FCNC Research Articles

Controlled flavour changing neutral couplings in two Higgs Doublet models

We propose a class of two Higgs doublet models where there are flavour changing neutral currents (FCNC) at tree level, but under control due to the introduction of a discrete symmetry in the full Lagrangian. It is shown that in this class of models, one can have simultaneously FCNC in the up and down sectors, in contrast to the situation encountered in the renormalisable and minimal flavour violating 2HDM models put forward by Branco et al. (Phys Lett B 380:119, 1996). The intensity of FCNC is analysed and it is shown that in this class of models one can respect all the strong constraints from experiment without unnatural fine-tuning. It is pointed out that the additional sources of flavour and CP violation are such that they can enhance significantly the generation of the Bbaryon asymmetry of the Universe, with respect to the standard model.

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.

Light charged Higgs boson scenario in 3-Higgs doublet models

The constraints from the measurements of the [Formula: see text] decay rate on the parameter space of 3-Higgs Doublet Models (3HDMs), where all the doublets have nonzero vacuum expectation values, are studied at the next-to-leading order in QCD. In order to naturally avoid the presence of flavour changing neutral currents at the tree level, we impose two softly-broken discrete [Formula: see text] symmetries. This gives rise to five independent types of 3HDMs that differ in their Yukawa couplings. We show that in all these 3HDMs (including the case of type-II-like Yukawa interactions) both masses of the two charged Higgs bosons [Formula: see text] and [Formula: see text] can be smaller than the top mass [Formula: see text] while complying with the constraints from [Formula: see text]. As an interesting phenomenological consequence, the branching ratios of the charged Higgs bosons decay into the [Formula: see text] final states can be as large as [Formula: see text] when their masses are taken to be below [Formula: see text] in two of the five 3HDMs (named as Type-Y and Type-Z). This light charged Higgs boson scenario provides a hallmark 3HDM signature that cannot be realised in [Formula: see text] symmetric 2-Higgs doublet models. We find that in the Type-Y and Type-Z 3HDMs the scenario with [Formula: see text], [Formula: see text] is ruled out by the direct searches at the LHC, but in the Type-Y 3HDM [Formula: see text] and [Formula: see text] is allowed by [Formula: see text] and direct searches at LEP2, Tevatron and LHC due to the reduced sensitivity of these searches to the degenerate case [Formula: see text]. The cases where only one or both charged Higgs bosons are above the top quark mass are also naturally allowed in the both Type-Y and Type-Z 3HDMs.

Analysis of Λb → Λl+ l− rare decays in a non-universal Z′ model

We investigate the rare baryonic Λb → Λl+l− decays in a non-universal Z′ model, which is one of the well-motivated extensions of the standard model (SM). Considering the effects of Z′-mediated flavour-changing neutral currents (FCNCs) we analyse the differential decay rate, forward-backward asymmetries and lepton polarisation asymmetries for the Λb → Λl+l− decays. We find significant deviations from their SM predictions, which could indicate new physics arising from the Z′ gauge boson.

Sensitivity of CLIC at 380 GeV to the top FCNC decay t → cH

In the Standard Model (SM), flavour changing neutral current (FCNC) top decays, possible at loop level only, are very strongly suppressed. Observation of any such decay would be a direct signature of physics beyond the SM. Large enhancements are possible in many “new physics” scenarios and the largest enhancement is in most cases expected for the t → cH decay. A full study for CLIC was based on the WHIZARD simulation of FCNC top decays within the 2HDM(III) model. Beam polarization and beam-induced background were taken into account. Top pair production events with the FCNC decay t → cH can be identified based on kinematic constrains and flavour tagging information. Due to a large overlap in the kinematic space with standard top pair events, the final signal selection-efficiency is small, at the 10% level. Expected limits on BR(t → cH) × BR(H → ) are compared with earlier results based on parton level simulation.

Using Spin Observables and Polarizations to Probe Top-Higgs FCNC Couplings at Colliders

We present how the polarized linear colliders can be used, complementary to the LHC, to fully determine the top-Higgs flavor changing neutral current (FCNC) couplings by using produced asymmetries and top spin polarizations and correlations.

Phenomenological prospects for two Higgs doublet models with controlled FCNC

Flavour changing processes like t → hu, hc, h → τe, τμ as well as hadronic decays h → bs, bd, are analysed within a class of two Higgs doublet models where flavour changing neutral scalar currents at tree level are present. The most remarkable characteristic of this class of models is that the flavour-violating couplings are entirely determined by the fermion masses, the ratio of vacuum expectation values tan and the CKM and PMNS matrices. The flavour structure of the scalar currents results from a symmetry of the Lagrangian and consequently it is natural and stable under the renormalization group evolution. We show that, in some of the models, the rates of the mentioned flavour changing processes can reach values at the discovery level for the LHC operating at 13 TeV, even taking into account stringent bounds on low energy processes.

Muon specific two-Higgs-doublet model

We investigate a new type of a two-Higgs-doublet model as a solution of the muon g − 2 anomaly. We impose a softly-broken Z4 symmetry to forbid tree level flavor changing neutral currents in a natural way. This Z4 symmetry restricts the structure of Yukawa couplings. As a result, extra Higgs boson couplings to muons are enhanced by a factor of tan β, while their couplings to all the other standard model fermions are suppressed by cot β. Thanks to this coupling property, we can avoid the constraint from leptonic τ decays in contrast to the lepton specific two-Higgs-doublet model, which can explain the muon g − 2 within the 2σ level but cannot within the 1σ level due to this constraint. We find that the model can explain the muon g − 2 within the 1σ level satisfying constraints from perturbative unitarity, vacuum stability, electroweak precision measurements, and current LHC data.

Constraints of chromoelectric dipole moments to natural SUSY type sfermion spectrum

We investigate the lower bounds of sfermion masses from the constraints of chromoelectric dipole moments (CEDMs) in the natural SUSY-type sfermion mass spectrum, in which stop mass $m_{\tilde t}$ is much smaller than the other sfermion masses $m_0$. The natural SUSY-type sfermion mass spectrum has been studied since the supersymmetric (SUSY) flavor-changing neutral currents (FCNC) are suppressed because of large sfermion masses of the first two generations, and the weak scale is stabilized because of the light stop. However, this type of sfermion mass spectrum is severely constrained by CEDM, because the light stop contributions to the up quark CEDM are not decoupled in the limit $m_0\rightarrow\infty$, while the down quark CEDM is decoupled in the limit. It is important that the constraints are severe even if SUSY-breaking parameters (and Higgsino mass) are taken to be real because complex diagonalizing matrices of Yukawa matrices, which are from complex Yukawa couplings, generate nonvanishing CP phases in off-diagonal elements of sfermion mass matrices. We calculate the CEDM of up and down quarks numerically in the minimal SUSY standard model, and give the lower bounds for stop mass and the other sfermion masses. We show that the lower bound of stop mass becomes 7 TeV to satisfy the CEDM constraints from Hg EDM. The result is not acceptable if the weak scale stability is considered seriously. We show that if the up-type Yukawa couplings are taken to be real at the grand unification scale, the CEDM constraints are satisfied even if $m_{\tilde t}\sim 1$ TeV.

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.

Vector-like quarks at the origin of light quark masses and mixing

We show how a novel fine-tuning problem present in the Standard Model can be solved through the introduction of a Z_6 flavour symmetry, together with three Q = - 1/3 quarks, three Q = 2/3 quarks, as well as a complex singlet scalar. The Z_6 symmetry is extended to the additional fields and it is an exact symmetry of the Lagrangian, only softly broken in the scalar potential, in order to avoid the domain-wall problem. Specific examples are given and a phenomenological analysis of the main features of the model is presented. It is shown that even for vector-like quarks with masses accessible at the LHC, one can have realistic quark masses and mixing, while respecting the strict constraints on processes arising from flavour changing neutral currents. The vector-like quark decay channels are also described.

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.

Flavour-changing neutral currents making and breaking the standard model.

The standard model of particle physics is our best description yet of fundamental particles and their interactions, but it is known to be incomplete. As yet undiscovered particles and interactions might exist. One of the most powerful ways to search for new particles is by studying processes known as flavour-changing neutral current decays, whereby a quark changes its flavour without altering its electric charge. One example of such a transition is the decay of a beauty quark into a strange quark. Here we review some intriguing anomalies in these decays, which have revealed potential cracks in the standard model-hinting at the existence of new phenomena.

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.

Dark sectors and enhanced h \u2192 \u03c4 \u03bc transitions

LHC searches with τ leptons in the final state are always inclusive in missing-energy sources. A signal in the flavor-violating Higgs decay search, h → τ μ, could therefore equally well be due to a flavor conserving decay, but with an extended decay topology with additional invisible particles. We demonstrate this with the three-body decay h → τ μφ, where φ is a flavorful mediator decaying to a dark-sector. This scenario can give thermal relic dark matter that carries lepton flavor charges, a realistic structure of the charged lepton masses, and explain the anomalous magnetic moment of the muon, (g − 2)μ, while simultaneously obey all indirect constraints from flavor-changing neutral currents. Another potentially observable consequence is the broadening of the collinear mass distributions in the h → τ μ searches.

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.

Phenomenology of the new physics coming from 2HDMs to the neutrino magnetic dipole moment

In several frameworks for leptons sectors of Two Higgs Doublet Models, we calculate the magnetic dipole moment for different flavor types of neutrino. Computations are carried out by assuming a normal hierarchy for neutrino masses, and analyzing the process [Formula: see text] with a charged Higgs boson into the loop. The analysis was performed by sweeping the charged Higgs mass and taking into account the experimental constraints for relevant parameters in Two Higgs Doublet Models with and without flavor changing neutral currents; obtaining magnetic dipole moments close to the experimental thresholds for tau neutrinos in type II and lepton-specific cases. In the neutrino-specific scenario, the contribution of new physics could be sizeable to the current measurement for flavor magnetic dipole moment. This fact leads to excluding possible zones in the parameter space of charged Higgs mass and vacuum expectation value of the second doublet.

Recent results on flavor physics by CMS

Differential production cross sections of the B+ meson and of J/ψ, ψ(2S), and ϒ(nS) (n=1,2,3) vector mesons are measured in pp collisions at 13 TeV, as a function of transverse momentum and rapidity, using data collected by the CMS detector in 2015. The B+ is reconstructed in the decay channel B+→J/ψK+ and the five S-wave quarkonium states are reconstructed in the dimuon decay channel.The simultaneous production of two ϒ(1S) mesons in observed at s=8 TeV in a data sample corresponding to an integrated luminosity of L=20.7 fb−1.The angular distributions and the differential branching fraction of the flavor changing neutral current decay B0→K⁎0μ+μ− are studied using data collected at s=8 TeV, as a function of the dimuon invariant mass squared.Flavor changing neutral currents in top quark couplings are searched through single top quark production in association with a photon, and in tt‾→Zq+Wb and tt‾→Hq+Wb decays, where Z→l+l− and H→γγ, WW, ZZ, τ+τ− and bb‾. The observed number of events agrees with the Standard Model prediction, and no evidence of flavor changing neutral currents is found.

Rare Higgs three body decay induced by top-Higgs FCNC coupling in the littlest Higgs model with T-parity**Supported by National Natural Science Foundation of China (11305049, 11405047), Startup Foundation for Doctors of Henan Normal University (11112, qd15207) and Education Department Foundation of Henan Province(14A140010)

Motivated by the search for flavor-changing neutral current (FCNC) top quark decays at the LHC, we calculate the rare Higgs three body decay H → Wbc induced by top-Higgs FCNC coupling in the littlest Higgs model with T-parity (LHT). We find that the branching ratio of H → Wbc in the LHT model can reach O(10−7) in the allowed parameter space.

Patterns of flavour violation in models with vector-like quarks

We study the patterns of flavour violation in renormalisable extensions of the Standard Model (SM) that contain vector-like quarks (VLQs) in a single complex representation of either the SM gauge group GSM or GSM′≡ GSM ⊗ U(1)Lμ − Lτ. We first decouple VLQs in the M = (1 − 10) TeV range and then at the electroweak scale also Z, Z′ gauge bosons and additional scalars to study the phenomenology. The results depend on the relative size of Z- and Z′-induced flavour-changing neutral currents, as well as the size of |ΔF | = 2 contributions including the effects of renormalisation group Yukawa evolution from M to the electroweak scale that turn out to be very important for models with right-handed currents through the generation of left-right operators. In addition to rare decays like Pto ell overline{ell},Pto {P}^{prime}ell overline{ell},Pto {P}^{prime}nu overline{nu} with P = K, Bs, Bd and |ΔF | = 2 observables we analyze the ratio ε′/ε which appears in the SM to be significantly below the data. We study patterns and correlations between these observables which taken together should in the future allow for differentiating between VLQ models. In particular the patterns in models with left-handed and right-handed currents are markedly different from each other. Among the highlights are large Z-mediated new physics effects in Kaon observables in some of the models and significant effects in Bs,d-observables. ε′/ε can easily be made consistent with the data, implying then uniquely the suppression of {K}_Lto {pi}^0nu overline{nu} . Significant enhancements of Brleft({K}^{+}to {pi}^{+}nu overline{nu}right) are still possible. We point out that the combination of NP effects to |ΔF | = 2 and |ΔF | = 1 observables in a given meson system generally allows to determine the masses of VLQs in a given representation independently of the size of VLQ couplings.