temperature changes much more slowly. It is also shown that when both ions and electrons are strongly magnetized (β_{i},β_{e}≫1), the ion-electron perpendicular relaxation rate dramatically decreases with magnetization strength.

Trap-integrated fluorescence detection with silicon photomultipliers for sympathetic laser cooling in a cryogenic Penning trap

We present a fluorescence-detection system for laser-cooled 9Be+ ions based on silicon photomultipliers (SiPMs) operated at 4 K and integrated into our cryogenic 1.9 T multi-Penning-trap system. Our approach enables fluorescence detection in a hermetically sealed cryogenic Penning-trap chamber with limited optical access, where state-of-the-art detection using a telescope and photomultipliers at room temperature would be extremely difficult. We characterize the properties of the SiPM in a cryocooler at 4 K, where we measure a dark count rate below 1 s-1 and a detection efficiency of 2.5(3)%. We further discuss the design of our cryogenic fluorescence-detection trap and analyze the performance of our detection system by fluorescence spectroscopy of 9Be+ ion clouds during several runs of our sympathetic laser-cooling experiment.

Observation of the effect of gravity on the motion of antimatter

Einstein's general theory of relativity from 19151 remains the most successful description of gravitation. From the 1919 solar eclipse2 to the observation of gravitational waves3, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac's theory4 appeared in 1928; the positron was observed5 in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted6 by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter7-10. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive 'antigravity' is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP.

Many-Body Theory Calculations of Positron Scattering and Annihilation in H_{2}, N_{2}, and CH_{4}

The recently developed ab initio many-body theory of positron molecule binding [22J. Hofierka et al., Many-body theory of positron binding to polyatomic molecules, Nature (London) 606, 688 (2022)NATUAS0028-083610.1038/s41586-022-04703-3] is combined with the shifted pseudostates method [A. R. Swann and G. F. Gribakin, Model-potential calculations of positron binding, scattering, and annihilation for atoms and small molecules using a Gaussian basis, Phys. Rev. A 101, 022702 (2020)PLRAAN2469-992610.1103/PhysRevA.101.022702] to calculate positron scattering and annihilation rates on small molecules, namely H_{2}, N_{2}, and CH_{4}. The important effects of positron-molecule correlations are delineated. The method provides uniformly good results for annihilation rates on all the targets, from the simplest (H_{2}, for which only a sole previous calculation agrees with experiment), to larger targets, where high-quality calculations have not been available.

Maxwellianization of Positrons Cooling in CF_{4} and N_{2} Gases

Positron cooling in CF_{4} and N_{2} gases via inelastic vibrational and rotational (de)excitations is simulated, importantly including elastic positron-positron collisions. For CF_{4}, it is shown that rotational (de)excitations play no role on the experimental timescale, and further, that in the absence of positron-positron collisions, cooling via excitation of the dipole-active ν_{3} and ν_{4} modes alone would lead to a non-Maxwellian positron momentum distribution, in contrast to the observations of experiment. It is shown that the observed Maxwellianization of the distribution may be effected by positron-positron collisions and/or cooling involving the combination of the dipole-inactive ν_{1} mode with the dipole-active modes. For N_{2}, rotational excitations alone are sufficient to Maxwellianize the distribution (vibrational effects are negligible).