Calculating fermion masses in superstring derived standard-like models (original) (raw)

The Minimal Supersymmetric Standard Model and Precision of W-Boson Mass and Top Quark Mass

Modern Physics Letters A, 1998

We argue that the present value and accuracy of MW and mt measurements tend to favor the MSSM, provided that the central values of MW and mt stay at the current values, over the SM. By speculating that a precision of order 40 MeV and 3 GeV respectively for MW and mt will be achieved at LEP2 and Tevatron, we show that the prospect for the MSSM will be further enhanced as long as the central values of MW and mt do not increase below the present values. In addition, we discuss how this scenario can constrain the Higgs boson mass and distinguish the Higgs boson of the MSSM type from that of the SM.

Precise Predictions for the Masses and Couplings in the Minimal Supersymmetric Standard Model

1995

We present selected results of our program to determine the masses, gauge couplings, and Yukawa couplings of the minimal supersymmetric model in a full oneloop calculation. We focus on the precise prediction of the strong coupling α s (M Z ) in the context of supersymmetric unification. We discuss the importance of including the finite corrections and demonstrate that the leading-logarithmic approximation can significantly underestimate α s (M Z ) when some superpartner masses are light. We show that if GUT thresholds are ignored, and the superpartner masses are less than about 500 GeV, the prediction for α s (M Z ) is quite large. We impose constraints from nucleon decay experiments and find that minimal SU(5) GUT threshold corrections increase α s (M Z ) over most of the parameter space. We also consider the missing-doublet SU(5) model and find that it predicts preferred values for the strong coupling, even for a very light superpartner spectrum. We briefly discuss predictions for the bottom-quark mass in the small tan β region. * M 0 is the universal scalar mass, M 1/2 is the universal gaugino mass, and A 0 is the universal A-term. † See Ref.

Phenomenological study of a minimal superstring standard model

Nuclear Physics B, 2001

Recently, we demonstrated the existence of heterotic-string solutions in which the observable sector effective field theory just below the string scale reduces to that of the MSSM, with the standard observable gauge group being just SU (3) C × SU (2) L × U (1) Y and the SU (3) C × SU (2) L × U (1) Y-charged spectrum of the observable sector consisting solely of the MSSM spectrum. Associated with this model is a set of distinct flat directions of vacuum expectation values (VEVs) of non-Abelian singlet fields that all produce solely the MSSM spectrum. In this paper, we study the effective superpotential induced by these choices of flat directions. We investigate whether sufficient degrees of freedom exist in these singlet flat directions to satisfy various phenomenological constraints imposed by the observed Standard Model data. For each flat direction, the effective superpotential is given to sixth order. The variations in the singlet and hidden sector low energy spectrums are analyzed. We then determine the mass matrices (to all finite orders) for the three generations of MSSM quarks and leptons. Possible Higgs µ-terms are investigated. We conclude by considering generalizations of our flat directions involving VEVs of non-Abelian fields.

Intermediate scales, μ parameter, and fermion masses from string models

Physical Review D, 1998

We address intermediate scales within a class of string models. The intermediate scales occur due to the SM singlets S i acquiring non-zero VEVs due to radiative breaking; the mass-square m 2 i of S i is driven negative at µ RAD due to O(1) Yukawa couplings of S i to exotic particles (calculable in a class of string models). The actual VEV of S i depends on the relative magnitude of the non-renormalizable terms of the typeŜ K+3 i /M K in the superpotential. We mainly consider the case in which the S i are charged under an additional non-anomalous U (1) ′ gauge symmetry and the VEVs occur along F -and Dflat directions. We explore various scenarios in detail, depending on the type of Yukawa couplings to the exotic particles and on the initial boundary values of the soft SUSY breaking parameters. We then address the implications of these scenarios for the µ parameter and the fermionic masses of the standard model.

The Higgs boson mass as a probe of the minimal supersymmetric standard model

Physics Letters B, 1998

Recently, the LEP collaborations have reported a lower bound on a Standard Model-like Higgs boson of order 89 GeV. We discuss the implications of this bound for the minimal supersymmetric extension of the Standard Model (MSSM). In particular, we show that the lower bound on tan β, which can be obtained from the presently allowed Higgs boson mass value, becomes stronger than the one set by the requirement of perturbative consistency of the theory up to scales of order M GU T (associated with the infrared fixed-point solution of the top quark Yukawa coupling) in a large fraction of the allowed parameter space. The potentiality of future LEP2 searches to further probe the MSSM parameter space is also discussed.

The lightest Higgs boson mass in the Minimal Supersymmetric Standard Model

Nuclear Physics B, 1995

We compute the upper bound on the mass of the lightest Higgs boson in the Minimal Supersymmetric Standard Model in a model-independent way, including leading (one-loop) and next-to-leading order (two-loop) radiative corrections. We find that (contrary to some recent claims) the two-loop corrections are negative with respect to the one-loop result and relatively small ( < ∼ 3%). After defining physical (pole) top quark mass M t , by including QCD self-energies, and physical Higgs mass M H , by including the electroweak self-energies Π M 2 H − Π(0), we obtain the upper limit on M H as a function of supersymmetric parameters. We include as supersymmetric parameters the scale of supersymmetry breaking M S , the value of tan β and the mixing between stops X t = A t + µ cot β (which is responsible for the threshold correction on the Higgs quartic coupling). Our results do not depend on further details of the supersymmetric model. In particular, for M S ≤ 1 TeV, maximal threshold effect X 2 t = 6M 2 S and any value of tan β, we find M H ≤ 140 GeV for M t ≤ 190 GeV. In the particular scenario where the top is in its infrared fixed point we find M H ≤ 86 GeV for M t = 170 GeV.

On Standard Model Higgs and Superstring Theories

Particles, Strings and Cosmology (PASCOS 99), 2000

It is shown that in the Standard Model, the property of charge quantization holds for a Higgs with arbitrary isospin and hypercharge. These defining quantum numbers of the Higgs remain unconstrained while the whole basic and fundamental structure of the Standard Model remains intact. Hence it is shown that the Higgs cannot be a physical particle. Higgs is the underlying 'vacuum' over which the whole edifice of the Standard Model stands. Also on most general grounds it is established here that as per the Standard Model there is no electric charge above the electro-weak phase transition temperature. Hence there was no electric charge present in the Early Universe. The Superstring Theories are flawed in as much as they are incompatible with this requirement.