[en] This paper presents new measurements in a case study of the minimal supergravity (SUGRA) model with m₀=100 GeV, m_1/2=300 GeV, A₀=0, tan β=2, and sgn μ=+ based on four-body distributions from three-step decays and on minimum masses in such decays. These measurements allow masses of supersymmetric particles to be determined without relying on a model. The feasibility of testing slepton universality at the ∼0.1% level at high luminosity is discussed. In addition, the effect of enlarging the parameter space of the minimal SUGRA model is discussed. The direct production of left-handed sleptons and the nonobservation of additional structure in the dilepton invariant mass distributions are shown to provide additional constraints. (c) 2000 The American Physical Society

[en] Precision electroweak data are generally believed to constrain the Higgs boson mass to lie below approximately 190 GeV at 95% confidence level. The standard Higgs model is, however, trivial and can only be an effective field theory valid below some high energy scale characteristic of the underlying nontrivial physics. Corrections to the custodial isospin violating parameter T arising from interactions at this higher energy scale dramatically enlarge the allowed range of Higgs mass. We perform a fit to precision electroweak data and determine the region in the (m_H, ΔT) plane that is consistent with experimental results. Overlaying the estimated size of corrections to T arising from the underlying dynamics, we find that a Higgs mass up to 500 GeV is allowed. (c) 2000 The American Physical Society

[en] We investigate whether multi-TeV (1-3 TeV) squarks can be natural in models of gauge-mediated SUSY breaking. The idea is that for some boundary condition of the scalar (Higgs boson and top squark) masses, the Higgs boson (mass)², evaluated at the renormalization scale ∼O(100) GeV, is not very sensitive to (boundary values of) the scalar masses (this has been called ''focusing'' in recent literature). Then, the top squark masses can be multi-TeV without leading to fine-tuning in electroweak symmetry breaking. Minimal gauge mediation does not lead to this focusing (for all values of tan β and the messenger scale): the (boundary value of) the Higgs boson mass is too small compared to the top squark masses. Also, in minimal gauge mediation, the gaugino masses are of the same order as the scalar masses so that multi-TeV scalars implies multi-TeV gauginos (especially the gluino) leading to fine-tuning. We discuss ideas to increase the Higgs boson mass relative to the top squark masses (so that focusing can be achieved) and also to suppress gaugino masses relative to scalar masses (or to modify the gaugino mass relations) in nonminimal models of gauge mediation--then multi-TeV (top and other) squarks can be natural. Specific models of gauge mediation which incorporate these ideas and thus have squarks (and, in some cases, the gluino) heavier than 1 TeV without resulting in fine-tuning are also studied and their collider signals are contrasted with those of other models which have multi-TeV squarks. (c) 2000 The American Physical Society

[en] We use 106 pb^-1 of data collected with the Collider Detector at Fermilab to search for narrow-width, vector particles decaying to a top and an antitop quark. Model independent upper limits on the cross section for narrow, vector resonances decaying to tt are presented. At the 95% confidence level, we exclude the existence of a leptophobic Z^' boson in a model of top-color-assisted technicolor with mass M_Z^'<480 GeV/c² for natural width Γ=0.012M_Z^' , and M_Z^'<780 GeV/c² for Γ=0.04M_Z^' . (c) 2000 The American Physical Society

[en] For a top quark mass fixed to its measured value, we find natural regions of minimal supergravity parameter space where all squarks, sleptons, and heavy Higgs scalars have masses far above 1 TeV and are possibly beyond the reach of the Large Hadron Collider at CERN. This result is simply understood in terms of ''focus point'' renormalization group behavior and holds in any supergravity theory with a universal scalar mass that is large relative to other supersymmetry breaking parameters. We highlight the importance of the choice of fundamental parameters for this conclusion and for naturalness discussions in general. (c) 2000 The American Physical Society

[en] A search for antiproton decay has been made at the Fermilab Antiproton Accumulator. Limits are placed on fifteen antiproton decay modes. The results are used to place limits on the characteristic mass scale m_X that could be associated with CPT violation accompanied by baryon number violation. (c) 2000 The American Physical Society

[en] In a generic supersymmetric extension of the standard model, there will be lepton flavor violation at a neutral gaugino vertex due to misalignment between the lepton Yukawa couplings and the slepton soft masses. Sleptons produced at the CERN LHC through the cascade decays of squarks and gluinos can give a sizable number of events with 4 leptons. This channel could give a clean signature of supersymmetric lepton flavor violation under conditions which are identified. (c) 2000 The American Physical Society

[en] We report on a search for R -parity-violating supersymmetry in pp(bar sign) collisions at √(s)=1.8 TeV using the D0 detector at Fermilab. Events with at least two electrons and four or more jets were studied. We observe two events in 99±4.4 pb^-1 of data, consistent with the expected background of 1.8±0.4 events. This result is interpreted within the framework of minimal low-energy supergravity supersymmetry models. Squarks with mass below 243 GeV/c² and gluinos with mass below 227 GeV/c² are excluded at the 95% C.L. for A₀=0 , μ<0 , tan β=2 , and a finite value for any one of the six R -parity-violating couplings λ^'_1jk ( j=1 , 2 and k=1 , 2, 3). (c) 1999 The American Physical Society

[en] With one extra dimension, current high precision electroweak data constrain the masses of the first Kaluza-Klein (KK) excitations of the standard model gauge fields to lie above ≅4 TeV. States with masses not much larger than this should be observable at the CERN LHC. However, even for first excitation masses close to this lower bound, the second set of excitations will be too heavy to be produced, thus eliminating the possibility of realizing the cleanest signature for KK scenarios. Previous studies of heavy Z^' and W^' production in this mass range at the CERN LHC have demonstrated that very little information can be obtained about their couplings to the conventional fermions given the limited available statistics and imply that the CERN LHC cannot distinguish an ordinary Z^' from the degenerate pair of the first KK excitations of the γ and Z. In this paper we discuss the capability of lepton colliders with center of mass energies significantly below the excitation mass to resolve this ambiguity. In addition, we examine how direct measurements obtained on and near the top of the first excitation peak at lepton colliders can confirm these results. For more than one extra dimension we demonstrate that it is likely that the first KK excitation is too massive to be produced at the CERN LHC. (c) 2000 The American Physical Society

[en] We present the first observation of the decay B→J/ψφK . Using 9.6x10⁶ BB meson pairs collected with the CLEO detector, we have observed ten fully reconstructed B→J/ψφK candidates, whereas the estimated background is 0.5±0.2 event. We obtain a branching fraction of B(B→J/ψφK)=( 8.8^+3.5_-3.0[stat] ±1.3[syst])x10^-5 . This is the first observed B meson decay requiring the creation of an additional ss quark pair. (c) 2000 The American Physical Society