Flavor Changing Neutral Current Effects Research Articles

Neutrino masses and mixing in models with large extra dimensions and localized fermions

Using a low-energy effective field theory approach, we study some properties of models with large extra dimensions, in which quarks and leptons have localized wave functions in the extra dimensions. We consider models with two types of gauge groups: (i) the Standard-Model gauge group, and (ii) the left-right symmetric (LRS) gauge group. Our main focus is on the lepton sector of models with $n=2$ extra dimensions, in particular, neutrino masses and mixing. We analyze the requisite conditions that the models must satisfy to be in accord with data and present a solution for lepton wave functions in the extra dimensions that fulfills these conditions. As part of our work, we also present a new solution for quark wave function centers. Issues with flavor-changing neutral current effects are assessed. Finally, we remark on baryogenesis and dark matter in these models.

Affleck-Dine inflation

The Affleck-Dine mechanism in its simplest form provides baryogenesis from the out-of-equilibrium evolution of a complex scalar field with a simple renormalizable potential. We show that such a model, supplemented by nonminimal coupling to gravity, can also provide inflation, consistent with Planck constraints, simultaneously with the generation of the baryon asymmetry. The predictions of the model include significant tensor-to-scalar ratio and possibly baryon isocurvature fluctuations. The reheating temperature is calculable, making the model fully predictive. We require color triplet scalars for reheating and transfering the primordial baryon asymmetry to quarks; these could be observable at colliders. They can also be probed at higher scales by searches for quark compositeness in dijet angular distributions, and flavor-changing neutral current effects.

Matching for FCNC effects in the flavour-symmetric SMEFT

We calculate the complete tree and one-loop matching of the dimension-six Standard Model Effective Field Theory (SMEFT) with unbroken U(3)5 flavour symmetry to the operators of the Weak Effective Theory (WET) which are responsible for flavour changing neutral current effects among down-type quarks. We also explicitly calculate the effects of SMEFT corrections to input observables on the WET Wilson coefficients, a necessary step on the way to a well-defined, complete prediction. These results will enable high-precision flavour data to be incorporated into global fits of the SMEFT at high energies, where the flavour symmetry assumption is widespread.

Study of Bs0 → ℓ+ℓ−γ decays in a nonuniversal Z′ model

We investigate [Formula: see text] decays in a nonuniversal [Formula: see text] model derived from the extension of the Standard Model (SM). Considering the [Formula: see text]-mediated flavor-changing neutral current (FCNC) effects, we calculate the branching ratio and forward–backward asymmetry [Formula: see text] for [Formula: see text] decay processes. We compare the obtained results with predictions of the SM and discuss the sensitivity of the observables to [Formula: see text] boson coupling parameters. We find that the branching ratios are enhanced by one order from SM predictions in [Formula: see text] model scenario. We also observe that the variation of forward–backward asymmetry with the [Formula: see text] boson coupling parameters portrays discrimination between NP effects and SM results.

Investigating the 331-Z′ mediated FCNC effects on b → sμ+μ− transitions

The rare semileptonic [Formula: see text] and [Formula: see text] decay modes mediated by the [Formula: see text] transition are studied in this paper. We analyze observables such as the differential branching ratio, lepton polarization asymmetry and forward–backward asymmetry within the standard model in the low [Formula: see text] region. We then perform an analysis of the same modes within the 331 or [Formula: see text] model in which a new heavy [Formula: see text] boson contributes. The effects of the [Formula: see text]-mediated flavor-changing neutral current (FCNC) transitions on these modes are investigated by varying the mass [Formula: see text] and the model parameter [Formula: see text]. The new [Formula: see text] couplings are constrained by flavor observables of [Formula: see text] mixing and branching ratio of [Formula: see text]. We find that for some values of these couplings, the observables are quite sensitive to [Formula: see text] effects and deviations from Standard Model can be large.

Search for doubly charged Higgs bosons through vector boson fusion at the LHC and beyond

Production and decays of doubly charged Higgs bosons at the LHC and future hadron colliders triggered by a vector boson fusion mechanism are discussed in the context of the minimal left-right symmetric model. Our analysis is based on the Higgs boson mass spectrum compatible with available constraints which include flavor changing neutral current (FCNC) effects and vacuum stability of the scalar potential. Though the parity breaking scale ${v}_{R}$ is large ($\ensuremath{\sim}$ few TeV) and scalar masses which contribute to FCNC effects are even larger, a consistent Higgs boson mass spectrum still allows us to keep doubly charged scalar masses below 1 TeV which is an interesting situation for LHC and future circular collider (FCC). We have shown that the allowed Higgs boson mass spectrum constrains the splittings (${M}_{{H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}}\ensuremath{-}{M}_{{H}_{1}^{\ifmmode\pm\else\textpm\fi{}}}$), closing the possibility of ${H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{W}_{1}^{\ifmmode\pm\else\textpm\fi{}}{H}_{1}^{\ifmmode\pm\else\textpm\fi{}}$ decays. Assuming that doubly charged Higgs bosons decay predominantly into a pair of same-sign charged leptons through the process $pp\ensuremath{\rightarrow}{H}_{1/2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}{H}_{1/2}^{\ensuremath{\mp}\ensuremath{\mp}}jj\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{\ensuremath{\mp}}{\ensuremath{\ell}}^{\ensuremath{\mp}}jj$, we find that for the LHC operating at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ with an integrated luminosity at the level of $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ (HL-LHC), there is practically no chance to detect such particles at the reasonable significance level through this channel. However, at 33 TeV HE-LHC and (or) 100 TeV FCC-hh, a wide region opens up for exploring the doubly charged Higgs boson mass spectrum. In FCC-hh, the doubly charged Higgs bosons mass up to 1 TeV can be easily probed.

Lowest limits on the doubly charged Higgs boson masses in the minimal left-right symmetric model

The doubly charged Higgs bosons ${H}_{1,2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$ would undoubtedly be clear messengers of the new physics. We discuss their mass spectrum and show how experimental data and relations between scalar masses put limits on it. In particular, both the masses of the particles ${H}_{1}^{0}$, ${A}_{1}^{0}$ that play a crucial role in flavor changing neutral current effects and the masses of the additional gauge bosons ${W}_{2}$, ${Z}_{2}$ are notably important. For instance, if ${M}_{{H}_{1}^{0},{A}_{1}^{0}}\ensuremath{\simeq}15\text{ }\text{ }\mathrm{TeV}$ and ${M}_{{W}_{2}}\ensuremath{\simeq}3.76\text{ }\text{ }\mathrm{TeV}$, then the lowest mass of ${H}_{1}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$ is 465 GeV. In contrast, because of the freedom in the parameter space of the full scalar potential, there is no lowest limit on the mass of ${H}_{2}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$. It is shown to which signals at hadron colliders such relatively light doubly charged scalars might correspond. LHC working at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ will enter into the region where the existence of such particles with minimal masses can be thoroughly explored for a much wider parameter space of the minimal and manifest version of the left-right symmetric model (MLRSM). Taking into account our considerations and present ATLAS and CMS exclusion limits on ${M}_{{H}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}}$, there exist already first partial bounds on some of the MLRSM scalar potential parameters.

Light charged Higgs boson in two-Higgs doublet model for CDFWjjanomaly

Motivated by recent anomalous CDF data on $Wjj$ events, we study a possible explanation within the framework of the two-Higgs doublet model. We find that a charged Higgs boson of mass $\sim$ 140 GeV with appropriate couplings can account for the observed excess. In addition, we consider the flavor-changing neutral current effects induced at loop level by the charged Higgs boson on the $B$ meson system to further constrain the model. Our study shows that the like-sign charge asymmetry $A_{s\ell}^b$ can be of ${\cal O}(10^{-3})$ in this scenario.

The physics of heavyZ′gauge bosons

The U(1)' symmetry associated with a possible heavy Z' would have profound implications for particle physics and cosmology. The motivations for such particles in various extensions of the standard model, possible ranges for their masses and couplings, and classes of anomaly-free models are discussed. Present limits from electroweak and collider experiments are briefly surveyed, as are prospects for discovery and diagnostic study at future colliders. Implications of a Z' are discussed, including an extended Higgs sector, extended neutralino sector, and solution to the mu problem in supersymmetry; exotic fermions needed for anomaly cancellation; possible flavor changing neutral current effects; neutrino mass; possible Z' mediation of supersymmetry breaking; and cosmological implications for cold dark matter and electroweak baryogenesis.

Suppressed flavor changing neutral current in new physics with shared flavor symmetry

Many extensions of the standard model (SM) generate contributions to flavor changing neutral current (FCNC) processes that must have sufficient flavor suppression to be consistent with experiments, if the new physics (NP) is associated with a scale of a TeV. Here we present a mechanism for suppressing the NP effects to FCNC processes while allowing for new non-SM $CP$ violating phases. We consider the possibility that the source of NP contributions to FCNC processes share the same flavor symmetry underlying the SM source of FCNC processes which are the quark and lepton mass matrices. We call this the principle of shared flavor symmetry. In the flavor symmetric limit, the quark and lepton mixing matrices have fixed forms and there are no NP FCNC processes. In the flavor symmetric limit, we take the quark mixing matrix to be the identity matrix and the lepton mixing matrix to be given by tri-bimaximal mixing. Realistic mixing matrices are obtained by the small breaking of the flavor symmetry. New contributions to FCNC processes arise because of nonuniversal breaking of the flavor symmetry in the quark and lepton mass matrices and the NP sources of FCNC processes. In particular, we will focus on new FCNC effects that arise due to the breaking of flavor symmetry only in the quark and charged lepton mass matrices but not in the NP sector. In this scenario, NP contributions to FCNC processes are linked to the source of flavor symmetry breaking in the quark and charged lepton mass matrices. The breaking of flavor symmetry in the NP sector is assumed to produce FCNC effects that are at most the size of NP FCNC effects due to the breaking of flavor symmetry in the quark and charged lepton mass matrices. To demonstrate the mechanism we use a two Higgs doublet model as an example of beyond the SM physics though one should be able to adapt this mechanism to other models of new physics.

Same-sign top pair production in an extra-dimension model of flavor at the CERN Large Hadron Collider

We study the same-sign top pair production mediated by the first Kaluza-Klein (KK) excitation of the gluon in the Randall-Sundrum model with flavor violation at the LHC, in which the nonuniversal couplings between fermions and KK gauge bosons will lead to observable tree-level flavor changing neutral current effects. We find that the same-sign top quarks produced in our case have property of high energy and high transverse momentum, and lead to an observable signal in the same-sign dilepton channel even when the mass of the KK gluon reach up to 3 TeV. We further investigate the potential of the LHC to probe the flavor violating parameters and find that the LHC can probe their values down to 0.06.

Quark masses in supersymmetricSU(3)color⊗SU(3)W⊗U(1)Xmodel with discreteT′flavor symmetry

We propose a supersymmetric model of quark flavor symmetry based on $\mathrm{SU}(3{)}_{\mathrm{color}}\ensuremath{\bigotimes}\mathrm{SU}(3{)}_{W}\ensuremath{\bigotimes}\mathrm{U}(1{)}_{X}\ensuremath{\bigotimes}{T}^{\ensuremath{'}}$ where ${T}^{\ensuremath{'}}$ (binary tetrahedral group) is a discrete flavor symmetry group. The third (heavy) generation of quarks are assigned to singlet representation of ${T}^{\ensuremath{'}}$ while light quark generations are treated differently and assigned to doublet representation of ${T}^{\ensuremath{'}}$. The model generates masses for heavy quarks as in 3-3-1 case. The quark mass terms for the second generation are obtained when ${T}^{\ensuremath{'}}$ symmetry is broken at leading order and for first generation with small higher order corrections. The flavor-changing neutral current effects are suppressed at leading order for $3\ensuremath{\rightarrow}2$ quark transitions due to ${T}^{\ensuremath{'}}$ breaking which is an attractive feature of the 3-3-1 $\ensuremath{\bigotimes}{T}^{\ensuremath{'}}$ model.

Top hypercharge

We propose a top hypercharge model with gauge symmetry SU(3)C × SU(2)L × U(1)1 × U(1)2 where the first two families of the Standard Model (SM) fermions are charged under U(1)1 while the third family is charged under U(1)2. The U(1)1 × U(1)2 gauge symmetry is broken down to the U(1)Ygauge symmetry, when a SM singlet Higgs field acquires a vacuum expectation value. We consider the electroweak constraints, and compare the fit to experimental observables to that of the SM. We study the quark CKM mixing between the first two families and the third family, the neutrino masses and mixing, the flavour changing neutral current effects in meson mixing and decays, the Z' discovery potential at the Large Hadron Collider, the dark matter with a gauged Z2 symmetry, and the Higgs boson masses.

Probing RS scenarios of flavour at the LHC via leptonic channels

We study a purely leptonic signature of the Randall-Sundrum scenario with Standard Model fields in the bulk at LHC: the contribution from the exchange of Kaluza- Klein (KK) excitations γ (n) , Z (n) of gauge bosons to the clear Drell-Yan reaction pp → γ/Z → l + l − . We show that, even with the low luminosities of the LHC initial regime, this contribution is detectable with the ATLAS apparatus for KK masses around the TeV scale and for sufficiently large lepton couplings to KK gauge bosons . Such large couplings can be compatible with electroweak precision data on the Z ¯ ff coupling in the framework of the custodial O(3) symmetry recently proposed, for specific configurations of lepton local- izations along the extra dimension. These configurations can simultaneously reproduce the correct lepton masses, while generating acceptably small flavour changing neutral current effects.

Complex CKM from spontaneous CP violation without flavor changing neutral current

We analyse the general constraints on unified gauge models with spontaneous CP breaking that satisfy the conditions that (i) CP violation in the quark sector is described by a realistic complex CKM matrix, and (ii) there is no significant flavor changing neutral current effects in the quark sector. We show that the crucial requirement in order to conform to the above conditions is that spontaneous CP breaking occurs at a very high scale by complex vevs of standard model singlet Higgs fields. Two classes of models are found, one consisting of pure Higgs extensions and the other one involving fermionic extensions of the standard model. We give examples of each class and discuss their possible embeddings into higher unified theories. One of the models has the interesting property that spontaneous CP violation is triggered by spontaneous P violation, thereby linking the scale of CP violation to the seesaw scale for neutrino masses.

Symmetries of two Higgs doublet model andCPviolation

We use the invariance of a physical picture under a change of Lagrangian, the reparameterization invariance in the space of Lagrangians and its particular case---the rephrasing invariance---for analysis of the two-Higgs-doublet extension of the standard model. We found that some parameters of theory like $\mathrm{tan}\ensuremath{\beta}$ are reparameterization dependent and therefore cannot be fundamental. We use the ${Z}_{2}$ symmetry of the Lagrangian, which prevents a ${\ensuremath{\phi}}_{1}\ensuremath{\leftrightarrow}{\ensuremath{\phi}}_{2}$ transition, and the different levels of its violation, soft and hard, to describe the physical content of the model. In general, the broken ${Z}_{2}$ symmetry allows for a $CP$ violation in the physical Higgs sector. We argue that the two-Higgs-doublet model with a soft breaking of ${Z}_{2}$ symmetry is a natural model in the description of electroweak symmetry breaking. To simplify the analysis, we choose among different forms of Lagrangian describing the same physical reality a specific one, in which the vacuum expectation values of both Higgs fields are real. A possible $CP$ violation in the Higgs sector is described by using a two-step procedure with the first step identical to a diagonalization of the mass matrix for $CP$-even fields in the $CP$-conserving case. We find a very simple, necessary, and sufficient condition for a $CP$ violation in the Higgs sector. We determine the range of parameters for which $CP$ violation and flavor-changing neutral current effects are naturally small---it corresponds to a small dimensionless mass parameter $\ensuremath{\nu}=\mathrm{Re}{m}_{12}^{2}/(2{v}_{1}{v}_{2})$. We show that for small $\ensuremath{\nu}$ some Higgs bosons can be heavy---with mass up to about 0.6 TeV---without violating of the unitarity constraints. If $\ensuremath{\nu}$ is large, all Higgs bosons except one can be arbitrarily heavy. We discuss, in particular, main features of this case, which corresponds for $\ensuremath{\nu}\ensuremath{\rightarrow}\ensuremath{\infty}$ to a decoupling of heavy Higgs bosons. In the model II for Yukawa interactions we obtain the set of relations among the couplings to gauge bosons and to fermions which allows us to analyze different physical situations (including $CP$ violation) in terms of these very couplings, instead of the parameters of Lagrangian.

Phenomenology of Twisted Moduli in Type I String Inspired Models

We make a first study of the phenomenological implications of twisted moduli in type I intersecting D5-brane models, focussing on the resulting predictions at the LHC using SOFTSUSY to estimate the Higgs and sparticle spectra. Twisted moduli can play an important role in giving a viable string realisation of sequestering in the limit where supersymmetry breaking comes entirely from the twisted moduli. We focus on a particular string inspired version of gaugino mediation in which the first two families are localised at the intersection between D5-branes, whereas the third family and Higgs doublets are allowed to move within the world-volume of one of the branes. The soft supersymmetry breaking third family sfermion mass terms are then in general non-degenerate with the first two families. We place constraints upon parameter space and predictions of flavour changing neutral current effects. Twisted moduli domination is studied and, as well as solving the most serious part of the SUSY flavour problem, is shown to be highly constrained. The constraints are weakened by switching on gravity-mediated contributions from the dilaton and untwisted T-moduli sectors. In the twisted moduli domination limit we predict a stop-heavy MSSM spectrum and quasi-degenerate lightest neutralino and chargino states with wino-dominated mass eigenstates.

Neutrino-induced lepton flavor violation in gauge-mediated supersymmetry breaking

Gauge-mediated supersymmetry breaking is known to greatly suppress flavor changing neutral current effects. However, we show that gauge mediation in the context of leptogenesis implies potentially large lepton flavor violating signals. If the heavy right-handed neutrinos that participate in leptogenesis are lighter than the messenger scale of gauge mediation, they will induce flavor off-diagonal masses to the sleptons which in turn can induce large effects in $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma},\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}\ensuremath{\gamma},$ and $\ensuremath{\mu}\ensuremath{-}e$ conversion in nuclei. We demonstrate this result and compute numerically the lepton-flavor violating decay and conversion rates in scenarios of direct gauge mediation.

Signals of supersymmetric flavor models inBphysics

If the mechanism of Supersymmetry breaking is not flavour blind, some flavour symmetry is likely to be needed to prevent excessive flavour changing neutral current effects. We discuss two flavour models (based respectively on a U(2) and on a SU(3) horizontal symmetry) providing a good fit to fermion masses and mixings and particularly constraining the supersymmetry soft breaking terms. We show that, while reproducing successfully the Standard Model fit of the unitarity triangle, it is possible to obtain sizable deviations from the Standard Model predictions for three very clean B-physics observables: the time dependent CP asymmetries in $B_d \to J/\psi K^0$ and in $B_s \to J/\psi \phi$ and the $B_s-\bar{B}_s$ mass difference. Our analysis exhibits with two explicit realizations that in supersymmetric theories with a new flavour structure in addition to the Yukawa matrices there exist concrete potentialities for revealing supersymmetry indirectly in theoretically clean $B$-physics observables.

CPViolation and Flavor Changing Effects inKandBMesons from Nonuniversal Soft Breaking Terms

We analyze CP violation and flavor changing effects in a minimal supersymmetric standard model with arbitrary nonuniversal soft breaking. Large flavor changing neutral current effects are naturally expected in the K system, even in the absence of quark-squark flavor misalignment. However, the B system is sensitive to new supersymmetric contributions only if nonuniversality implies, not only different soft terms for the three generations but also a large quark-squark misalignment. The only exceptions are processes with a leading chirality changing contribution proportional to tanbeta (as b-->sgamma).