Flavor-changing Neutral-current Problem Research Articles

A Two-Higgs-Doublet Model without Flavor-Changing Neutral Currents at Tree-Level

The flavor-changing neutral current (FCNC) problem at tree-level is a very critical defect of the two Higgs doublet extension of standard model (SM). In this article, a two-Higgs-doublet model (2HDM) in which such defects do not exist at all is to be demonstrated. The general pattern of matrix pairs which can be diagonalized simul-taneously by a same unitary transformation is proposed without extra constraints like symmetries or zeros in M matrices. Only an assumption of the hermiticity of mass matrices is employed in the derivation. With this assumption, number of parameters in the mass matrix of a specific fermion type is reduced from eighteen down to five. Eigenvalues and eigenvectors are analytically derived and it is surprising that unitary transformation matrix thus derived depends on only two parameters. It is a very general and elegant way to solve the tree-level FCNC problem radically and it includes previous similar models as special cases with specific parameter values.

Splitting mass spectra and muon g− 2 in Higgs-anomaly mediation

We propose a scenario where only the Higgs multiplets have direct couplings to a supersymmetry (SUSY) breaking sector. The standard model matter multiplets as well as the gauge multiples are sequestered from the SUSY breaking sector; therefore, their masses arise via anomaly mediation at the high energy scale with a gravitino mass of $\sim$100 TeV. Due to renormalization group running effects from the Higgs soft masses, the masses of the third generation sfermions become O(10) TeV at the low energy scale, while the first and second generation sfermion masses are O(0.1-1) TeV, avoiding the tachyonic slepton problem and flavor changing neutral current problem. With the splitting mass spectrum, the muon g-2 anomaly is explained consistently with the observed Higgs boson mass of 125 GeV. Moreover, the third generation Yukawa couplings are expected to be unified in some regions.

Phenomenological aspects of gauge mediation with sequestered supersymmetry breaking in light of dark matter detection

In a recent work, a model of gauge mediation with sequestered supersymmetry (SUSY) breaking was proposed. In this model, the mass of the gravitino is O(100) GeV without causing the flavor-changing neutral-current problem. In contrast to traditional gauge mediation, the gravitino is not the lightest SUSY particle and the neutralino is the candidate of the dark matter. In this paper, we investigate phenomenological aspects of this model and discuss the possibility of the direct detection of the dark matter. In particular, we focus on the light neutralino case and find that the light-Higgsino scenario such as the focus point is interesting, taking account of the recent CDMS result.

Top quark flavor-changing neutral-current decays and productions at LHC in the littlest Higgs model withTparity

In the littlest Higgs model with $T$-parity (LHT) the newly introduced mirror quarks have flavor-changing couplings with the standard model (SM) quarks and may enhance the flavor-changing neutral-current (FCNC) top-quark interactions which are extremely suppressed in the SM. In this work we perform a comprehensive study for the contributions of these mirror fermions to various top-quark FCNC decays and productions at the LHC, which includes the decays $t\ensuremath{\rightarrow}cV$ ($V=g$, $\ensuremath{\gamma}$, $Z$), $t\ensuremath{\rightarrow}cgg$ and the productions proceeding through the parton processes $cg\ensuremath{\rightarrow}t$, $gg\ensuremath{\rightarrow}t\overline{c}$, $cg\ensuremath{\rightarrow}tg$, $cg\ensuremath{\rightarrow}t\ensuremath{\gamma}$ and $cg\ensuremath{\rightarrow}tZ$. We find that although these FCNC processes can be greatly enhanced by the LHT contributions, they are hardly accessible at the LHC. Therefore, the LHT model may not cause the FCNC problem in the top-quark sector if the top-quark property is proved to be SM-like at the LHC.

Radius stabilization by constant boundary superpotentials in warped space

A warped space model with a constant boundary superpotential has been an efficient model both to break supersymmetry and to stabilize the radius, when a hypermultiplet, a compensator, and a radion multiplet are taken into account. In such a model of radius stabilization, the radion and moduli masses, the gravitino mass, and the induced soft masses are studied. We find that a lighter physical mode composed of the radion and the moduli can have mass of the order of a TeV and that the gravitino mass can be of the order of 10{sup 7} GeV. It is also shown that soft mass induced by the anomaly mediation can be of the order of 100 GeV and can be dominant compared to that mediated by bulk fields. Localized F-terms and D-terms are discussed as candidates for canceling the cosmological constant. We find that there is no flavor changing neutral current problem in a wide range of parameters.

Composite Higgs from higher representations

We investigate new models of dynamical electroweak symmetry breaking resulting from the condensation of fermions in higher representations of the technicolor group. These models lie close to the conformal window, and are free from the flavor-changing neutral current problem despite small numbers of flavors and colors. Their contribution to the S parameter is small and not excluded by precision data. The Higgs itself can be light and narrow.

Top-color-assisted supersymmetry

It has been known that the supersymmetric flavor changing neutral current problem can be avoided if the squarks take the following mass pattern, namely the first two generations with the same chirality are degenerate with masses around the weak scale, while the third generation is very heavy. We realize this scenario through the supersymmetric extension of a topcolor model with gauge mediated supersymmetry breaking.

On the soft supersymmetry-breaking parameters in gauge-mediated models

Gauge mediation of supersymmetry breaking in the observable sector is an attractive idea, which naturally alleviates the flavor changing neutral current problem of supersymmetric theories. Quite generally, however, the number and quantum number of the messengers are not known; nor is their characteristic mass scale determined by the theory. Using the recently proposed method to extract supersymmetry-breaking parameters from wave-function renormalization, we derived general formulae for the soft supersymmetry-breaking parameters in the observable sector, valid in the small and moderate tan β regimes, for the case of split messengers. The full leading-order effects of top Yukawa and gauge couplings on the soft supersymmetry-breaking parameters are included. We give a simple interpretation of the general formulae in terms of the renormalization group evolution of the soft supersymmetry-breaking parameters. As a by-product of this analysis, the one-loop renormalization group evolution of the soft supersymmetry-breaking parameters is obtained for arbitrary boundary conditions of the scalar and gaugino mass parameters at high energies.

Gauge-mediated SUSY breaking at an intermediate scale

Gauge-mediated SUSY breaking with a messenger scale of order $10^{15}$ GeV has some interesting features. It can solve the flavor changing neutral current problem of supersymmetric models with predictions for superpartner masses which are identical to those of minimal supergravity models. It can however also lead to theories with novel experimental signatures. For example, we present a model in which the gluino is the lightest supersymmetric particle. We also review the present experimental status of a stable gluino.

Effective potential approach to technicolor theories with slowly running coupling constant

We discuss the slowly running coupling constant hypothesis in technicolor, as proposed in order to avoid the flavor changing neutral current problem. We find that these theories are very sensitive to the location of the critical point for chirality breaking. We use the effective potential approach for composite operators to determine the phase structure and the fermion condensates for various groups and representations, and we find, within usual approximations, that only a few cases remain acceptable.

Discriminating technicolor theories through flavor-changing neutral currents: Slowly varying or fixed coupling constants?

On the basis of the analytical study of the ladder Schwinger-Dyson equation of the technifermion self-energy we find crucial constraints on the running coupling constants in technicolor theories for the flavor-changing neutral currents to be dynamically suppressed. It is unlikely that the slowly varying coupling constants in the asymptotically free technicolor theory can solve the flavor-changing neutral-current problem. Fixed-point theories may be the only viable possibility.

A walk with technicolor

We propose an ETC model based on SO(10) 2 which naturally accomodates a complete family of technifermions in the 7 of SO(7) TC plus three generations of quarks and leptons. The technicolor sector corresponds to one of the examples recently shown by Appelquist et al. to avoid the flavor-changing neutral-current problem. In order to obtain an interesting fermion mass spectrum we propose a composite version of the ETC model based on SO(10) 3.

Technidilaton

Phenomenological issues are investigated of the technidilaton which was predicted in a recently proposed scale invariant technicolor model whose large anomalous dimension at a nontrivial ultraviolet fixed point naturally resolves the flavor-changing neutral current problem. In particular, the possibility is examined that the technidilaton may be identified with ζ(2.23) observed by the Mark III experiment.

Hierarchic chiral condensate

Using the Schwinger-Dyson gap equation a chiral condensate is discussed in theories having a slowly varying running-coupling constant. It is shown that if the coupling strength λ remains close to a certain critical value λ c up to a large scale Λ, a chiral condensate may occur with the hierarchic nature σ(0)/Λ ≈ exp[ − π/(λ − λ c) 1 2 ] , σ(0) being the fermion d ynamical mass at zero-momentum. It is pointed out that such a condensate provides a possible solution to the flavor-changing neutral-current problem in the technicolor theory.

Scale-invariant hypercolor model and a dilaton.

We propose a scale-invariant hypercolor model with a nontrivial ultraviolet fixed point having large anomalous dimension, which resolves the notorious flavor-changing neutral-current problem in hypercolor models, and at the same time predicts a ${J}^{\mathrm{PC}}={0}^{++}$ Nambu-Goldstone boson (dilaton) associated with the spontaneous breakdown of the scale invariance.