No abstract

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

We consider the parameters of an entry-level neutrino factory designed to make the first observation of e → oscillations, measure the corresponding amplitude sin 2 2 13 , and determine the sign of the atmospheric-scale ␦m 32 2 via matter effects. A 50 kt detector, a stored muon energy E у20 GeV and 10 19 muon decays would enable these goals to be met provided sin 2 2 13 Ͼ0.01. The determination of the sign of ␦m 32 2 also requires a baseline Lу2000 km. An upgraded neutrino factory with O(10 20 ) decays would enable the first observation of e → oscillations. With O(10 21 ) decays the effects of a large CP phase could be measured in the case of the large angle matter oscillation solution to the solar neutrino anomaly. Our analysis includes a family of three-neutrino models that can account for the atmospheric and solar neutrino oscillation indications.

We consider the medium-and long-baseline oscillation physics capabilities of intense muon-neutrino and muon-antineutrino beams produced using future upgraded megawatt-scale high-energy proton beams. In particular we consider the potential of these conventional neutrino ''superbeams'' for observing → e oscillations, determining the hierarchy of neutrino mass eigenstates, and measuring CP violation in the lepton sector. The physics capabilities of superbeams are explored as a function of the beam energy, baseline, and the detector parameters ͑fiducial mass, background rates, and systematic uncertainties on the backgrounds͒. The trade-offs between very large detectors with poor background rejection and smaller detectors with excellent background rejection are illustrated. We find that, with an aggressive set of detector parameters, it may be possible to observe → e oscillations with a superbeam provided that the amplitude parameter sin 2 2 13 is larger than a few ϫ10 Ϫ3 . If sin 2 2 13 is of order 10 Ϫ2 or larger, then the neutrino mass hierarchy can be determined in long-baseline experiments, and if in addition the large mixing angle MSW solution describes the solar neutrino deficit, then there is a small region of parameter space within which maximal CP violation in the lepton sector would be observable ͑with a significance of a few standard deviations͒ in a low-energy mediumbaseline experiment. We illustrate our results by explicitly considering massive water Cherenkov and liquid argon detectors at superbeams with neutrino energies ranging from 1 GeV to 15 GeV, and baselines ranging from 295 km to 9300 km. Finally, we compare the oscillation physics prospects at superbeams with the corresponding prospects at neutrino factories. The sensitivity at a neutrino factory to CP violation and the neutrino mass hierarchy extends to values of the amplitude parameter sin 2 2 13 that are one to two orders of magnitude lower than at a superbeam.tions at a mass-squared-difference scale Ͼ10 Ϫ3 eV 2 with amplitude Ͼ0.1. Furthermore, large amplitude → s oscillations at the ␦m atm 2 scale are also excluded by SuperK. This is because → s oscillations are expected to be significantly affected by propagation through matter ͓7,8͔, causing a distortion in the zenith-angle distribution at large angles ͑corresponding to long path lengths͒ that is not present in the data ͓9͔. The zenith-angle distribution observed by SuperK excludes → s oscillations of maximal amplitude at 99% confidence level ͓9͔. We conclude that, if the oscillation interpretation of the atmospheric neutrino deficit is correct, the dominant mode must be → oscillation, with the possibility of some smaller amplitude muon-neutrino oscillations to sterile and/or electron-neutrinos ͓10͔.An exotic alternative interpretation ͓11͔ of the atmospheric neutrino disappearance results is that a neutrino mass eigenstate, which is a dominant component of the state, decays to a lighter mass-eigenstate and a Majoron ͓12͔. The first oscillation minimum in → must be observed or excluded t...

We determine the top quark mass m t using t t pairs produced in the DO " detector by ͱsϭ1.8 TeV pp collisions in a 125 pb Ϫ1 exposure at the Fermilab Tevatron. We make a two constraint fit to m t in t t→bW ϩ b W Ϫ final states with one W boson decaying to qq and the other to e or . Likelihood fits to the data yield m t (lϩjets)ϭ173.3Ϯ5.6 (stat) Ϯ 5.5 (syst) GeV/c 2 . When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m t ϭ172.1Ϯ5.2 (stat) Ϯ 4.9 (syst) GeV/c 2 . An alternate analysis, using three constraint fits to fixed top quark masses, gives m t (lϩjets)ϭ176.0 Ϯ7.9 (stat)Ϯ 4.8 (syst) GeV/c 2 , consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. ͓S0556-2821͑98͒06815-5͔