LEP Constraints on 5-Dimensional Extensions of the Standard Model

We study minimal 5-dimensional extensions of the Standard Model, in which all or only some of the SU(2)$_L$ and U(1)$_Y$ gauge fields and Higgs bosons propagate in the fifth compact dimension. In all the 5-dimensional settings, the fermions are assumed to be localized on a 3-brane. In addition, we present the consistent procedure for quantizing 5-dimensional models in the generalized $R_\xi$ gauge. Bounds on the compactification scale between 4 and 6 TeV, depending on the model, are established by analyzing electroweak precision measurements and LEP2 cross sections.Comment: 6 pages, talk given at the 10th international conference on ``Supersymmetry and Unification of Fundamental Interactions,'' Hamburg, Germany, 17--23 June 200

Probing the Noncommutative Standard Model at Hadron Colliders

We study collider signals for the noncommutative extension of the standard model using the Seiberg-Witten maps for SU(3)_C x SU(2)_L x U(1)_Y to first order in the noncommutativity parameters theta_munu. In particular, we investigate the ensitivity of Z-gamma-production at the Tevatron and the LHC to the components of theta_munu. We discuss the range of validity of this approximation and estimate exclusion limits from a Monte Carlo simulation.Comment: 18 pages LaTeX, 23 figures. Slightly expanded introduction and additional references. Accepted for publication in Physical Review

Supersymmetric Signals in e−e−e^-e^- Collisions

We consider the production and decay of selectrons and charginos in $e^-e^-$ collisions. The advantage over usual $e^+e^-$ collisions is the very low level of standard model backgrounds which should make the discovery of selectrons or charginos relatively straightforward. The use of polarized beams provides an additional powerful tool to determine the supersymmetry parameters.Comment: 18 pages including 8 figures, included as uufiled .ps files. LaTeX with axodraw.sty for Feynman graphs, epsf.sty for figures; the texfile and complete PSfile are available via ftp from pss058.psi.ch. CERN-TH.6807/93, MPI-Ph/93-28, PSI-93-1

Supersymmetric Signals in Electron-Photon Collision

Associated selectron-neutralino production in the process $e^-\gamma\to\tilde e^-\tilde\chi^0$ provides a striking supersymmetric signal: events with a single high $p_\perp$ electron and otherwise only invisible particles. For $e^-\gamma$ collisions obtained at high energy linear colliders through back-scattering of a laser beam, this reaction is shown to be complementary to selectron pair production in the processes $e^+e^-\to\tilde e^+\tilde e^-$ and $e^-e^-\to\tilde e^-\tilde e^-$, and to be a probe of heavy selectrons beyond the kinematical limit of pair production. The standard model background from $e^-\gamma\to e^-Z^0$ and $W^-\nu$ is studied and substantially reduced by rapidity and transverse momentum cuts. The minimum required integrated luminosities for observing this \susic\ signal are given as functions of several model parameters and collider energies.Comment: 15 page