Quark-lepton Symmetric Models Research Articles

The parameters of the fermion-mixing and search restriction on a mass leptoquark.

It is shown that the inclusion of fermion mixing in the framework of fourcolour quark-lepton symmetric model can be greatly reduces the restrictions on the mass vector leptoquark. The lower limit for the mass of the vector leptoquark obtained from processes K L 0 → l i + l j − , B 0 → l i + l j − and B s → l i + l j − can be of the order of 85 TeV.

Quark-lepton symmetric model at the LHC

We investigate the quark-lepton symmetric model of Foot and Lew in the context of the Large Hadron Collider (LHC). In this `bottom-up' extension to the Standard Model, quark-lepton symmetry is achieved by introducing a gauged `leptonic colour' symmetry which is spontaneously broken above the electroweak scale. If this breaking occurs at the TeV scale, then we expect new physics to be discovered at the LHC. We examine three areas of interest: the Z$'$ heavy neutral gauge boson, charge $\pm1/2$ exotic leptons, and a colour triplet scalar diquark. We find that the LHC has already explored and/or will explore new parameter space for these particles over the course of its lifetime.

QUARK-LEPTON SYMMETRY AND QUARTIFICATION IN FIVE DIMENSIONS

We outline some features of higher dimensional models possessing a Quark-Lepton (QL) symmetry. The QL symmetric model in five dimensions is discussed, with particular emphasis on the use of split fermions. An interesting fermionic geography which utilises the QL symmetry to suppress the proton decay rate and to motivate the flavor differences in the quark and leptonic sectors is considered. We discuss the quartification model in five dimensions and contrast the features of this model with traditional four dimensional constructs.

Quark-lepton symmetry in five dimensions

We construct a complete five dimensional quark-lepton symmetric model, with all fields propagating in the bulk. The extra dimension forms an ${S}^{1}/{Z}_{2}\ifmmode\times\else\texttimes\fi{}{Z}_{2}^{\ensuremath{'}}$ orbifold with the zero mode fermions corresponding to standard model quarks localized at one fixed point. Zero modes corresponding to left (right)-chiral leptons are localized at (near) the other fixed point. This localization pattern is motivated by the symmetries of the model. Shifting the right-handed neutrinos and charged leptons slightly from the fixed point provides a new mechanism for understanding the absence of relations of the type ${m}_{e}={m}_{u}$ or ${m}_{e}={m}_{d}$ in quark-lepton symmetric models. Flavor changing neutral currents resulting from Kaluza-Klein gluon exchange, which typically arise in the quark sector of split fermion models, are suppressed due to the localization of quarks at one point. The separation of quarks and leptons in the compact extra dimension also acts to suppress the proton decay rate. This permits the extra dimension to be much larger than that obtained in a previous construct, with the bound $1/R\ensuremath{\gtrsim}30\text{ }\text{ }\mathrm{TeV}$ obtained.

Orbifold reduction of the quark-lepton symmetric model

We investigate the quark-lepton symmetric gauge group in five dimensions, with the gauge symmetry broken by a combination of orbifold compactification of the extra dimension and the Higgs mechanism. The gauge sector of the model is investigated and contrasted with the four dimensional case. We obtain lower bounds on the mass of the exotic gauge bosons, the inverse compactification scale and the exotic leptons. Light neutrinos are obtained without requiring any scale larger than a TeV. However an ultraviolet cutoff of order ${10}^{11}\text{ }\text{ }\mathrm{GeV}$ is required to suppress proton decay inducing nonrenormalizable operators.

New and improved quark-lepton symmetric models

We show how the use of a see-saw mechanism based on a 3 × 3 neutrino mass matrix texture can considerably simplify Higgs sectors for quark-lepton symmetric models (and for Standard Model extensions generally). The main theory we discuss also incorporaties a previously considered scenario whereby the charged lepton of a particular generation is necessarily less massive than the corresponding charge +2 3 quark, provided intergenerational mixing is small.

Phenomenology of the two Higgs doublet sector of a quark-lepton symmetric model.

In the simplest examples of models with a discrete quark-lepton symmetry, an electroweak symmetry breaking sector with more than one Higgs doublet is necessary to obtain the correct mass relations between quarks and leptons. A two Higgs doublet model has flavour-nonconserving Yukawa couplings, which are proportional to the masses of the quark-lepton symmetric partners of the fermions. We describe how flavour changing leptonic decays can occur, with branching ratios not far beyond that currently measureable, enabling investigation of the phenomenology of such models.

Study of the simplest realistic Higgs sector in quark-lepton symmetric models.

A discrete symmetry between quarks and (generalized) leptons can exist in nature, and its spontaneous symmetry breaking scale can be as low as a few TeV. Such a discrete symmetry also has interesting implications for how electroweak symmetry is spontaneously broken, because the simplest version of the theory requires two electroweak Higgs doublets rather than one in order to provide acceptable values for quark and lepton masses. The effective theory generated at electroweak-scale energies is thus a particular type of two-Higgs-doublet model. We point out in this paper that the broken discrete symmetry imposes very interesting constraints on the form of the Yukawa couplings between physical Higgs bosons and quarks and leptons. In particular, we find that the flavour-changing neutral Higgs couplings to down-sector quarks are proportional to the neutrino Dirac mass matrix. If neutrinos are Dirac particles, then the severe experimental upper bounds on their mass values renders tree-level neutral flavour-changing Higgs effects on down-quark systems like $K^0-\bar{K}^0$ negligibly small. We also discuss minimization of some relevant Higgs potentials and some other pertinent phenomenological issues.

Baryogenesis, domain walls and the role of gravity

It has recently been speculated that global symmetries are broken by gravity. We propose a scenario for the generation of the baryon asymmetry in the early Universe in which the domain walls predicted by theories with discrete symmetries become unstable due to these Planck scale effects. The relative motion of the decaying walls can provide a mechanism for the departure from thermal equilibrium necessary to have a nonvanishing baryon asymmetry. In particular, we implement this idea within the frameworks of the left-right and quark-lepton symmetric models.

GLUEBALL MEDIATED DECAY OF MASSIVE NEUTRINOS IN QUARK-LEPTON SYMMETRIC MODELS

The mysterious similarity of quarks and leptons may be invoked to motivate the hypothesis of an exact discrete symmetry between the quarks and the leptons which is broken spontaneously. This hypothesis predicts the existence of heavy bound states and light glueballs associated with an unbroken SU(2)′ gauge symmetry. We show that some of these bound states can mix significantly with the ordinary leptons. An interesting consequence of this mixing is that a massive neutrino can decay surprisingly quickly via real or virtual SU(2)′ glueball emission. Hence, we argue that massive neutrinos in generic quark-lepton symmetric models may decay fast enough to be consistent with the energy density bound in the standard big bang model of cosmology.

Discrete quark-lepton symmetry need not pose a cosmological domain wall problem.

Quarks and leptons may be related to each other through a spontaneously broken discrete symmetry. Models with acceptable and interesting collider phenomenology have been constructed which incorporate this idea. However, the standard Hot Big Bang model of cosmology is generally considered to eschew spontaneously broken discrete symmetries because they often lead to the formation of unacceptably massive domain walls. We point out that there are a number of plausible quark-lepton symmetric models which do not produce cosmologically troublesome domain walls. We also raise what we think are some interesting questions concerning anomalous discrete symmetries.

Extended weak isospin and fermion masses in a unified model.

It is interesting to ask whether the weak-isospin gauge group ${\mathrm{SU}(2)}_{L}$ is embedded in a larger symmetry group. From a model-building perspective there are usually too many possible answers to this question, because extensions invariably introduce new gauge anomalies. New fermions then have to be postulated, usually in an ad hoc manner, to cancel these anomalies. However, the quark-lepton symmetric models of Foot and Lew allow one to extend the weak-isospin group in a disciplined way, because the new anomalies can now cancel between quarks and generalized leptons. The basics of a model with gauge group $G={[\mathrm{SU}(3)]}^{4}$ are presented in this paper. We find that, through left-right symmetry, a partial unification of the gauge coupling constants of the theory naturally suggests itself. We also find that the symmetries of the model can impose restrictions on fermion masses and mixing angles at the tree level, which renormalization effects and mixing phenomena may modify in a predictive manner.

Models of extended Pati-Salam gauge symmetry.

The possibility of constructing nonminimal models of the Pati-Salam type is investigated. The most interesting examples are found to have an SU(6){direct product}SU(2){sub {ital L}}{direct product}SU(2){sub {ital R}} gauge invariance. Two interesting symmetry-breaking patterns are analyzed: one leading to the theory of SU(5) color at an intermediate scale, the other to the quark-lepton symmetric model. In both cases there is ample room for new physics in the interesting energy range of 100 GeV to a few TeV. We also identify a new candidate for dark matter.