We describe analytical and numerical methods for calculating forces between conductors due to variations of electrostatic surface potential across their surfaces. In the simple case where the spatial variation of surface potential gives rise to uniform power spectra, we show that the electrostatic force can be large in comparison with, and scale in approximately the same way with distance of closest approach as, the Casimir force. Patch potentials that are consistent with existing experimental data could give rise to forces with a magnitude of 4% of the Casimir force at separations of 0.1 microm.

We present a proposal to search for QCD axions with mass in the 200 µeV range, assuming that they make a dominant component of dark matter. Due to the axion-electron spin coupling, their effect is equivalent to the application of an oscillating rf field with frequency and amplitude fixed by the axion mass and coupling respectively. This equivalent magnetic field would produce spin flips in a magnetic sample placed inside a static magnetic field, which determines the resonant interaction at the Larmor frequency. Spin flips would subsequently emit radio frequency photons that can be detected by a suitable quantum counter in an ultra-cryogenic environment. This new detection technique is crucial to keep under control the thermal photon background which would otherwise produce a too large noise.

We describe the design of the suspension systems for the major optics for Advanced LIGO, the upgrade to LIGO—the Laser Interferometric Gravitational-Wave Observatory. The design is based on that used in GEO600—the German/UK interferometric gravitational wave detector, with further development to meet the more stringent noise requirements for Advanced LIGO. The test mass suspensions consist of a four-stage or quadruple pendulum for enhanced seismic isolation. To minimize suspension thermal noise, the final stage consists of a silica mirror, 40 kg in mass, suspended from another silica mass by four silica fibres welded to silica ears attached to the sides of the masses using hydroxide-catalysis bonding. The design is chosen to achieve a displacement noise level for each of the seismic and thermal noise contributions of 10−19 m/√Hz at 10 Hz, for each test mass. We discuss features of the design which has been developed as a result of experience with prototypes and associated investigations.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) has performed the fourth science run, S4, with significantly improved interferometer sensitivities with respect to previous runs. Using data acquired during this science run, we place a limit on the amplitude of a stochastic background of gravitational waves. For a frequency independent spectrum, the new Bayesian 90% upper limit is GW ; H 0 / 72 km s À1 Mpc À1 À Á Â Ã 2 < 6:5 ; 10 À5 . This is currently the most sensitive result in the frequency range 51Y150 Hz, with a factor of 13 improvement over the previous LIGO result. We discuss the complementarity of the new result with other constraints on a stochastic background of gravitational waves, and we investigate implications of the new result for different models of this background.

To account for the dark matter content in our Universe, post-inflationary scenarios predict for the QCD axion a mass in the range (10 − 10 3 ) µeV. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of Cu resonant cavities. In this paper we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a 36 cm 3 NbTi superconducting cavity. The cavity worked at T = 4 K in a 2 T magnetic field and exhibited a quality factor Q0 = 4.5 × 10 5 for the TM010 mode at 9 GHz. With such values of Q the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup we set the limit gaγγ < 1.03 × 10 −12 GeV −1 on the axion photon coupling for a mass of about 37 µeV. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.