N.D. Tracas

Search for New Quarks Suggested by the Superstring

We discuss the possible phenomenology of the additional charge (−13) colour triplet particles D12 of spin 12 and D0,D0c of spin 0 which are contained in each matter generation if the superstring is compactified on a Calabi-Yau manifold. In the minimal model with no intermediate mass scales and supersymmetry breaking fed into the observable sector via a gaugino mass, either a spin-12 or a spin-0 D particle could be the lightest, and either could be as light as the present lower bound from e+e− experiments of about 20 GeV. The D particles could behave as leptoquarks coupling to quarks and leptons, in which case the single production process ep → D0(D0c) + X would occur, and D0(D0c) → lq, D12 → lqχ decays would dominate, where χ is the lightest supersymmetric particle. Alternatively, the D particles could behave as disquarks coupling (p)p → D0(D0(Dc,D0D0c) + K to pairs of antiquarks, in which case the single production process would occur, and D0(D0c) → qq, D0c(D0) → qq, D12 → qqχ decays would dominate. We present cross sections for ep → D0(D0c + X,ep → D0(D0c) + X, (p)p → D0(D0c, D0c, D0c, D0) + X, e+e− → D0D0(D0cD0c) and D12D12, (p)p → D0D0(D0cD0c and D12D12 + X. We calculate the experimental signals and estimate backgrounds for D production and decay in these processes. The decays D0(D0c) → lq and D12 → lqχ or qqχ would be detectable in most of these reactions, but D0(D0c → qq decays may only be detectable in e+e− → D0D0(D0cD0c) collisions.

Effects of a superstring gauge boson on high energy e+e− annihilation and ep scattering

Abstract The effects at LEP and HERA are calculated of the second neutral gauge boson appearing in phenonenological superstring models with an effective SU(3) C ×SU(2) L ×U(1) Y L ×U(1) Y R gauge group. Present phenomenological constraints leave open the possibility of a measurable shift in the first Z mass, and of observable modifications to the total e + e − cross section and forward-backward asymmetries at the Z peak and beyond. High energy ep scattering assymetries may also differ significantly from the standard model predictions.

Reduction of couplings in the MSSM

In this Letter, we first demonstrate the existence of renormalization group invariant relations among the top, bottom Yukawa and the gauge colour couplings in the minimal supersymmetric SM. Based on this observation and assuming furthermore the existence of a renormalization group invariant relation among the trilinear couplings in the superpotential and the soft supersymmetry breaking sector, we obtain predictions for the Higgs masses and the supersymmetric spectrum.

GUT, supersymmetry and top threshold corrections in the SU(4)×SU(2)L×SU(2)R model

We investigate the interplay between GUT, SUSY and top threshold effects in the context of the string-derived model based on SU(4)×SU(2)L×SU(2)R. The SUSY and top thresholds demand a top mass somewhat high (mt > 140 GeV) and a GUT scale 1015 GeV. This is lower than the GUT scale demanded in the model, where these thresholds are not taken into account.

Indirect searches for leptoquarks at present and future colliders

We explore the sensitivities of e+e− colliders (LEP, CLIC) and hadron-hadron colliders (CERN , FNAL , LHC pp and SSC pp) to the indirect effects of virtual scalar particles with superstring-inspired leptoquark and diquark couplings. We find that e+e− colliders have better sensitivities via total cross section measurements of than via asymmetry measurements, and that LEP 2 is much more sensitive than LEP 1. Hadron-hadron colliders are sensitive via lepton-pair production cross sections ( μ+μ−, τ+τ−).

Modular Weights, U(1)'s and Mass Matrices ⋆

Abstract We derive the scalar mass matrices in effective supergravity models with the standard gauge group augmented by a U (1) F family symmetry. Simple relations between U (1) F charges and modular weights of the superfields are derived and used to express the matrices with a minimum number of parameters. The model predicts a branching ratio for the μ → eγ process close to the present experimental limits.

Supersymmetric radiative corrections at the Z0 peak

Abstract We give compact and explicit formulae for the corrections of asymmetry measurements on the Z 0 peak induced by vacuum polarization due to supersymmetric particles. We investigate their magnitudes in realistic minimal models based on supergravity and inspired by the superstring. We find that the corrections to the forward-backward asymmetry are unobservably small, whilst corrections to the left-right polarization asymmetry are observable if particles have masses in the range accessible to LEP 200.

Contribution of scalar diquarks to the ΔI = 12 rule

Abstract One of the novel features of the superstring inspired models is the possible appearance of colour triplet particles with diquark couplings. They can mediate hadronic decays of baryons and mesons. We investigate their contribution to the ΔI= 1 2 rule through the operator product expansion.

Reduction of the parameters in MSSM

A bstractIn the present work we search for renormalization group invariant relations among the various massless and massive parameters of the Minimal Supersymmetric Standard Model. We find that indeed several of the previously free parameters of the model can be reduced in favor of few, the unique gauge coupling and the gaugino mass at the unification scale among them. Taking into account the various experimental constraints, including the B-physics ones, we predict the Higgs and the supersymmetric spectrum. We find that the lightest Higgs mass is in comfortable agreement with the measured value and its experimental and theoretical uncertainties, while the electroweak supersymmetric spectrum starts at 1.3 TeV and the colored at ∼4 TeV. Thus the reduced MSSM is in natural agreement with all LHC measurements and searches. The supersymmetric and heavy Higgs particles will likely escape the detection at the LHC, as well as at ILC and CLIC. However, the FCC-hh will be able to fully test the predicted parameter space.

Duality constraints on supersymmetric unified models and radiative symmetry breaking

Abstract Motivated from unified models with string origin, we analyse the constraints from duality invariance on effective supergravity models with intermediate gauge symmetry. Requiring vanishing vacuum energy and invariance of the superpotential couplings, the modular weights are subject to various constraints. Further, the intermediate gauge symmetry breaking scale M U is related to the values of modular weights of the matter and Higgs fields. For certain regions of values of the latter, M U can be close to the conventional unification scale of the minimal supersymmetric standard model. We also examine particular examples where the intermediate gauge symmetry breaks down to the standard gauge group radiatively.