The gravitational acceleration of antimatter, $\overline{g}$ , has yet to be directly measured; an unexpected outcome of its measurement could change our understanding of gravity, the universe, and the possibility of a fifth force. Three avenues are apparent for such a measurement: antihydrogen, positronium, and muonium, the last requiring a precision atom interferometer and novel muonium beam under development. The interferometer and its few-picometer alignment and calibration systems appear feasible. With 100 nm grating pitch, measurements of $\overline{g}$ to 10%, 1%, or better can be envisioned. These could constitute the first gravitational measurements of leptonic matter, of 2nd-generation matter, and possibly, of antimatter.

DOI: https://doi.org/10.1103/PhysRevD.53.570

We report measurements of the ratios $\frac{K^{+}}{{\pi }^{+}}$, $\frac{p}{{\pi }^{+}}$, $\frac{K^{-}}{{\pi }^{-}}$, $\frac{\overline{p}}{{\pi }^{-}}$, $\frac{{\pi }^{-}}{{\pi }^{+}}$, $\frac{K^{-}}{K^{+}}$, and $\frac{\overline{p}}{p}$ for hadrons with $0.19<\mathrm{}{x}_t<0.62\mathrm{}$ produced in $p-\mathrm{Be}$ and $p-W$ collisions at $\sqrt{s}=38.8\mathrm{}$ GeV. The $\frac{K^{+}}{{\pi }^{+}}$ ratio at high $x_t$ gives the fragmentation-function ratio $\frac{D_u^{K^{+}}}{D_u^{{\pi }^{+}}}$ at high $z$. The high-$x_t$ $\frac{K^{-}}{{\pi }^{-}}$ ratio gives an upper limit for $\frac{D_d^{K^{-}}}{D_d^{{\pi }^{-}}}$ at high $z$. The $p_t$ dependence of $\frac{p}{{\pi }^{+}}$ suggests that scattered constituent diquarks are the primary source of protons with $p_t<6\mathrm{}$ GeV/c. We also present species correlations in high-mass $\frac{h^{+}}{h^{-}}$ pairs. Strong $\frac{K^{+}}{K^{-}}$ and $p\overline{p}$ correlations were observed.

Ionization cooling, a method for shrinking the size of a particle beam, is an essential technique for future particle accelerators that use muons. In this technique, muons lose energy in all three directions by passing through an absorber while only the longitudinal energy is regenerated by RF cavities. Thus the beam phase space area decreases down to the limit of multiple scattering in the energy absorber. Hydrogen is the material of choice for ionization cooling because of its long radiation length relative to its energy loss. In the application discussed here, dense gaseous hydrogen also suppresses RF breakdown by virtue of the Paschen effect, thereby allowing higher accelerating gradients and a shorter and less‐expensive cooling channel. As described in this paper, a channel of RF cavities pressurized with about 3 tons of cold hydrogen gas could provide transverse muon cooling for a Muon Collider or Neutrino Factory. The present status of this research effort and several issues related to the use of hydrogen in this application are described.

We describe new results from the semiconductor-laser tracking frequency gauge, an instrument that can perform sub-picometer distance measurements and has applications in gravity research and in space-based astronomical instruments proposed for the study of light from extrasolar planets. Compared with previous results, we have improved incremental distance accuracy by a factor of two, to 0.9 pm in 80 s averaging time, and absolute distance accuracy by a factor of 20, to 0.17 mu m in 1000 s. After an interruption of operation of a tracking frequency gauge used to control a distance, it is now possible, using a nonresonant measurement interferometer, to restore the distance to picometer accuracy by combining absolute and incremental distance measurements.