Tree-level Flavor Changing Neutral Currents Research Articles

Lepton flavor specific extended Higgs model

In extended Higgs models, a discrete symmetry is needed in the quark sector to avoid tree-level flavor-changing neutral currents. However, this is not necessary the case in the lepton sector. We consider a model in which one Higgs couples to quarks and three others couple to the electron, muon and tau, respectively. This four-doublet model is presented with the full scalar potential and the gauge and Yukawa couplings. The constraints from boundedness, perturbativity and oblique parameters are incorporated as well as constraints from meson-antimeson mixing, radiative B-decays and the diphoton Higgs decay rate. We also consider bounds from searches for heavy neutral and charged scalars at the LHC. Since the Standard Model Higgs couplings match predictions very well, we focus on the alignment limit of the model. It is shown that for a wide range of parameters, the lightest additional scalar, pseudoscalar and charged scalar can have substantial decays into electrons and muons (in contrast to the usual leptonic decays into taus). An interesting signature in the neutral sector would be the production, through vector boson fusion, of a pair of scalars, each of which decays into an electron or muon pair.

Flavor-changing neutral currents in the Higgs sector

This review considers models with extended Higgs sectors in which there are tree-level flavor-changing neutral currents (FCNC) mediated by scalars. After briefly reviewing models without tree-level FCNC, several models with such currents are discussed. A popular mass-matrix ansatz, in which the flavor-changing couplings are the geometric mean of the individual flavor couplings, is presented. While it provided a target for experimentalists for three decades, it is now being severely challenged by experiments. Couplings expected to be of [Formula: see text] must be substantially smaller and the ansatz is now not favored. The minimal flavor violation hypothesis is introduced. Then specific models are presented, including the Branco–Grimus–Lavoura models. These models are not yet excluded experimentally, but they are highly predictive and will be tested once heavy Higgs bosons are discovered. We then turn to flavorful models and flavor-changing decays of heavy Higgs bosons, and it is shown that in many of these models, the heavy Higgs could predominantly decay in a flavor-changing manner (such as [Formula: see text] or [Formula: see text]) and experimentalists are encouraged to include these possibilities in their searches.

Axion-neutrino interplay in a gauged two-Higgs-doublet model

We propose a gauged two-Higgs-doublet model featuring an anomalous Peccei-Quinn symmetry, $U(1{)}_{PQ}$. Dangerous tree-level flavor-changing neutral currents, common in two-Higgs-doublet models, are forbidden by the extra gauge symmetry, $U(1{)}_{X}$. In our construction, the solutions to the important issues of neutrino masses, dark matter, and the strong $CP$ problems are interrelated. Neutrino masses are generated via a Dirac seesaw mechanism and are suppressed by the ratio of the $U(1{)}_{X}$ and the $U(1{)}_{PQ}$ breaking scales. Naturally small neutrino masses suggest that the breaking of $U(1{)}_{X}$ occurs at a relatively low scale, which may lead to observable signals in near-future experiments. Interestingly, spontaneous symmetry breaking does not lead to mixing between the $U(1{)}_{X}$ gauge boson, ${Z}^{\ensuremath{'}}$, and the standard $Z$. For the expected large values of the $U(1{)}_{PQ}$ scale, the associated axion becomes ``invisible,'' with Dine-Fischler-Srednicki-Zhitnitsky-like couplings, and may account for the observed abundance of cold dark matter. Moreover, a viable parameter space region, which falls within the expected sensitivities of forthcoming axion searches, is identified. We also observe that the flavor-violating process of kaon decaying into pion plus axion, ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}a$, is further suppressed by the $U(1{)}_{X}$ scale, providing a rather weak lower bound for the axion decay constant ${f}_{a}$.

Search for a heavy neutral Higgs boson in a left-right model with an inverse seesaw mechanism at the LHC

We develop a low scale left-right symmetric model based on $SU(3)_C\times SU(2)_L\times SU(2)_R\times U(1)_{B-L}\times Z_2$ with a simplified Higgs sector consisting of only one bidoublet and one $SU(2)_R$ doublet. In this model, the tiny values of light neutrino masses are generated through an inverse-seesaw mechanism. We emphasize that in this setup, the tree-level flavor changing neutral current can be strongly suppressed, consistent with the current experimental constraints. We show that the lightest $CP$-even Higgs boson, which is like the standard model Higgs boson, and the next lightest Higgs boson, $h'$, are generated from the neutral components of the bidoublet. We show that the mass of the next lightest Higgs boson can be of an order a few hundred~\text{GeVs}. We analyze the detection of $h'$ at the Large Hadron Collider (LHC) for a center-of-mass energy $\sqrt{s}=14~\text{TeV}$ and integrated luminosity $L_{\text{int}}=300~{\text{fb}}^{-1}$ via di-Higgs channel: $h'\to hh\to b\bar{b}\gamma\gamma$ and also in the $ZZ$ channel: $h'\to ZZ\to 4\ell~(\ell=e,~\mu)$ at an integrated luminosity $L_{\text{int}}=3000~{\text{fb}}^{-1}$. We consider three benchmark points for this analysis with $m_{h'}=250~\text{GeV},~400~\text{GeV}$, and $600~\text{GeV}$. We show that promising signals with good statistical significances can be obtained in di-Higgs channel, with $2\gamma+2b$-jets final states.

Suppression of the Higgs boson dimuon decay

It is often stated that elimination of tree-level flavor-changing neutral currents in multi-Higgs models requires that all fermions of a given charge to couple to the same Higgs boson. A counterexample was provided by Abe, Sato and Yagyu in a muon-specific two-Higgs doublet model with a softly-broken $Z_4$ symmetry. In this model, all fermions except the muon couple to one Higgs and the muon couples to the other. We study the phenomenology of the model and show that there is a wide range of parameter-space in which the branching ratios of the 125 GeV Higgs are very close to their Standard Model values, with the exception of the branching ratio into muons, which can be substantially suppressed -- this is an interesting possibility, since the current value of this branching ratio is $0.5 \pm 0.7$ times the Standard Model value. We also study the charged Higgs boson and show that, if it is lighter than $200$ GeV, it could have a large branching ratio into $\mu\nu$ - even substantially larger than the usual decay into $\tau\nu$. The decays of the heavy neutral scalars are also studied. The model does have a relationship between the branching ratios of the 125 GeV Higgs into $Z$'s, $\tau$'s and $\mu$'s, which can be tested in future accelerators.

The direct CP violation in a general two Higgs doublet model

In this paper, we study the CP violating processes in a general two-Higgs-doublet model (2HDM) with tree-level flavor changing neutral currents. In this model, sizable Yukawa couplings involving top and charm quarks are still allowed by the collider and flavor experiments, while the other couplings are strongly constrained experimentally. The sizable couplings, in general, have imaginary parts and could largely contribute to the CP violating observables concerned with the B and K mesons. In particular, the contribution may be so large that it affects the direct CP violating K meson decay, where the discrepancy between the experimental result and the Standard Model prediction is reported. We discuss how well the anomaly is resolved in the 2HDM, based on study of the other flavor observables. We also propose the way to test our 2HDM at the LHC.

Multi-Higgs-doublet models and singular alignment

We consider a 4-Higgs-doublet model in which each Higgs doublet gives mass to one of the fermion sets {mt}, {mb, mτ, mc}, {mμ, ms}, and {md, mu, me}. The sets have the feature that within each of them the masses are similar. Our model explains the mass hierarchies of the sets by hierarchies of the vacuum expectation values of the Higgs doublets associated to them. All Yukawa couplings are therefore of order one. Neutrino masses are generated by a type-I seesaw mechanism with PeV-scale singlet neutrinos. To avoid the appearance of tree-level flavour changing neutral currents, we assume that all Yukawa matrices are singularly aligned in flavour space. We mean by this that the Yukawa matrices are given as linear combinations of the rank 1 matrices that appear in the singular value decomposition of the mass matrix. In general, singular alignment allows to avoid flavour changing neutral currents in models with multiple Higgs doublets.

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.

Charged Higgs boson contribution to Bq−→ℓν¯ and B¯→(P,V)ℓν¯ in a generic two-Higgs doublet model

We comprehensively study the charged Higgs boson contributions to the leptonic ${B}_{q}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\ell}\overline{\ensuremath{\nu}}$ ($q=u$, $c$) and semileptonic $\overline{B}\ensuremath{\rightarrow}{X}_{q}\ensuremath{\ell}\overline{\ensuremath{\nu}}$ (${X}_{u}=\ensuremath{\pi}$, $\ensuremath{\rho}$; ${X}_{c}=D$, ${D}^{*}$) decays in the type-III two-Higgs doublet model. We employ the Cheng-Sher ansatz to suppress the tree-level flavor-changing neutral currents in the quark sector. When the strict constraints from the $\mathrm{\ensuremath{\Delta}}B=2$, $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$, and $pp(b\overline{b})\ensuremath{\rightarrow}H/A\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ processes are considered, parameters ${\ensuremath{\chi}}_{tq}^{u}$ from the quark couplings and ${\ensuremath{\chi}}_{\ensuremath{\ell}}^{\ensuremath{\ell}}$ from the lepton couplings dictate the leptonic and semileptonic $B$ decays. It is found that, when the measured ${B}_{u}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\tau}\overline{\ensuremath{\nu}}$ and indirect bound of ${B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\tau}\overline{\ensuremath{\nu}}$ obtained by LEP1 data are taken into account, $R(D)$ and $R(\ensuremath{\pi})$ can have broadly allowed ranges; however, the values of $R(\ensuremath{\rho})$ and $R({D}^{*})$ are limited to approximately the standard model (SM) results. We also find that the same behaviors also occur in the $\ensuremath{\tau}$-lepton polarizations and forward-backward asymmetries (${A}_{FB}^{{X}_{q},\ensuremath{\tau}}$) of the semileptonic decays, with the exception of ${A}_{FB}^{{D}^{*},\ensuremath{\tau}}$, for which the deviation from the SM due to the charged Higgs boson effect is still sizable. In addition, the ${q}^{2}$-dependent ${A}_{FB}^{\ensuremath{\pi},\ensuremath{\tau}}$ and ${A}_{FB}^{D,\ensuremath{\tau}}$ can be very sensitive to the charged Higgs boson effects and have completely different shapes from the SM.

Charged lepton flavor violation and electric dipole moments in the inert Zee model

The inert Zee model is an extension of the Zee model for neutrino masses to allow for a solution to the dark matter problem that involves two vector-like fields, a doublet and a singlet of SU(2)L, and two scalars, also a doublet and a singlet of SU(2)L, all of them being odd under an exact Z2 symmetry. The introduction of the Z2 guarantees one-loop neutrino masses, forbids tree-level Higgs-mediated flavor changing neutral currents and ensures the stability of the dark matter candidate. Due to the natural breaking of lepton numbers in the inert Zee model and encouraged by the ambitious experimental program designed to look for charged lepton flavor violation signals and the electron electric dipole moment, we study the phenomenology of the processes leading to these kind of signals, and establish which are the most promising experimental perspectives on that matter.

Heavy charged scalars from coverline{s} fusion: a generic search strategy applied to a 3HDM with U(1) \xd7 U(1) family symmetry

We describe a class of three Higgs doublet models (3HDMs) with a softly broken U(1) × U(1) family symmetry that enforces a Cabibbo-like quark mixing while forbidding tree-level flavour changing neutral currents. The hierarchy in the observed quark masses is partly explained by a softer hierarchy in the vacuum expectation values of the three Higgs doublets. As a consequence, the physical scalar spectrum contains a Standard Model (SM) like Higgs boson h125 while exotic scalars couple the strongest to the second quark family, leading to rather unconventional discovery channels that could be probed at the Large Hadron Collider. In particular, we describe a search strategy for the lightest charged Higgs boson H±, through the process coverline{s}to {H}^{+}to {W}^{+}{h}_{125} , using a multivariate analysis that leads to an excellent discriminatory power against the SM background. Although the analysis is applied to the proposed class of 3HDMs, we employ a model-independent formulation such that it can be applied to any other model with the same discovery channel.

Constraints and implications on Higgs FCNC couplings from precision measurement of Bs→μ+μ− decay

We study constraints and implications of the recent LHCb measurement of ${\cal B}(B_s \to \mu^+\mu^-)$ for tree-level Higgs-mediated flavor-changing neutral current (FCNC) interactions. Combined with experimental data on $B_s$ mass difference $\Delta m_s$, the $h \to \mu \tau$, and the $h \to \tau^+\tau^-$ decay branching ratios from the LHC, we find that the Higgs FCNC couplings are severely constrained. The allowed regions for $B_s \to \mu \tau$, $\tau\tau$ and $h \to sb$ decays are obtained. Current data allow large CP violation in the $h \to \tau^+ \tau^-$ decay. Consequences of the Cheng-Sher ansatz for the Higgs Yukawa couplings are discussed in some detail.

High scale flavor alignment in two-Higgs doublet models and its phenomenology

The most general two-Higgs doublet model (2HDM) includes potentially large sources of flavor changing neutral currents (FCNCs) that must be suppressed in order to achieve a phenomenologically viable model. The flavor alignment ansatz postulates that all Yukawa coupling matrices are diagonal when expressed in the basis of mass-eigenstate fermion fields, in which case tree-level Higgs-mediated FCNCs are eliminated. In this work, we explore models with the flavor alignment condition imposed at a very high energy scale, which results in the generation of Higgs-mediated FCNCs via renormalization group running from the high energy scale to the electroweak scale. Using the current experimental bounds on flavor changing observables, constraints are derived on the aligned 2HDM parameter space. In the favored parameter region, we analyze the implications for Higgs boson phenomenology.

Strong CP problem and spontaneous generation of the CP violating phase in the CKM matrix

We show that in a complementary two-Higgs doublet model (C2HDM) the $CP$ violating phase in the Cabibbo-Kobayashi-Maskawa matrix can be generated spontaneously, dangerous flavor changing neutral currents (FCNC) can be naturally suppressed and the strong $CP$ problem can also be avoided. The two Higgs doublets in the model are complementary in the sense that none of them is enough to describe masses of a given type of quarks. We find that the strength of FCNC is suppressed by the strength of Yukawa couplings of the first generation quark and the tree-level FCNC is sufficiently small. Using an explicit example, we show that radiative correction to the assumed Yukawa couplings can modify the discussion about the strong $\ensuremath{\theta}$. The correction to the strong $\ensuremath{\theta}$ is estimated to be less than around ${10}^{\ensuremath{-}12}\ensuremath{\sim}{10}^{\ensuremath{-}10}$ which can be tested in future experiments.

Phenomenology of the SU(3)C⊗SU(2)L⊗SU(3)R⊗U(1)X gauge model

We study the left-right asymmetric model based on SU(3)_C \otimes SU(2)_L \otimes SU(3)_R \otimes U(1)_X gauge group, which improves the theoretical and phenomenological aspects of the known left-right symmetric model. This new gauge symmetry yields that the fermion generation number is three, and the tree-level flavor-changing neutral currents arise in both gauge and scalar sectors. Also, it can provide the observed neutrino masses as well as dark matter automatically. Further, we investigate the mass spectrum of the gauge and scalar fields. All the gauge interactions of the fermions and scalars are derived. We examine the tree-level contributions of the new neutral vector, Z'_R, and new neutral scalar, H_2, to flavor-violating neutral meson mixings, say K-\bar{K}, B_d-\bar{B}_d, and B_s-\bar{B}_s, which strongly constrain the new physics scale as well as the elements of the right-handed quark mixing matrices. The bounds for the new physics scale are in agreement with those coming from the \rho-parameter as well as the mixing parameters between W, Z bosons and new gauge bosons.

Constraints on a Class of Two-Higgs Doublet Models with tree level FCNC

We analyse a class of two Higgs doublet models where flavour-changing neutral currents (FCNC) are present at tree level in a mixing-suppressed manner. In this class of models, because of a discrete symmetry imposed on the lagrangian, the FCNC couplings in the quark and lepton sector are fixed in terms of the corresponding mixing matrix (CKM or PMNS), the fermion masses and the ratio v2/v1 of the vacuum expectation values of the neutral scalars. A large number of processes, including tree and loop level transitions mediated by the new charged or neutral scalars are used as constraints. It is shown that among the interesting phenomenological prospects for these models, the new scalars may have masses within experimental reach.

The simplest 3-3-1 model

A simple extension of the Standard Model (SM), based on the gauge group [Formula: see text] with [Formula: see text] being the hypercharge, is considered. We show that, by imposing an approximate global [Formula: see text] custodial symmetry at the SM energy scale, the [Formula: see text] mixing is absent at tree level and the value of the [Formula: see text] parameter can be kept close to one. Tree-level flavor-changing neutral currents (FCNCs) are also reduced to three particles, namely [Formula: see text], a CP-odd Higgs and a CP-even Higgs. The model predicts new leptons with electric charges of ±1/2e and new quarks with ±1/6e charges as well as new gauge and scalar bosons with ±1/2e charges. Electric charge conservation requires that one of them must be stable. Their masses are unfortunately free parameters.

Higgs boson-μ−τcoupling at high and low energy colliders

There is no tree-level flavor changing neutral current (FCNC) in the standard model (SM) which contains only one Higgs doublet. If more Higgs doublets are introduced for various reasons, the tree-level FCNC would be inevitable, except that extra symmetry was imposed. Therefore, FCNC processes are an excellent probe for physics beyond the SM (BSM). In this paper, we study the lepton flavor violated decay processes $h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ and $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$ induced by the Higgs boson-$\ensuremath{\mu}\text{\ensuremath{-}}\ensuremath{\tau}$ vertex. For $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$, its branching ratio is also related to the $ht\overline{t}$, $h{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$, and $h{W}^{+}{W}^{\ensuremath{-}}$ vertices. We categorize the BSM into two scenarios for the Higgs boson coupling strengths near or away from the SM. For the latter scenario, we take the spontaneously broken two Higgs doublet model (the Lee model) as an example. We consider the constraints by recent data from the LHC and B factories, and we find that the measurements give weak constraints. At LHC run II, $h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ will be confirmed or will have a stricter limit set on its branching ratio. Accordingly, $\mathrm{Br}(\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma})\ensuremath{\lesssim}\mathcal{O}({10}^{\ensuremath{-}10}\ensuremath{-}{10}^{\ensuremath{-}8})$ for general chosen parameters. For the positive case, $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$ can be discovered with $\mathcal{O}({10}^{10})$ $\ensuremath{\tau}$ pair samples at the SuperB factory, the Super $\ensuremath{\tau}$-charm factory, and the new Z factory. The future measurements for $\mathrm{Br}(h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau})$ and $\mathrm{Br}(\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma})$ will be used to distinguish these two scenarios or set strict constraints on the correlations among different Higgs couplings.

Higgs and dark matter physics in the type-II two-Higgs-doublet model inspired by E 6 GUT

We study Higgs and dark matter physics in the type-II two-Higgs-doublet model (2HDM) with an extra U(1) H gauge symmetry, inspired by the E 6 grand unified theory (GUT). From the viewpoint of the bottom-up approach, the additional U(1) H gauge symmetry plays a crucial role in avoiding the tree-level flavor changing neutral currents mediated by neutral Higgs bosons in general 2HDMs. In the model with U(1) H gauge symmetry, which has Type-II Yukawa couplings, we have to introduce additional chiral fermions that are charged under the U(1) H gauge symmetry as well as under the Standard-Model (SM) gauge symmetry in order to cancel chiral gauge anomalies. For the U(1) H charge assignment and the extra matters, we adopt the ones inspired by the E 6 GUT: the extra quark-like and lepton-like fermions with the non-trivial U(1) H charges. We discuss their contributions to the physical observables, such as the measurements of Higgs physics and electro-weak interactions, and investigate the consistency with the experimental results. Furthermore, we could find extra neutral particles like the SM neutrinos after the electro-weak symmetry breaking, and they could be stable, because of the remnant symmetry after U(1) H symmetry breaking. We also discuss the thermal relic density and the (in)direct-detections of this dark matter candidate.

Measuring CP violation in two-Higgs-doublet models in light of the LHC Higgs data

In Two-Higgs-Doublet models, the conditions for CP violation can be expressed in terms of invariants under U(2) rotations among the two SU(2) Higgs doublet fields. In order to design a strategy for measuring the invariants we express them in terms of observables, i.e., masses and couplings of scalar bosons. We find amplitudes directly sensitive to the invariants. Observation of the Standard-Model-like Higgs boson at the LHC severely constrains the models. In particular, in the model with Z_2 symmetry imposed on dimension-4 terms (in order to eliminate tree-level flavour-changing neutral currents), CP violation is strongly suppressed. On the other hand, the most general Two-Higgs-Doublet model (without Z_2 symmetry) would still allow for CP violation to be present in the model, without being in conflict with the LHC data. Consequently, also flavour-changing neutral currents would in general be expected. We briefly sketch a strategy for measuring the remaining CP violation.