The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3-4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from 1098-4402͞99͞2(8)͞081001(73)$15.00 © 1999 The American Physical Society 081001-1 PRST-AB 2 CHARLES M. ANKENBRANDT et al. 081001 (1999) a heavy-Z target and proceeding through the phase rotation and decay (p ! m n m ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96

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We report measurements of the temperature dependence of the surface resistivity of highquality YBa2Cu307 crystals at 5.95 GHz. A narrow transition with 89-K onset is observed.Resistivity in the normal state is found to be R", 126&10 Q. %'e establish an upper limit for the resistivity at 77 K of 1.0&10 0 and at 2 K of 1.5&10 A. The dissipation in four crystals with a total area of about 20 mm2 is measured to be nearly independent of peak microwave magnetic field over the range of 0.5 to 93 G in sharp contrast to the strong field dependence observed in bulk sintered samples. Dissipation of microwaves in superconductors is due to the interaction of the oscillating fields with the unpaired

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