There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

There is almost universal agreement among astronomers that most of the mass in the Universe and most of the mass in the Galactic halo is dark. Many lines of reasoning suggest that the dark matter consists of some new, as yet undiscovered, weakly-interacting massive particle (WIMP). There is now a vast experimental effort being surmounted to detect WIMPS in the halo. The most promising techniques involve direct detection in low-background laboratory detectors and indirect detection through observation of energetic neutrinos from annihilation of WIMPs that have accumulated in the Sun and/or the Earth. Of the many WIMP candidates, perhaps the best motivated and certainly the most theoretically developed is the neutralino, the lightest superpartner in many supersymmetric theories. We review the minimal supersymmetric extension of the Standard Model and discuss prospects for detection of neutralino dark matter. We review in detail how to calculate the cosmological abundance of the neutralino and the event rates for both direct- and indirect-detection schemes, and we discuss astrophysical and laboratory constraints on supersymmetric models. We isolate and clarify the uncertainties from particle physics, nuclear physics, and astrophysics that enter at each step in the calculation. We briefly review other related dark-matter candidates and detection techniques.

Supersymmetric Dark Matter

We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

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Phase separation in confined systems

The techniques of optical trapping and manipulation of neutral particles by lasers provide unique means to control the dynamics of small particles. These new experimental methods have played a revolutionary role in areas of the physical and biological sciences. This paper reviews the early developments in the field leading to the demonstration of cooling and trapping of neutral atoms in atomic physics and to the first use of optical tweezers traps in biology. Some further major achievements of these rapidly developing methods also are considered.

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Optical trapping and manipulation of neutral particles using lasers

Microcalorimeters operating at low temperatures have proven to be suitable tools for high resolution x-ray spectroscopy. Metallic magnetic calorimeters (MMCs) are one kind of such detectors. The operation of a MMC is based on the temperature dependence of the magnetization of a paramagnetic material such as Au doped with Er in a small magnetic field. The temperature of a x-ray absorber, in thermal contact with the paramagnetic material, can be monitored with high sensitivity by using a dc superconducting quantum interference device. An energy resolution of one part in 1000 at 6 keV has now been demonstrated for MMCs. We report on the recent progress in the development of MMCs and discuss the limitations of such devices resulting from fundamental noise sources.

Magnetic calorimeters for high resolution x-ray spectroscopy

The dependence on applied electric field (0--40 kV/cm) of the scintillation light produced by fast electrons and $\ensuremath{\alpha}$ particles stopped in liquid helium in the temperature range of 0.44 K to 3.12 K is reported. For both types of particles, the reduction in the intensity of the scintillation signal due to the applied field exhibits an apparent temperature dependence. Using an approximate solution of the Debye-Smoluchowski equation, we show that the apparent temperature dependence for electrons can be explained by the time required for geminate pairs to recombine relative to the detector signal integration time. This finding indicates that the spatial distribution of secondary electrons with respect to their geminate partners possesses a heavy, non-Gaussian tail at larger separations and has a dependence on the energy of the primary ionization electron. We discuss the potential application of this result to pulse shape analysis for particle detection and discrimination.

Effect of an electric field on liquid helium scintillation produced by fast electrons

We have constructed a new apparatus designed to study the exotic negative ions in superfluid helium-4 previously observed. Our apparatus is similar to that used by Ihas and Sanders, and by Eden and McClintock. The ions are generated from an electrical discharge in the vapor above the surface of the liquid and the mobility is measured by the time-of-flight method. We have detected eleven exotic ions with distinct mobility, and have measured their mobility as a function of temperature. In addition to the peaks in the time-of-flight signal due to the different exotic ions, there is a smoothly varying and continuous background signal. The variation of the background with field and temperature appears to be inconsistent with any explanation invoking the decay of one exotic ion into another, and supports the idea it arises from negative ions with a continuous distribution of mobility. This is a striking result because it would indicate that these ions have a continuous size distribution.

Experimental Investigation of Exotic Negative Ions in Superfluid Helium

We have constructed an apparatus that is able to maintain a helium‐free surface at low temperature (T≤0.1 K) in a cell containing superfluid helium. We discuss the considerations involved in the design of this device, and describe tests that we have made to confirm that a film‐free surface has been produced.

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Removal of superfluid helium films from surfaces below 0.1 K

Investigations of quantum evaporation from the surface of superfluid4He at 20mK have been carried out using both an electrical heater and a 5.5MeV α source as generators of quasiparticles. The angular dependence of the evaporation signals for these two sources is found to be similar. The angular dependence and the total efficiency of the process are discussed in terms of models of quantum evaporation.

George M. Seidel

Papers

Magnetic calorimeters for high resolution x-ray spectroscopy

Effect of an electric field on liquid helium scintillation produced by fast electrons

Experimental Investigation of Exotic Negative Ions in Superfluid Helium

Removal of superfluid helium films from surfaces below 0.1 K

Quantum evaporation of4He: Angular dependence and efficiency