Flavor Changing Neutral Current Processes Research Articles

CP violation and FCNC in a warped A4 flavor model

We recently proposed a spontaneous A4 flavor symmetry breaking scheme implemented in a warped extra dimensional setup to explain the observed pattern of quark and lepton masses and mixings. The quark mixing is induced by bulk A4 flavons mediating "cross-brane" interactions and a "cross-talk" between the quark and neutrino sectors. In this work we explore the phenomenology of RS-A4 and systematically obtain bounds on the Kaluza-Klein (KK) mass scale implied by flavor changing neutral current (FCNC) processes. In particular, we study the constraints arising from Re(\epsilon'/\epsilon_K), b->s\gamma, the neutron EDM and Higgs mediated FCNCs, while the tree level contribution to \epsilon_K through a KK gluon exchange vanishes. We find an overall lower bound on the Kaluza-Klein mass scale M_{KK} > 1.3 TeV from FCNCs, induced by b->s\gamma, differently from flavor anarchic models. This bound is still weaker than the bound M_{KK} > 4.6 TeV induced by Zb_L\bar{b}_L in RS-A4. The little CP problem, related to the largely enhanced new physics contributions to the neutron EDM in flavor anarchic models, is absent. The subtleties of having the Higgs and flavons in the bulk are taken into account and final predictions are derived in the complete three-generation case.

Flavor physics in an SO(10) grand unified model

In supersymmetric grand-unified models, the lepton mixing matrix can possibly affect flavor-changing transitions in the quark sector. We present a detailed analysis of a model proposed by Chang, Masiero and Murayama, in which the near-maximal atmospheric neutrino mixing angle governs large new b -> s transitions. Relating the supersymmetric low-energy parameters to seven new parameters of this SO(10) GUT model, we perform a correlated study of several flavor-changing neutral current (FCNC) processes. We find the current bound on B(tau -> mu gamma) more constraining than B(B -> X_s gamma). The LEP limit on the lightest Higgs boson mass implies an important lower bound on tan beta, which in turn limits the size of the new FCNC transitions. Remarkably, the combined analysis does not rule out large effects in B_s-B_s-bar mixing and we can easily accomodate the large CP phase in the B_s-B_s-bar system which has recently been inferred from a global analysis of CDF and DO data. The model predicts a particle spectrum which is different from the popular Constrained Minimal Supersymmetric Standard Model (CMSSM). B(tau -> mu gamma) enforces heavy masses, typically above 1 TeV, for the sfermions of the degenerate first two generations. However, the ratio of the third-generation and first-generation sfermion masses is smaller than in the CMSSM and a (dominantly right-handed) stop with mass below 500 GeV is possible.

The Flavour Symmetry: S3

By introducing in the theory three Higgs fields that are SU(2) doublets and a flavour permutational symmetry, S3, we extend the concepts of flavour and generations to the Higgs sector and formulate a Minimal S3–Invariant Extension of the Standard Model. The mass matrices of the neutrinos and charged leptons are re-parameterized in terms of their eigenvalues, then the neutrino mixing matrix, VPMNS, is computed and exact, explicit analytical expressions for the neutrino mixing angles as functions of the masses of neutrinos and charged leptons are obtained in excellent agreement with the latest experimental data. We also compute the branching ratios of some selected flavour-changing neutral current (FCNC) processes, as well as the contribution of the exchange of neutral flavour-changing scalars to the anomaly of the magnetic moment of the muon, as functions of the masses of charged leptons and the neutral Higgs bosons. We find that the S3 × Z2 flavour symmetry and the strong mass hierarchy of the charged leptons strongly suppress the FCNC processes in the leptonic sector, well below the present experimental bounds by many orders of magnitude. The contribution of FCNC's to the anomaly of the muon's magnetic moment is small, but non-negligible.

Flavor mixing in gauge-Higgs unification

We discuss flavor mixing and resultant flavor changing neutral current processes in the SU(3) ⊗ SU(3)color gauge-Higgs unification scenario. To achieve flavor violation is a challenging issue in the scenario, since the Yukawa couplings are originally higher dimensional gauge interactions. We argue that the presence of Z 2-odd bulk masses of fermions plays a crucial role as the new source of flavor violation. Although introducing brane-localized mass terms in addition to the bulk masses is necessary to realize flavor mixing, if the bulk masses were universal among generations, the flavor mixing and flavor changing neutral current processes are known to disappear. We also discuss whether natural flavor conservation is realized in the scenario. It is shown that the new source of flavor violation leads to flavor changing neutral current processes at the tree level due to the exchange of non-zero Kaluza-Klein gauge bosons. As a typical example we calculate the rate of \( {K^0} - {\overline K^0} \) mixing due to the non-zero Kaluza-Klein gluon exchange at the tree level. The obtained result for the mass difference of neutral kaon is suppressed by the inverse powers of the compactification scale. By comparing our prediction with the data we obtain the lower bound of the compactification scale as a function of one unfixed parameter of the theory, which is of \( \mathcal{O}\left( {10} \right) \) TeV, except for some extreme cases. We argue that the reason to get such rather mild lower bound is the presence of “GIM-like” mechanism, which is a genuine feature of GHU scenario.

Radiative corrections in K→π ℓ + ℓ − decays

We calculate radiative corrections to the flavor-changing neutral current process K --> pi l+ l-, both for charged and neutral kaon decays. While the soft-photon approximation is shown to work well for the muon channels, we discuss the necessity of further phase space cuts with electrons in the final state. It is also shown how to transfer our results to other decays such as eta --> gamma l+ l- or omega --> pi0 l+ l-.

An A4 flavor model for quarks and leptons in warped geometry

We propose a spontaneous A4 flavor symmetry breaking scheme implemented in a warped extra dimensional setup to explain the observed pattern of quark and lepton masses and mixings. The main advantages of this choice are the explanation of fermion mass hierarchies by wave function overlaps, the emergence of tribimaximal neutrino mixing and zero quark mixing at the leading order and the absence of tree-level gauge mediated flavor violations. Quark mixing is induced by the presence of bulk flavons, which allow for cross-brane interactions and a cross-talk between the quark and neutrino sectors, realizing the spontaneous symmetry breaking pattern A4 --> nothing first proposed in [X.G.\,He, Y.Y.\,Keum, R.R.\,Volkas, JHEP{0604}, 039 (2006)]. We show that the observed quark mixing pattern can be explained in a rather economical way, including the CP violating phase, with leading order cross-interactions, while the observed difference between the smallest CKM entries V_{ub} and V_{td} must arise from higher order corrections. We briefly discuss bounds on the Kaluza-Klein scale implied by flavor changing neutral current processes in our model and show that the residual little CP problem is milder than in flavor anarchic models.

Chirally enhanced corrections to flavor-changing neutral current processes in the generic MSSM

Chirally enhanced supersymmetric QCD corrections to flavor-changing neutral current processes are investigated in the framework of the minimal supersymmetric standard model with generic sources of flavor violation. These corrections arise from flavor-changing self-energy diagrams and can be absorbed into a finite renormalization of the squark-quark-gluino vertex. In this way enhanced two-loop and even three-loop diagrams can be efficiently included into a leading-order calculation. Our corrections substantially change the values of the parameters ${\ensuremath{\delta}}_{23}^{d\mathrm{LL}}$, ${\ensuremath{\delta}}_{23}^{d\mathrm{LR}}$, ${\ensuremath{\delta}}_{23}^{d\mathrm{RL}}$, and ${\ensuremath{\delta}}_{23}^{d\mathrm{RR}}$ extracted from $\mathrm{Br}[B\ensuremath{\rightarrow}{X}_{s}\ensuremath{\gamma}]$ if $\mathrm{tan}\ensuremath{\beta}$ is large. We find stronger (weaker) constraints compared to the leading-order result for negative (positive) values of $\ensuremath{\mu}$. The constraints on ${\ensuremath{\delta}}_{13}^{d\mathrm{LR},\mathrm{RL}}$ and ${\ensuremath{\delta}}_{23}^{d\mathrm{LR},\mathrm{RL}}$ from ${B}_{d}\ensuremath{-}{\overline{B}}_{d}$ and ${B}_{s}\ensuremath{-}{\overline{B}}_{s}$ mixing change drastically if the third-generation squark masses differ from those of the first two generations. $K\ensuremath{-}\overline{K}$ mixing is more strongly affected by the chirally enhanced loop diagrams and even subpercent deviations from degenerate down and strange squark masses lead to profoundly stronger constraints on ${\ensuremath{\delta}}_{12}^{d\mathrm{LR},\mathrm{RL}}$.

Experimental constraints on heavy fermions in Higgsless models

Using an effective Lagrangian approach we analyze a generic Higgsless model with composite heavy fermions, transforming as SU(2)_{L+R} Doublets. Assuming that the Standard Model fermions acquire mass through mixing with the new heavy fermions, we constrain the free parameters of the effective Lagrangian studying Flavour Changing Neutral Current processes. In so doing we obtain bounds that can be applied to a wide range of models characterized by the same fermion mixing hypothesis.

Flavor Physics Constraints for Physics Beyond the Standard Model

Over the past decade, much progress in experimentally measuring and theoretically understanding flavor physics has been achieved. Specifically, the accuracy of the determination of the CKM elements has been greatly improved, and a large number of (a) flavor-changing neutral-current processes involving b→d, b→s, and c→u transitions and (b) CP-violating asymmetries have been measured. No evidence for new physics has been established. Consequently, strong constraints on new physics at a high scale apply. In particular, the flavor structure of new physics at the teraelectronvolt scale is strongly constrained. We review these constraints and discuss future prospects to better understand the flavor structure of physics beyond the Standard Model.

Flavor in supersymmetry: anarchy versus structure

Future high-precision flavor experiments may discover a pattern of deviations from the standard model predictions for flavor-changing neutral current processes. One of the interesting questions that can be answered then will be whether the flavor structure of the new physics is related to that of the standard model or not. We analyze this aspect of flavor physics within a specific framework: supersymmetric models where the soft breaking terms are dominated by gauge-mediation but get non-negligible contributions from gravitymediation. We compare the possible patterns of non-minimally flavor-violating effects that arise if the gravity-mediated contributions are anarchical vs. the case that they are structured by a Froggatt-Nielsen symmetry. We show that combining information on flavor and CP violation from meson mixing and electric dipole moments is indicative for the flavor structure of gravity-mediation.

CPviolation and the fourth generation

Within the Standard model with the 4th generation quarks b' and t' we have analyzed CP-violating flavor changing neutral current processes t -> cX; b'-> sX, b'-> bX,t'-> cX, and t'-> tX, with X=Z,H,gamma,g, by constructing and employing global, unique fit for the 4th generation mass mixing matrix CKM4 at 300 < m_t' < 700 GeV. All quantities appearing in the CKM4 were subject to our fitting procedure. We have found that our fit produces the following CP partial rate asymmetry dominance: a_CP(b'-> s(Z,H,gamma,g))= (90,73,52,30)%, at m_t' ~ 300,300,380,400 GeV, respectively. From the experimental point of view the best decay mode, out of the above four, is certainly b'-> s gamma, because of the presence of a clean high energy single final state photon. We have also obtained relatively large a_CP(t -> c g) ~ 15 (10)% for t' running in the loops with the mass m_t'= 650(500) GeV. There are fair chances that the 4th generation quarks will be discovered at Tevatron or LHC and that some of their decay rates shall be measured. If b' and t' exist at energies we assumed, with well executed tagging, large a_CP could be found too.

Flavor changing neutral currents in the 3-3-1 model with right-handed neutrinos

Flavor changing neutral currents coming from a new non-universal neutral Gauge-Boson and from the non-unitary quark mixing matrix for the $SU(3)_c\otimes SU(3)_L\otimes U(1)_X$ model with right handed neutrinos are studied. By imposing as experimental constraints the measured values of the 3x3 quark mixing matrix, the neutral meson mixing, and the bounds measured values for direct flavor changing neutral current processes, the largest mixing of the known quarks with the exotic ones can be established, with new sources of flavor changing neutral currents being identified. Our main result is that for a $|V_{tb}|$ value smaller than one, large rates of rare top decays such as $t\to c\gamma$, $t\to cZ$, and $t\to cg$ (where g stands for the gluon field) are obtained; but if $|V_{tb}|\approx 1$ the model can survive present experimental limits only if the mass of the new neutral Gauge Boson becomes larger that 10 TeV.

Correlation between direct dark matter detection andBr(Bs→μμ)with a large phase ofBs−B¯smixing

We combine the analyses for flavor changing neutral current processes and dark matter solutions in minimal-type supersymmetric grand unified theory models, SO(10) and SU(5), with a large B s -B s mixing phase and large tanβ. For large tanβ, the double-penguin diagram dominates the supersymmetry contribution to the B s -B s mixing amplitude. Also, the Br(B s → μμ) constraint becomes important as it grows as tan 6 β, although it can still be suppressed by a large pseudoscalar Higgs mass m A . We investigate the correlation between B s → μμ and the dark matter direct detection cross section through their dependence on m A . In the minimal-type of SU(5) with type I seesaw, the large mixing in neutrino Dirac couplings results in a large lepton flavor violating decay process τ → μγ, which in turn sets the upper bound on m A . In the SO(10) case, the large mixing can be chosen to be in the Majorana couplings instead, and the constraint from Br(τ → μγ) can be avoided. The heavy Higgs funnel region turns out to be an interesting possibility in both cases and the direct dark matter detection should be possible in the near future in these scenarios.

Resummation of tan β-enhanced supersymmetric loop corrections beyond the decoupling limit

We study the Minimal Supersymmetric Standard Model with Minimal Flavour Violation for the case of a large parameter tanbeta and arbitrary values of the supersymmetric mass parameters. We derive several resummation formulae for tanbeta-enhanced loop corrections, which were previously only known in the limit of supersymmetric masses far above the electroweak scale. Studying first the renormalisation-scheme dependence of the resummation formula for the bottom Yukawa coupling, we clarify the use of the sbottom mixing angle in the supersymmetric loop factor Delta_b. As a new feature, we find tan-beta-enhanced loop-induced flavour-changing neutral current (FCNC) couplings of gluinos and neutralinos which in turn give rise to new effects in the renormalisation of the Cabibbo-Kobayashi-Maskawa matrix and in FCNC processes of B mesons. For the chromomagnetic Wilson coefficient C_8, these gluino-squark loops can be of the same size as the known chargino-squark contribution. We discuss the phenomenological consequences for the mixing-induced CP asymmetry in B_d -> phi K_S. We further quote formulae for B_s -> mu^+ mu^- and B_s-B_s-bar mixing valid beyond the decoupling limit and find a new contribution affecting the phase of the B_s-B_s-bar mixing amplitude. Our resummed tan-beta-enhanced effects are cast into Feynman rules permitting an easy implementation in automatic calculations.

Composite Higgs-mediated flavor-changing neutral current

We discuss how, in the presence of higher-dimensional operators, the standard model fermion masses can be misaligned in flavor space with the Yukawa couplings to the Higgs boson, even with only one Higgs doublet. Such misalignment results in flavor-violating couplings to the Higgs and hence flavor-changing neutral current processes from tree-level Higgs exchange. We perform a model-independent analysis of such an effect. Specializing to the framework of a composite Higgs with partially composite standard model gauge and fermion fields, we show that the constraints on the compositeness scale implied by ${ϵ}_{K}$ can be generically as strong as those from the exchange of heavy spin-1 resonances if the Higgs is light and strongly coupled to the new states. In the special and well-motivated case of a composite pseudo-Goldstone Higgs, we find that the shift symmetry acting on the Higgs forces an alignment of the fermion mass terms with their Yukawa couplings at leading order in the fermions' degree of compositeness, thus implying much milder bounds. As a consequence of the flavor-violating Higgs couplings, we estimate $\mathrm{BR}(t\ensuremath{\rightarrow}ch)\ensuremath{\sim}{10}^{\ensuremath{-}4}$ and $\mathrm{BR}(h\ensuremath{\rightarrow}tc)\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ both for a pseudo-Goldstone (if ${t}_{R}$ is fully composite) and for a generic composite Higgs. By virtue of the AdS/CFT correspondence, our results directly apply to 5-dimensional Randall-Sundrum compactifications.

Viable and testable SUSY GUTs with Yukawa unification: the case of split trilinears

We explore general SUSY GUT models with exact third-generation Yukawa unification, but where the requirement of universal soft terms at the GUT scale is relaxed. We consider the scenario in which the breaking of universality inherits from the Yukawa couplings, i.e. is of minimal flavor violating (MFV) type. In particular, the MFV principle allows for a splitting between the up-type and the down-type soft trilinear couplings. We explore the viability of this trilinear splitting scenario by means of a fitting procedure to electroweak observables, quark masses as well as flavor-changing neutral current processes. Phenomenological viability singles out one main scenario. This scenario is characterized by a sizable splitting between the trilinear soft terms and a large μ term. Remarkably, this scenario does not invoke a partial decoupling of the sparticle spectrum, as in the case of universal soft terms, but instead it requires part of the spectrum, notably the lightest stop, the gluino and the lightest charginos and neutralinos to be very close to the current experimental limits. The above mechanism is mostly triggered by a non-trivial interplay between the requirements of negative, sizable SUSY threshold corrections to mb and an instead negligible modification of the B → Xsγ decay rate, in presence of various other constraints, most notably a successful EWSB and a not too large BR(Bs → μ+μ−). We present a model-building interpretation of our discussed scenario and emphasize the crucial role of SUSY spectrum determinations at the LHC for either falsifying Yukawa unification or else providing important hints on the mechanism of SUSY breaking at work.

Lepton FCNC in Type III seesaw model

In Type III seesaw model, there are tree level flavor changing neutral currents (FCNC) in the lepton sector, due to mixing of charged particles in the leptonic triplet introduced to realize seesaw mechanism, with the usual charged leptons. In this work we study these FCNC effects in a systematic way using available experimental data. Several FCNC processes have been studied before. The new processes considered in this work include: lepton flavor violating processes τ → Pl, τ → Vl, V → l', P → l', M → M'l' and muonium-antimuonium oscillation. Results obtained are compared with previous results from li → ljlkl, li → ljγ, Z → l' and μ−e conversion. Our results show that the most stringent constraint on the e-to-τ FCNC effect comes from τ → π0e decay. τ → ρ0μ and τ → π0μ give very stringent constraints on the μ-to-τ FCNC effect, comparable with that obtained from τ → μμ studied previously. The constraint on the e-to-μ FCNC effect from processes considered in this work is much weaker than that obtained from processes studies previously, in particular that from μ−e conversion in atomic nuclei. We find that in the canonical seesaw models the FCNC parameters, due to tiny neutrino masses, are all predicted to be much smaller than the constraints obtained here, making such models irrelevant. However, we also find that in certain special circumstances the tiny neutrino masses do not directly constrain the FCNC parameters. In these situations, the constraints from the FCNC studies can still play important roles.

Probing new physics in charm couplings with flavor-changing neutral currents

Low-energy experiments involving kaon, B-meson, D-meson, and hyperon flavor-changing neutral transitions have confirmed the loop-induced flavor-changing neutral current picture of the standard model. The continuing study of these processes is essential to further refine this picture and ultimately understand the flavor dynamics. In this paper we consider deviations from the standard model in the charm sector and their effect on flavor-changing neutral current processes. Specifically, we parametrize new physics in terms of left- and right-handed anomalous couplings of the W boson to the charm quark. We present a comprehensive study of existing constraints and point out those measurements that are most sensitive to new physics of this type.

Implication of the HyperCP bosonX0(214 MeV) in the flavor changing neutral current processes

We analyze the inclusive $b(c)\ensuremath{\rightarrow}s(u){\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and exclusive $B({D}^{+})\ensuremath{\rightarrow}K({\ensuremath{\pi}}^{+}){\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ flavor changing neutral current decays in light of the HyperCP boson ${X}^{0}$ of mass 214 MeV recently observed in the hyperon decay ${\ensuremath{\Sigma}}^{+}\ensuremath{\rightarrow}p{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$. Using the branching ratio data of the respective inclusive and exclusive decays, we obtain constraints on ${g}_{1}({h}_{1})$ and ${g}_{2}({h}_{2})$, the scalar and pseudoscalar coupling constants of the $b\ensuremath{-}s\ensuremath{-}{X}^{0}(c\ensuremath{-}u\ensuremath{-}{X}^{0})$ vertices.

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

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