Ordinary Fermions Research Articles

Production of heavy spin-(3/2 fermions in colliders.

If very heavy quarks exist in the standard model, it is likely that spin-(3/2) excited states will arise. Moreover, a recent model indicates that deeply bound nucleonlike bound states having a nearby spin-(3/2) excitation will be present. We calculate production rates for creating pairs of color-singlet or -triplet spin-(3/2) fermions in proton-(anti)proton colliders. In pointlike approximation, the rates turn out to be considerably larger than those for ordinary fermions. We estimate the influence of form factors, making some reasonable guesses. We show that very different decay patterns emerge depending whether or not these states are pointlike. We estimate that the Superconducting Super Collider could uncover spin-(3/2) quarks up to a mass around 1 TeV.

Weak-isospin violation in "walking" technicolor.

We show that the mechanism which enhances ordinary fermion masses and raises the masses of pseudo Goldstone bosons in walking-technicolor models may also lead to an enhancement of weak-isospin--violating effects of extended-technicolor interactions. We discuss the constraints on these models due to limits on the deviation of the \ensuremath{\rho} parameter from 1.

Mixing between ordinary and exotic fermions.

The results of a comprehensive analysis of the limits on mixings between ordinary fermions and possible heavy fermions with exotic SU(2)\ifmmode\times\else\texttimes\fi{}U(1) assignments (e.g., left-handed singlets and/or right-handed doublets) are presented. A general formalism for describing such mixings is given. It is shown that a variety of constraints, including the relation between the W and Z masses and the Fermi constant, charged-current universality, limits on induced right-handed charged currents, and flavor-diagonal neutral currents suffice to limit all directions in parameter space that are not excluded by the absence of flavor-changing neutral currents. Limits on ${s}^{2}$, the square of the mixing between ordinary and exotic fermions, are quite stringent for the ${\ensuremath{\nu}}_{\ensuremath{\mu}L}$, ${\ensuremath{\mu}}_{L}^{\mathrm{\ensuremath{-}}}$, ${u}_{L}$, and ${d}_{L}$ (${s}^{2}$\ensuremath{\le}0.002--0.005) if only one particle is allowed to mix at a time, but are weaker by an order of magnitude if fine-tuned cancellations between different mixings are allowed. Similar statements apply to quark mixings with heavy sequential doublets. Limits on ${s}^{2}$ for the other light fermions (${\ensuremath{\nu}}_{\mathrm{eL}}$, ${\mathit{e}}_{\mathit{L}}^{\mathrm{\ensuremath{-}}}$,${\mathit{e}}_{\mathit{R}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\mu}}}_{\mathit{R}}^{\mathrm{\ensuremath{-}}}$, ${\mathit{u}}_{\mathit{R}}$,${\mathit{d}}_{\mathit{R}}$) are in the range 0.02--0.06, while those for the s, c, b, ${\ensuremath{\nu}}_{\ensuremath{\tau}}$, and ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}}$ are considerably weaker. Slightly stronger limits are found in specific models (e.g., ${\mathrm{E}}_{6}$). Implications for the masses of the heavy exotic fermions are discussed.

Nonperturbative unification of technicolor.

Nonperturbative unification of technicolor (TC) with the standard-model forces can be achieved in the scheme of Maiani, Parisi, and Petronzio, where all interactions become strong at ${M}_{\mathrm{GUT}}$, if the TC perturbative \ensuremath{\beta} function has a nontrivial infrared fixed point. SU(4) with a single family of technifermions in the 6 emerges as the only plausible TC group. The low-energy couplings are computed. The scale of interactions responsible for ordinary fermion masses might be raised up to ${M}_{\mathrm{GUT}}$.

Implications of the E6SU

Besides the ordinary 27-multiplet fermions in the usual ${\mathrm{E}}_{6}$ grand unified theories, two superheavy 27-multiplet fermions are added to form the ${\mathrm{E}}_{6}$\ifmmode\times\else\texttimes\fi{}SU(3${)}_{\mathrm{H}}$ model. After both vertical and horizontal spontaneous symmetry breaking, the Fritzsch mass matrices of the ordinary fermions arise as a result of mixing of the ordinary fermions with the superheavy fermions. The charged-fermion masses and Kobayashi-Maskawa mixing follow as a consequence. The neutrino mass in this model is also discussed.

Parisi-Sourlas fermions from Ramond superstring

We show that the zero energy limit of the Ramond superstring naturally describes Dirac fermions with an OSP( D + 1,1¦2) or Parisi-Sourlas supersymmetry, and establish an equivalence between ordinary Dirac fermions and a massive (0 + 1)-dimensional supergravity theory.

Right-handed currents produced by mixing in E6 theories.

We explore the possibility that leptonic right-handed currents (RHC's) can arise in ${\mathrm{E}}_{6}$ theories with Dirac neutrinos due to mixing between the ordinary fermions and new, exotic fermions in the 27 representation. These RHC's can lead to an additional, potentially large, contribution to the electron and muon anomalous magnetic moments (g-2), and can produce a large magnetic moment (\ensuremath{\kappa}) for Dirac ${\ensuremath{\nu}}_{e}$ and ${\ensuremath{\nu}}_{\ensuremath{\mu}}$, as well as new phenomenology. The constraints on the existence of RHC's from \ensuremath{\mu} decay and the size of g-2 in addition to reactor accelerator, and astrophysical constraints on \ensuremath{\kappa} can be used to tightly constrain the model parameters.

Excited fermions and low-energy parity-violating phenomena

A calculation is made of the parity-violating contributions of hypothetical excited quarks and leptons to two examples of low-energy phenomena. In particular, the contributions to polarized-electron--deuteron scattering and the parity-violating nuclear potential are studied. The parity-violating contributions are induced by the magnetic coupling of photons or gluons with virtual excited fermions and left-handed ordinary fermions. It is concluded that the mass of an excited quark, if it exists, must be larger than approx.3 TeV. The restriction on the mass of an excited lepton is less stringent.

Breaking of isospin symmetry in theories with a dynamical Higgs mechanism.

The sensitivity of physical amplitudes to the observed isospin-symmetry breaking in the fermion mass matrix is analyzed in theories where the electroweak symmetry is dynamically broken. As a first step toward discussing dynamical theories in a model-independent way, we consider a strongly interacting Higgs theory. The nonlinear \ensuremath{\sigma} model coupled to an SU(2)\ifmmode\times\else\texttimes\fi{}U(1) Yang-Mills theory and to fermions is used to generate the low-dimensional operators induced by the quantum theory. The strengths of these operators, in particular those that break isospin symmetry, are estimated using the Higgs-boson mass of the linear \ensuremath{\sigma} model as a regulator. Technicolor models are next considered as a specific example of the strong-interaction physics which leads to electroweak symmetry breaking. Emphasis is placed on the effective four-fermion interactions that are natural partners of those which give mass to the ordinary fermions. New sources of isospin-symmetry breaking are found; in particular there is one which contributes directly to the \ensuremath{\rho} parameter with strength that is linear in the effective Yukawa coupling of the heaviest ordinary fermion. This effect is qualitatively different from the quadratic dependence found in Higgs theories.

Instantons in grand unification

A class of instantons in four-dimensional Euclidean grand unification is discussed. These solutions in the classical limit have nonzero Lagrangian density at only one single point. These instantons can generate proton decay processes, whose rate has an extremely feeble effect in theSU5 model. However, it can be observable in unification with technifermions that embedded instantons induce technifermions decay to ordinary fermion. We also consider the instantons in aSO10 model.

Matter couplings in non-minimal supergravity and massive superspace

We show that the non-minimal supergravity theory can accomodate matter couplings different from the canonical ones. These couplings generate mass terms for the superpartners of the ordinary matter fermions and gauge vectors, thus leading to the concept of the massive superspace introduced earlier.

Mirror neutrinos in standard cosmology

Right-handed mirror fermions having suppressed interactions with ordinary fermions exist in many gauge symmetry groups. We study the bounds on the mass, lifetime and mixing angles of an unstable mirror muon neutrino N μ, imposed by astrophysics, cosmology and low-energy particle physics phenomenology.

Fractional charge and automatic U(1) symmetry without domain walls

All fermion representations compatible with fractional charge in asymptotically free SU( N) gauge theories are constructed. With a survival hypothesis argument it is shown that none of them have three light ordinary fermion families. Automatic U(1) symmetry is constructed for each case, but all of them are plagued with domain-wall problems, unresolvable by the Lazarides and Shafi method.

Possible candidates for finite supersymmetric grand unified theories

A realistic finite grand unified model has to accomodate at least three generations of ordinary fermions. When this criterion is imposed, we found that only a few models of the SU( N), SO(2 N), SO(2 N+1) and exceptional groups can survive as candidates for possible realistic finite supersymmetric grand unified models. All possible solutions are obtained, but the SU(5) model is excluded.

Possible Candidates for Finite Supersymmetric Grand Unified Theories

A realistic finite grand unified model has to accommodate at least three generations of ordinary fermions. When this criterion is imposed, we found that only few models of and exceptional groups can survive as candidates for possible realistic finite supersymmetric grand unified models. All possible solutions are obtianed, but the model is excluded.

Composite Higgs scalars

We construct models in which the Higgs doublet whose vacuum expectation breaks SU(2) × U(10 is a bound state of massive strongly interacting fermions. The couplings of the composite Higgs to ordinary fermions are induced by heavy gauge boson exchange in the manner of extended technicolor. Other heavy gauge bosons generate a negative mass term for the Higgs.

Dynamical interpretation of the technifermion approach motivated by composite gravity

The algebraic connection of ordinary and technifermions by an extended technicolour group is reinterpreted as an analytic connection involving their dilatation properties. An embedding of the ordinary fermions into such a « dilatation proliferation » is motivated by the conformal part of the gravitational interaction. Newton’s constant (or the Planck mass) gives the scale for the interaction of the ordinary and technifermions in this extended scheme. The internal gauge bosons arise as composites. The mass splitting of the technifermions, appearing locally in parity doublets, is the relevant scale for the Higgs phenomenon.

On a self-consistent chiral doublet composite scheme

A composite scheme with a chiral doublet of charges Q=( 2 3 , − 1 3 ) is considered. The scheme is self-consistent in the sense that the constituent spectator chiral symmetry SE(2) × U(1) coincides with the chiral electroweak symmetry. Due to the mirror fermions they both are anomaly free. The chiral colour symmetry arises as an effective symmetry of composite quarks. The scheme contains exotic composite fermions with isospin I= 3 2 . The common maximal effective symmetry of both ordinary and exotic composite fermions is the chiral SU(2) × SU(4) one, the latter being a possible partial unified symmetry.

Technicolor

The need for reexamination of the standard model of strong, weak, and electromagnetic interactions is discussed, especially with regard to 't Hooft's criterion of naturalness. It has been argued that theories with fundamental scalar fields tend to be unnatural at relatively low energies. There are two solutions to this problem: (i) a global supersymmetry, which ensures the absence of all the naturalness-violating effects associated with scalar fields, and (ii) composite structure of the scalar fields, which starts showing up at energy scales where unnatural effects would otherwise have appeared. With reference to the second solution, this article reviews the case for dynamical breaking of the gauge symmetry and the technicolor scheme for the composite Higgs boson. This new interaction, of the scaled-up quantum chromodynamic type, keeps the new set of fermions, the technifermions, together in the Higgs particles. It also provides masses for the electroweak gauge bosons ${W}^{\ifmmode\pm\else\textpm\fi{}}$ and ${Z}^{0}$ through technifermion condensate formation. In order to give masses to the ordinary fermions, a new interaction, the extended technicolor interaction, which would connect the ordinary fermions to the technifermions, is required. The extended technicolor group breaks down spontaneously to the technicolor group, possibly as a result of the "tumbling" mechanism, which is discussed here. In addition, the author presents schemes for the isospin breaking of mass matrices of ordinary quarks in the technicolor models. In generalized technicolor models with more than one doublet of technifermions or with more than one technicolor sector, we have additional low-lying degrees of freedom, the pseudo-Goldstone bosons. The pseudo-Goldstone bosons in the technicolor model of Dimopoulos are reviewed and their masses computed. In this context the vacuum alignment problem is also discussed. An effective Lagrangian is derived describing colorless low-lying degrees of freedom for models with two technicolor sectors in the combined limits of chiral symmetry and large number of colors and technicolors. Finally, the author discusses suppression of flavor-changing neutral currents in the extended technicolor models.

SU(8) gut models with fractionally charged color singlet fermions and low mass magnetic monopoles

Two SU(8) GUT models (one with fractionally charged color singlet fermions, another with low mass magnetic monopoles) are proposed. Both of them can accomodate three generations of ordinary light fermions.