Flavor-changing Couplings Research Articles

Same-sign taus signatures of maximally flavor-violating scalars at the LHC

We explore single and double flavor-violating scalar (flavon) production at the 13 and 14 TeV LHC in an effective field theory formulation where flavons always change the flavor of the Standard Model fermions. When those scalars couple to mass, their flavor-changing couplings to top quarks and tau leptons are favored. Focusing on the mass region below the top-quark mass, we find couplings that fit the muon (g−2) discrepancy and avoid several current experimental constraints. We determine the potential of the LHC to exclude or discover such a new physics scenario with clean signatures consisting of same-sign tau leptons and the simultaneous observation of resonances in the tau plus electron or muon invariant mass. We found that in the double production mode, effective couplings down to order 10−2 TeV−1 can be probed for flavon masses in the 10–170 GeV range at the 14 TeV HL-LHC, but couplings down to 0.1 TeV−1 can already be excluded at 95% confidence level with data collected from the 13 TeV LHC in the same mass interval. We also explore the impact of sizeable diagonal flavon couplings on the prospects of LHC for the signals we propose. Published by the American Physical Society 2024

Spontaneous CP violation and mu -tau symmetry in two-Higgs-doublet models with flavour conservation

In multi-Higgs-doublet models, requiring simultaneously that (i) CP violation only arises spontaneously, (ii) tree level scalar flavour changing couplings are absent and (iii) the fermion mixing matrix is CP violating, can only be achieved in a very specific manner. A general approach with new clarifying insights on the question is presented. Considering the quark sector, that peculiar possibility is not viable on phenomenological grounds. We show that, considering the lepton sector, it is highly interesting and leads to viable models with mu -tau symmetric PMNS matrices. Phenomenological implications of the models, both for Dirac and Majorana (in a type I seesaw scenario) neutrinos, are analysed.

Flavor probes of axion-like particles

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ratios. We further calculate quark flavor-changing effects that are unavoidably induced by running and matching between the new physics scale and the scale of the measurements. This allows us to derive bounds on benchmark ALP models in which only a single (flavorless or flavor-universal) ALP coupling to SM particles is present at the new physics scale, and in this context we highlight the complementarity and competitiveness of flavor bounds with constraints derived from collider, beam dump and astrophysical measurements. We find that searches for ALPs produced in meson decays provide some of the strongest constraints in the MeV-GeV mass range, even for the most flavorless of ALP models. Likewise, we discuss the interplay of flavor-conserving and flavor-violating couplings of the ALP to leptons, finding that constraints from lepton flavor-violating observables generally depend strongly on both. Additionally, we analyze whether an ALP can provide an explanation for various experimental anomalies including those observed in rare B-meson decays, measurements at the ATOMKI and KTeV experiments, and in the anomalous magnetic moments of the muon and the electron.

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.

Explaining the b → sℓ+ℓ− anomalies in Z′ scenarios with top-FCNC couplings

Motivated by the recent anomalies in b→sℓ+ℓ− transitions, we explore a minimal Z′ scenario, in which the Z′ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a Z′ boson can explain the current b→sℓ+ℓ− anomalies, while satisfying other flavour and collider constraints simultaneously. The Z′ boson can be as light as few hundreds GeV. In this case, the t→cμ+μ− decay and the tZ′ associated production at the LHC could provide sensitive probes of such a Z′ boson. As a special feature, the Z′ contributions to all rare B- and K-meson processes are controlled by one parameter. This results in interesting correlations among these processes, which could provide further insights into this scenario. In addition, an extended scenario, in which the Z′ boson interacts with the SU(2)L fermion doublets with analogous couplings as in the minimal scenario, is also investigated.

CP Violation for the Heavens and the Earth

Electroweak baryogenesis can be driven by the top quark in a general two Higgs doublet model with extra Yukawa couplings. Higgs quartics provide the first order phase transition, while extra top Yukawa coupling ρtt can fuel the cosmic baryon asymmetry through the λtImρtt product, with flavor-changing ρtc coupling as backup. The impressive ACME 2018 bound on the electron electric dipole moment calls for an extra electron coupling ρee for exquisite cancellation among dangerous diagrams, broadening the baryogenesis solution space. The mechanism suggests that extra Yukawa couplings echo the hierarchical structure of standard Yukawa couplings. Phenomenological consequences in the Higgs search and flavor physics are discussed, with μ and τ EDM touched upon.

Higgs flavor phenomenology in a supersymmetric left-right model with parity

In this paper, we focus on the supersymmetric model with left-right (LR) symmetry, that is especially proposed in our previous work [1]. In this model, there are four Higgs doublets in order to realize the Standard Model (SM) fermion masses and the Cabibbo-Kobayashi-Maskawa matrix. The heavy Higgs doublets unavoidably have flavor changing couplings to the SM fermions and induce flavor-changing neutral currents at tree level. We study broader parameter space than the previous work with including the renormalization group corrections to the Yukawa couplings between the LR breaking scale, mathcal{O} (1013) GeV, and the supersymmetry breaking scales, mathcal{O} (100) TeV. The CP violating observable in K– overline{K} mixing, ϵK, strongly constrains the model, so that heavy Higgs mass should be heavier than mathcal{O} (100) TeV. We study the lepton flavor violating (LFV) processes setting heavy Higgs masses to be 170 TeV. The branching ratios of μ → 3e and the μ–e conversion can be larger than 10−16 that could be covered by the future experiments. We also study the degree of fine-tuning in the parameter region that predicts testable LFV processes.

Flavor structure of anomaly-free hidden photon models

Extensions of the Standard Model with an Abelian gauge group are constrained by gauge anomaly cancellation, so that only a limited number of possible charge assignments is allowed without the introduction of new chiral fermions. For flavour universal charges, couplings of the associated hidden photon to Standard Model fermions are flavour conserving at tree-level. We show explicitly that even the flavour specific charge assignments allowed by anomaly cancellation condition lead to flavour conserving tree-level couplings of the hidden photon to quarks and charged leptons \emph{if} the CKM or PMNS matrix can be successfully reconstructed. Further, loop-induced flavour changing couplings are strongly suppressed. As a consequence, the structure of the Majorana mass matrix is constrained and flavour changing tree-level couplings of the hidden photon to neutrino mass eigenstates are identified as a means to distinguish the $U(1)_{B-L}$ gauge boson from any other anomaly-free extension of the Standard Model without new chiral fermions. We present a comprehensive analysis of constraints and projections for future searches for a $U(1)_{B-3L_i}$ gauge boson, calculate the reach of resonance searches in $B$ meson decays and comment on the implications for non-standard neutrino interactions.

Contributions to ZZV^* (V=gamma ,Z,Z') couplings from CP violating flavor changing couplings

The one-loop contributions to the trilinear neutral gauge boson couplings ZZV^* (V=gamma ,Z,Z'), parametrized in terms of one CP-conserving f_5^{V} and one CP-violating f_4^{V} form factors, are calculated in models with CP-violating flavor changing neutral current couplings mediated by the Z gauge boson and an extra neutral gauge boson Z'. Analytical results are presented in terms of Passarino-Veltman scalar functions. Constraints on the vector and axial couplings of the Z gauge boson left| g_{{VZ}}^{tu}right| < 0.0096 and left| g_{{VZ}}^{tc}right| <0.011 are obtained from the current experimental data on the trightarrow Z q decays. It is found that in the case of the ZZgamma ^* vertex the only non-vanishing form factor is f_5^{gamma }, which can be of the order of 10^{-3}, whereas for the ZZZ^* vertex both form factors f_5^{Z} and f_4^{Z} are non-vanishing and can be of the order of 10^{-6} and 10^{-5}, respectively. Our estimates for f_5^{gamma } and f_5^{Z} are smaller than those predicted by the standard model, where f_4^{Z} is absent up to the one loop level. We also estimate the ZZ{Z'}^{*} form factors arising from both diagonal and non-diagonal Z' couplings within a few extension models. It is found that in the diagonal case f_{5}^{Z'} is the only non-vanishing form factor and its real and imaginary parts can be of the order of 10^{-1}–10^{-2} and 10^{-2}–10^{-3}, respectively, with the dominant contributions arising from the light quarks and leptons. In the non-diagonal case f_{5}^{Z^prime } can be of the order of 10^{-4}, whereas f_4^{Z'} can reach values as large as 10^{-7}–10^{-8}, with the largest contributions arising from the Z'tq couplings.

Vector leptoquarks beyond tree level. III. Vectorlike fermions and flavor-changing transitions

Extending previous work on this subject, we evaluate the impact of vector-like fermions at next-to-leading order accuracy in models with a massive vector leptoquark embedded in the $SU(4)\times SU(3)^\prime\times SU(2)_L\times U(1)_X$ gauge group. Vector-like fermions induce new sources of flavor symmetry breaking, resulting in tree-level flavor-changing couplings for the leptoquark not present in the minimal version of the model. These, in turn, lead to a series of non-vanishing flavor-changing neutral-current amplitudes at the loop level. We systematically analyze these effects in semileptonic, dipole and $\Delta F=2$ operators. The impact of these corrections in $b\to s\nu\nu$ and $b\to c\tau\nu$ observables are discussed in detail. In particular, we show that, in the parameter region providing a good fit to the $B$-physics anomalies, the model predicts a $10\%$ to $50\%$ enhancement of $\mathcal{B}(B\to K^{(*)}\nu\nu)$.

Electric dipole moment of the neutron in two-Higgs-doublet models with flavor changing couplings

I consider contributions to the neutron electric dipole moment within two-Higgs-doublet models which allow for small flavor changing neutral Higgs couplings. In a previous paper, I considered flavor changing interactions for the Standard Model Higgs boson to first order in the flavor changing coupling. In that paper I found that the obtained value of the neutron electric dipole moment was below the present experimental limit, given previous restrictions on such couplings. Because this was an effective theory, the result depended on an ultraviolet cut off $\mathrm{\ensuremath{\Lambda}}$, parametrized as $\mathrm{ln}({\mathrm{\ensuremath{\Lambda}}}^{2})$. In the present paper I demonstrate that, when going to two-Higgs-doublet models, the result stays the same as in the previous paper, up to ${M}_{\mathrm{SM}}^{2}/{M}_{H}^{2}$ corrections, where ${M}_{\mathrm{SM}}$ is the mass of the top quark or the $W$ boson. ${M}_{H}$ is the mass of the heavy neutral scalar Higgs boson $H$ which is much heavier than the light neutral Higgs boson $h$ with mass ${M}_{h}$. In the limit ${M}_{H}^{2}\ensuremath{\gg}{M}_{h}^{2}$, the $\mathrm{ln}({\mathrm{\ensuremath{\Lambda}}}^{2})$ behavior in the previous paper is replaced by $\mathrm{ln}({\stackrel{\texttildelow{}}{{M}_{H}}}^{2})$, where $\stackrel{\texttildelow{}}{{M}_{H}}$ is of order ${M}_{H}$. I also explain how some divergences due to exchange of the pseudoscalar Higgs $A$ are canceled by similar contributions from the scalar heavy Higgs $H$, and that these contributions, and finite contributions from $A$ exchange, are suppressed.

CP violation in B decays

L’étude expérimentale de la brisure de la symétrie CP dans les désintégrations de mésons beaux (B) a apporté des contributions majeures à notre compréhension de la nature : (i) Elle a établi que le mécanisme de Kobayashi-Maskawa est la source dominante de brisure de la symétrie CP dans les systèmes des mésons K et B. (ii) Elle a amélioré significativement la précision des paramètres qui décrivent la matrice de mélange des quarks Cabibbo–Kobayashi–Maskawa. (iii) Elle a prouvé que l’échelle d’énergie d’une nouvelle physique avec des couplages de changement de saveurs d’ordre unité, devait être supérieure à 𝒪(10 3 TeV). Des progrès sont attendus, dans cette décennie et la suivante, des expériences du LHC et Belle II. L’état de l’art de cette Physique et les perspectives des expériences futures sont discutés dans cet article.

Kaon decays shedding light on massless dark photons

We explore kaon decays with missing energy carried away by a massless dark photon, {{overline{gamma }}}, assumed to have flavor-changing dipole-type couplings to the d and s quarks. We consider in particular the neutral-kaon modes K_Lrightarrow gamma {{overline{gamma }}} and K_Lrightarrow pi ^0gamma {{overline{gamma }}} and their K_S counterparts, as well as the charged-kaon channel K^+rightarrow pi ^+gamma {{overline{gamma }}}, each of which also has an ordinary photon, gamma , in the final state. In addition, we look at K_{L,S}rightarrow pi ^+pi ^-{{overline{gamma }}} and K^+rightarrow pi ^+pi ^0{{overline{gamma }}}. Interestingly, the same ds{{overline{gamma }}} interactions give rise to the flavor-changing two-body decays of hyperons with missing energy and are subject to model-independent constraints that can be inferred from the existing hyperon data. Taking this into account, we obtain branching fractions {{{mathcal {B}}}}(K_Lrightarrow gamma {{overline{gamma }}}) and mathcal{B}(K_Lrightarrow pi ^0gamma {{overline{gamma }}}) which can be as high as 10^{-3} and 10^{-6}, respectively, one or both of which may be within the sensitivity reach of the KOTO experiment. Furthermore, we find that {{{mathcal {B}}}}(K^+rightarrow pi ^+gamma {{overline{gamma }}}) and mathcal{B}(K^+rightarrow pi ^+pi ^0{{overline{gamma }}}) are allowed to be maximally of order 10^{-6} as well, which may be probed by NA62. Complementarily, the hyperon modes can have rates which are potentially accessible by BESIII. Thus, these ongoing experiments could soon be able to offer significant tests on the existence of the massless dark photon.

Investigation of the Higgs boson anomalous FCNC interactions in the simple 3-3-1 model

We study phenomenological constraints on the simple 3-3-1 model with flavor-violating Yukawa couplings. Both Higgs triplets couple to leptons and quarks, which generates flavor-violating signals in both lepton and quark sectors. We have shown that this model allows for a large Higgs lepton flavor-violating rate decay h rightarrow mu tau and also may reach perfect agreement with other experimental constraints such as tau rightarrow mu gamma and (g-2)_mu . The contributions of flavor-changing neutral current couplings, Higgs–quark–quark couplings, mixing to the mesons are investigated. Br(h rightarrow q q^prime ) can be enhanced acknowledging the measurements of meson mixing. The branching ratio for t rightarrow q h can reach up to 10^{-3}, but it could be as low as 10^{-8}.

Constraints on long-lived light scalars with flavor-changing couplings and the KOTO anomaly

Recently, the KOTO experiment at J-PARC has observed three anomalous events in the flavor-changing rare decay $K_L \to \pi^0 \nu \bar\nu$, which indicates that the corresponding branching ratio is almost two orders of magnitude larger than the Standard Model (SM) prediction. Taking this intriguing result at face value, we explore model implications of its viable explanation by a long-lived light SM-singlet scalar ($S$) emission, i.e. $K_L \to \pi^0 S$, with $S$ decaying outside the KOTO detector. We derive constraints on the parameter space of such a light scalar in the context of three simple models: (i) a real singlet scalar extension of the SM; (ii) a $B-L$ extension where neutrino masses arise via type-I seesaw mechanism from $B-L$ breaking; and (iii) a TeV-scale left-right symmetric model. The flavor-changing couplings needed to explain the KOTO excess in models (i) and (ii) originate from tree-level mixing of the scalar with SM Higgs field ($h$), and in model (iii), from the mixing of $S$ and $h$ with the neutral component of the heavy bidoublet Higgs field. After taking into account the stringent constraints from high-precision searches for flavor-changing charged and neutral kaon decays at NA62, E949, KOTO and CHARM experiments, as well as the astrophysical and cosmological constraints on a light scalar, such as those from supernova energy loss, big bang nucleosynthesis and relativistic degrees of freedom, we find that the light scalar interpretation of the KOTO excess is allowed in all these models. Parts of the parameter range can be tested in future NA62 and DUNE experiments.

Flavons and LFV decays and productions of pseudoscalar mesons

Flavons are the dynamic agent of flavor symmetry breaking and have flavor changing couplings to the Standard Model (SM) fermions. We consider their contributions to the lepton flavor violating (LFV) decays [Formula: see text] and [Formula: see text] with [Formula: see text], [Formula: see text] or [Formula: see text] and [Formula: see text] in the simplest flavon model without Higgs-flavon mixing. We find that flavons can produce significant contributions to some of these LFV decay processes.

Point-Particle Catalysis

We use the point-particle effective field theory (PPEFT) framework to describe particle-conversion mediated by a flavour-changing coupling to a point-particle. We do this for a toy model of two non-relativistic scalars coupled to the same point-particle, on which there is a flavour-violating coupling. It is found that the point-particle couplings all must be renormalized with respect to a radial cut-off near the origin, and it is an invariant of the flow of the flavour-changing coupling that is directly related to particle-changing cross-sections. At the same time, we find an interesting dependence of those cross-sections on the ratio k_out/k_in of the outgoing and incoming momenta, which can lead to a 1/k_in enhancement in certain regimes. We further connect this model to the case of a single-particle non-self-adjoint (absorptive) PPEFT, as well as to a PPEFT of a single particle coupled to a two-state nucleus. These results could be relevant for future calculations of any more complicated reactions, such as nucleus-induced electron-muon conversions, monopole catalysis of baryon number violation, as well as nuclear transfer reactions.

Probing the top quark flavor-changing couplings at CEPC * * Supported by CNRS via the LIA FCPPL, by the CEPC theory study grant, and by IHEP (Y7515540U1)

We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider (CEPC) in China. We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark, which can be tested at CEPC in the single top production channel. Compared with the current bounds from the LEP2 data and the projected limits at the high-luminosity LHC, we find that CEPC could improve the limits of the four-fermion flavor-changing coefficients by one to two orders of magnitude, and would also provide similar sensitivity for the two-fermion flavor-changing coefficients. Overall, CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the LHC upgrade. We show that the c-jet tagging capacity at CEPC could further improve its sensitivity to top-charm flavor-changing couplings. If a signal is observed, the kinematic distribution as well as the c-jet tagging could be exploited to pinpoint the various flavor-changing couplings, providing valuable information about the flavor properties of the top quark.

Study of the FCNH Coupling with Boosted Higgs at LHC**Supported by the National Science Foundation of China under Grant No. 11405095

We present a study about the flavor changing coupling of the top quark with the Higgs boson through the channel with at LHC. The final states considered for the such process are . We focus on the boosted region in the phase space of the Higgs boson. The backgrounds and events are simulated and analyzed. The sensitivities for the FCNH couplings are estimated. It is found that it is more sensitive for than at LHC. The upper limits of the FCNH couplings can be set at LHC with 3000 integrated luminosity as and at 95% C.L.

Electric dipole moment of the neutron from a flavor changing Higgs-boson

I consider neutron electric dipole moment contributions induced by flavor changing Standard Model Higgs boson couplings to quarks. Such couplings might stem from non-renormalizable SU(2)_L times U(1)_Y invariant Lagrange terms of dimension six, containing a product of three Higgs doublets. We extend previous one loop analysis to two loops. The divergent loops, due to non-renormalisabillity, are parametrized in terms of an ultraviolet cut-off Lambda . I also consider QCD corrections. Using the current experimental bound on the neutron electric dipole moment, then for cut offs from one to seven TeV, I find a constraint of order 10^{-3} for the imaginary part of the product of the Higgs flavor changing coupling for (d rightarrow b)-transition and the CKM element V_{td}. Assuming that the previous bound of the absolute value of the Higgs flavor changing coupling for (d rightarrow b)-transition obtained from B_d - bar{B_d}-mixing is saturated, the experimental bound on the neutron electric dipole moment would be reached for the bare result, if the cut off were extended up to about ca 20 TeV. However, QCD corrections suppress this result by a factor of order ten, and keep the nEDM below the experimental bound.