Suppression Of Flavor-changing Neutral Currents Research Articles

Standard model flavor from an SU(2) symmetry

We propose that the flavor structure of the standard model is based on a horizontal $SU(2)$ symmetry. It generically predicts (i) a parametrically small mass for the lightest charged fermions, (ii) small mixings in the quark sector, and (iii) suppression of flavor-changing neutral currents. Supplemented with the assumption of a strong hierarchy between the second- and third- generation masses, it also predicts (iv) a large $CP$-violating phase in the quark sector. Only Majorana neutrinos allow for large mixings in the lepton sector. In this case, this framework further predicts (v) near-maximal $\theta_{23}^l$, (vi) a normal hierarchy of neutrino masses, and (vii) large $CP$ violation in the lepton sector.

FLAVOR SYMMETRY AND VACUUM ALIGNED MASS TEXTURES

A texture-zeros is an approach to reduce the number of free parameters in Yukawa couplings and it is one of the most attractive ones. In our paper, we discuss the origin of zero-structure in texture-zeros by S3 flavor symmetry approach. Some of electroweak doublet Higgs fields have vanishing vacuum expectation value (VEV) which leads to vanishing elements in quark and lepton mass matrices. Then, the structure of supersymmetric scalar potential is analyzed and Higgs fields have non-trivial S3 charges. As a prediction of our paper, a lower bound of a MNS matrix element, Ue3 ≥ 0.04, is obtained. The suppression of flavor-changing neutral currents (FCNC) mediated by the Higgs fields is discussed and lower bounds of the Higgs masses are derived.

Flavor Symmetry and Vacuum Aligned Mass Textures

A texture-zeros is an approach to reduce the number of free parameters in Yukawa couplings and it is one of the most attractive ones. In our paper, we discuss the origin of zero-structure in texture-zeros by S3 flavor symmetry approach. Some of electroweak doublet Higgs fields have vanishing vacuum expectation value (VEV) which leads to vanishing elements in quark and lepton mass matrices. Then, the structure of supersymmetric scalar potential is analyzed and Higgs fields have non-trivial S3 charges. As a prediction of our paper, a lower bound of a MNS matrix element, Ue3 ≥ 0.04, is obtained. The suppression of flavor-changing neutral currents (FCNC) mediated by the Higgs fields is discussed and lower bounds of the Higgs masses are derived.

Double suppression of FCNCs in supersymmetric models

A mechanism for double suppression of flavor-changing neutral currents (FCNCs) and CP violating phases in supersymmetric models is suggested. At M_{SUSY} they are suppressed due to a nonabelian discrete flavor symmetry, and the infrared attractive force of gauge interactions in extra dimensions are used to suppress them at the compactification scale. We present a concrete model, which is a simple extension of S_{3} invariant minimal supersymmetric standard model, where only SU(2)_{L} and SU(3)_{C} gauge multiplets are assumed to propagate in the bulk. We find that a disorder of two orders of magnitude in soft supersymmetry breaking parameters above the compactification scale may be allowed to satisfy experimental constraints on FCNC processes and CP violating phenomena.

Improved single sector supersymmetry breaking

Building on recent work by Arkani-Hamed and the present authors, we construct realistic models that break supersymmetry dynamically and give rise to composite quarks and leptons, all in a single strongly coupled sector. The most important improvement compared to earlier models is that the second-generation composite states correspond to dimension-2 ``meson'' operators in the ultraviolet. This leads to a higher scale for flavor physics and gives a completely natural suppression of flavor-changing neutral currents. We also construct models in which the hierarchy of Yukawa couplings is explained by the dimensionality of composite states. These models provide an interesting and viable alternative to gravity- and gauge-mediated models. The generic signatures are unification of scalar masses with different quantum numbers at the compositeness scale, and lighter gaugino, Higgsino, and third-generation squark and slepton masses. We also analyze large classes of models that give rise to local vacua with both compositeness and supersymmetry breaking, based on gauge theories with confining, fixed-point, or free-magnetic dynamics.

Quarks, squarks, and textures

By studying symmetric mass textures for the up and down quark sectors, and expanding in a small parameter $\ensuremath{\lambda}\ensuremath{\sim}\mathrm{sin}{\ensuremath{\theta}}_{C},$ bounds are set on entries commonly assumed to vanish. Consequences of a 2+1 family structure which can result from horizontal symmetry are examined. Generalizing to squarks, we study suppression of flavor-changing neutral currents by mass degeneracy and/or small mixing angles.

Family symmetry, fermion mass matrices and cosmic texture

The observed replication of fermions in three families is undoubtedly a reflection of a deeper symmetry underlying the standard model. In this paper we investigate one very elementary possibility, that physics above the grand unification scale is described by the symmetry group G × SU(3) fam with G a gauged grand unified group, and SU(3) fam a global family symmetry. The breaking of this symmetry at the GUT scale produces global texture, providing a mechanism for structure formation in the universe, and sets strong constraints on the low energy fermion mass matrix. With the addition of a 45 Higgs and certain assumptions about the relative strength of Higgs couplings, the simplest SU(5) theory yields an eight-parameter form for the fermion mass matrices, which we show is consistent with the thirteen observable masses and mixing angles. We discuss the natural suppression of flavour-changing neutral currents (FCNCs) and emphasise the rich low energy Higgs sector. With minor assumptions, the theory unifies consistently with the recent LEP data. We consider the extension to G = SO(10), where some simplification and further predictiveness emerges. Finally we discuss the dynamics of SU(3) fam texture and show that the known constraints on unification “predict” a GUT symmetry-breaking scale of the order required for cosmic structure formation, and by the recent detection of cosmic microwave anisotropy by COBE.

Raising condensates beyond the ladder

We expand the Cornwall-Jackiw-Tomboulis effective potential for the fermion propagator > {;Z(p)[ ▪+Σ(p)] >;}; −1 in powers of Σ( p)/ p, to all orders in the coupling α. We need only expand to Σ 2 when the running coupling is effectively constant over a large enough momentum range. Renormalization group arguments applied to the kernel of the resulting Schwinger-Dyson equation allow us to deduce properties of Σ( p), and in particular we obtain Σ( M) where M is an ultraviolet cutoff. This is useful for demonstrating the enhancement of technifermion condensates and the suppression of flavor-changing neutral currents in technicolor theories.

Chiral hierarchies and flavor-changing neutral currents in hypercolor.

Dynamical chiral-symmetry breaking in asymptotically free theories with a slowly running coupling is analyzed. When the confinement scale ..lambda.. is much less than the cutoff M beyond which the theory cannot be used in isolation, the dynamical mass ..sigma..(p) starts from a value approx. =..lambda.. for momenta papprox. ..lambda... It then takes on the asymptotic form (lnp)/sup a//p/sup 2/ where a>1. In hypercolor theories this behavior generates sufficiently large fermion masses for a higher value of M than naively expected. This in turn could allow for an adequate suppression of flavor-changing neutral currents.

New procedure for the estimation of the extended-hypercolor boson masses

It is found that the natural suppression of flavor-changing neutral currents and CP-violating processes which occur in theories with elementary scalars cannot occur in extended-hypercolor (EHC) theories, thereby confirming the serious flaw encountered with these theories. The procedure is based on the calculation of the EHC gauge-boson masses using Feynman rules in the mass-feed-down mechanism and renders these masses independent of a loop cutoff in the expected mass range.

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.

Natural suppression of flavour-changing neutral currents in supersymmetric gauge theories

Induced flavour-changing neutral currents (FCNC) in supersymmetric unified theories are investigated both in models with the standard SU(2) L × U(1) gauge symmetry and in models with an extra Ũ(1) gauge symmetry. Supersymmetric extension of the natural flavour conservation laws for neutral currents is obtained by adding a condition regarding the assumed type of supersymmetry breaking. This condition ensures no direct flavour-changing couplings of neutral gauge-Higgs fermions and at the same time is necessary and sufficient for the natural suppression of the induced FCNC. It is found that in the class of models satisfying the new condition the contribution of the scalar partners of quarks to the induced strangeness-changing neutral current is comparable to that of the quarks in K L → π π , while it is negligibly small in K L − K S mass difference.

Challenges for extended technicolour theories

We develop the dialectic between extended technicolour theories and phenomenological constraints on rare processes. The natural suppression of flavour-changing neutral currents and CP-violating processes which can occur in models with elementary scalars is not a general feature of naïve extended technicolour (ETC) models. We study the extent to which naïve ETC estimates must in fact be suppressed in order to be compatible with the phenomenology of such rare processes. We emphasize the potential significance of the exchanges of neutral flavour-changing bosons and find that the strongest constraints arise from considering the combination of the D 1 0-D 2 0 and K 1 0-K 2 0 systems. CP-violating effects in ETC models must be severely suppressed if they are to be compatible with the observational facts. We point to several rare processes whose further experimental study is of particular concern to ETC theories.

Little groups, discrete symmetry, and an Sp(4) × U(1) theory of the weak and electromagnetic interactions

We introduce a graphical method for determining the little groups of vacuum expectation values of Higgs mesons in $G\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ gauge theories. This method is particularly useful when rank $G=2$. A general method for natural suppression of intramultiplet mixings of equally charged fermions using discrete symmetries is given. For concreteness we develop these methods using the gauge group Sp(4) \ifmmode\times\else\texttimes\fi{} U(1) and present a quasivectorlike theory of the weak and electromagnetic interactions based on this group. This theory insures $e\ensuremath{-}\ensuremath{\mu}$ and Cabibbo universality, the absence of right-handed currents in neutron, hyperon, and muon decay, suppression of flavor-changing neutral currents and suppression of untenably large contributions to various weak processes such as the ${K}_{L}{K}_{S}$ mass difference and lepton-number-nonconserving decays. Discrete symmetries are used to discuss the fermion mass spectrum of this theory and, finally, to predict Cabibbo type mixing angles in terms of ratios of fermion masses.