Search for flavor changing neutral currents in top quark decays in pp collisions at 7 TeV

The results of a search for flavor changing neutral currents in top quark decays $\mathrm{t}\to \mathrm{Zq}$ in events with a topology compatible with the decay chain $\mathrm{t}\overline{\mathrm{t}}\to \mathrm{Wb}+\mathrm{Zq}\to ?\nu \mathrm{b}+??\mathrm{q}$ are presented. The search is performed with a data sample corresponding to an integrated luminosity of $5.0{\text{fb}}^{?1}$ of proton–proton collisions at a center-of-mass energy of 7 TeV, collected with the CMS detector at the LHC. The observed number of events agrees with the standard model prediction and no evidence for flavor changing neutral currents in top quark decays is found. A $\mathrm{t}\to \mathrm{Zq}$ branching fraction greater than 0.21% is excluded at the 95% confidence level.     

An investigation into the lepton flavor changing neutral current at a Z-factory

The potential for detecting the lepton flavor changing neutral current (LFCNC) via the Z → eτ rare decay at a Z-factory is investigated.Practicable event selection strategies are designed to increase signal significance over SM contributions.Assuming no apparent deviation from SM predictions,one can conclude that the most general model-independent LFCNC effective form factors would be constrained at an unprecedent level,and accordingly a 3-order improvement from LEP1 measurements on the upper limit of BR(Z → eτ) could be achieved,as precisely as 10-8 with 100 fb-1 at Z-factory.     

Loop induced flavor changing neutral decays of the top quark in a general two-Higgs-doublet model

Decays of the top quark induced by flavor changing neutral currents (FCNC) are known to be extremely rare events within the Standard Model. This is so not only for the decay modes into gauge bosons, but most notably in the case of the Higgs channels, e.g., t→H_SM+c, with a branching fraction of 10⁻¹³ at most. Therefore, detection of FCNC top quark decays in a future high-energy, and high-luminosity, machine like the LHC or the LC would be an indisputable signal of new physics. In this paper we show that within the simplest extension of the SM, namely the general two-Higgs-doublet model, the FCNC top quark decays into Higgs bosons, t→(h⁰,H⁰,A⁰)+c, can be the most favored FCNC modes — comparable or even more efficient than the gluon channel t→g+c. In both cases the optimal results are obtained for Type II models. However, only the Higgs channels can have rates reaching the detectable level (10⁻⁵), with a maximum of order 10⁻⁴ which is compatible with the charged Higgs bounds from radiative B-meson decays. We compare with the previous results obtained in the Higgs sector of the MSSM.     

Z′ mediated flavor changing neutral currents in B meson decays

We study the effects of an extra U(1)′ gauge boson with flavor changing couplings with fermion mass eigenstates on certain B meson decays that are sensitive to such new physics contributions. In particular, we examine to what extent the current data on B_d→φK and B_d→η′K decays may be explained in such models, concentrating on the example in which the flavor changing couplings are left-chiral. We find that within reasonable ranges of parameters, the Z′ contribution can readily account for the anomaly in S_φKS but is not sufficient to explain large branching ratio of B_d→η′K with the same parameter value. S_φKS and S_η′KS are seen to be the dominant observables that constrain the extra weak phase in the model.     

Flavour changing neutral currents in predictive SUSY-GUTS

Constraints from flavour changing neutral currents are discussed in the context of predictive Grand Unified Supersymmetric Theories. Uncertainties in the estimation of their amplitudes are minimized by using a successful mass matrix ansatz which predicts the arbitrary parameters of the Standard Model. Furthermore, Renormalization Group Equations are used in order to express the sparticle masses involved, in terms of the gaugino massm _1/2, the universal scalar massm ₀ and the trilinear couplingA at the GUT scale. Modifications in the low energy expressions for the squark masses due to a large top Yukawa coupling are also considered. The above results, reconciled with experimental limits are used to put lower bounds on the parametersm _1/2,m ₀ and the low energy scalar masses.     

Sterile neutrinos in lepton number and lepton flavor violating decays

We study the contribution of massive dominantly sterile neutrinos, N, to the lepton number and lepton flavor violating semileptonic decays of τ and B, D, K-mesons. We focus on special domains of sterile neutrino masses mN where it is close to its mass-shell. This leads to an enormous resonant enhancement of the decay rates of these processes. This allows us to derive stringent limits on the sterile neutrino mass mN and its mixing UαN with active flavors. We apply a joint analysis of the existing experimental bounds on the decay rates of the studied processes. In contrast to other approaches in the literature our limits are free from ad hoc assumptions on the relative size of the sterile neutrino mixing parameters. We analyze the impact of this sort of assumptions on the extraction of the limits on mN and UαN, and discuss the effect of finite detector size. Special attention was paid to the limits on meson decays with e^±e^± in final state, derived from non-observation of 0νββ-decay. We point out that observation of these decays may, in particular, shed light on the Majorana phases of the neutrino mixing matrix.     

Grand unification and mirror particles

The grand unification model containing standard and mirror particles is constructed. In models of this kind there exist such strings that a standard particle transforms into a mirror particle in going around the string.     

Grand unification and B&L conservation

A brief summary of some of the recent developments in grand unification and B&;L conservation is given. Proton stability in supergravity unified models is discussed. Also discussed are the effects of supersymmetric dark matter constraints and the constraints of proton lifetime on the SUSY spectrum. Other topics reviewed include Planck scale effects and p decay, effects of textures, and extension to nonminimal models including models with many Higgs triplets. Recent developments in GUTs and strings are also discussed.     

Grand unification and heavy axion

We argue that sufficiently complex grand unified theories involving extra strong intractions that confine at very short distances may lead to a heavy axion solution of the CP problem of QCD. This axion may have a mass within the accessible energy range, and its low-energy interactions emerge through mixing with axial Higgs boson(s). Another signature of this scenario is softly broken Peccei-Quinn symmetry in the electroweak Higgs sector. We present a toy GUT exhibiting these features. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 590–593 (25 April 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Unitarity boomerangs of quark and lepton mixing matrices

The most popular way to present mixing matrices of quarks (CKM) and leptons (PMNS) is the parametrization with three mixing angles and one CP-violating phase. There are two major options in this kind of parametrizations, one is the original Kobayashi–Maskawa (KM) matrix, and the other is the Chau–Keung (CK) matrix. In a new proposal by Frampton and He, a unitarity boomerang is introduced to combine two unitarity triangles, and this new presentation displays all four independent parameters of the KM parametrization in the quark sector simultaneously. In this Letter, we study the relations between KM and CK parametrizations, and also consider the quark–lepton complementarity (QLC) in the KM parametrization. The unitarity boomerang is discussed in the situation of the CK parametrization for comparison with that in the KM parametrization in the quark sector. Then we extend the idea of unitarity boomerang to the lepton sector, and check the corresponding unitarity boomerangs in the two cases of parametrizations.     

Radiative transmission of lepton flavor hierarchies

We discuss a one loop model for neutrino masses which leads to a seesaw-like formula with the difference that the charged lepton masses replace the unknown Dirac mass matrix present in the usual seesaw case. This is a considerable reduction of parameters in the neutrino sector and predicts a strong hierarchical pattern in the right handed neutrino mass matrix that is easily derived from a U(1)_H family symmetry. The model is based on the left–right gauge group with an additional Z₄ discrete symmetry which gives vanishing neutrino Dirac masses and finite Majorana masses arising at the one loop level. Furthermore, it is one of the few models that naturally allow for large (but not necessarily maximal) mixing angles in the lepton sector. A generalization of the model to the quark sector requires three iso-spin singlet vector-like down type quarks, as in E₆. The model predicts an inert doublet type scalar dark matter.     

Unifying CP violations of quark and lepton sectors

A preliminary determination of the Dirac phase in the PMNS matrix is \(\delta _\mathrm{PMNS}\approx -\frac{\pi }{2}\). A rather accurately determined Jarlskog invariant J in the CKM matrix is close to the maximum. Since the phases in the CKM and PMNS matrices will be accurately determined in the future, it is an interesting problem to relate these two phases. This can be achieved in a families-unified grand unification if the weak CP violation is introduced spontaneously à la Froggatt and Nielsen at a high energy scale, where only one meaningful Dirac CP phase appears.     

Di-Higgs production as a probe of flavor changing neutral Yukawa couplings

Top partners are well motivated in many new physics models.Usually,vector like quarks,T_(L,R),are introduced to circumvent the quantum anomaly.Therefore,it is crucial to probe their interactions with standard model particles.However,flavor changing neutral couplings are always difficult to detect directly in current and future experiments.In this paper,we demonstrate how to constrain the flavor changing neutral Yukawa coupling Tth indirectly,via the di-Higgs production.We consider the simplified model,including a pair of gauge singlet T_(L,R).Under the perturbative unitarity and experimental constraints,we select m_T=400 GeV,s_L=0.2,and m_T=800 GeV,s_L=0.1 as benchmark points.After the analysis on the amplitude and evaluation of the numerical cross sections,we infer that the present constraints from di-Higgs production have already surpassed the unitarity bound because of the(y_(L,R)~(tT))~4 behavior.For the case of m_T=400 GeV and s_L=0.2,Rey_(L,R)~(tT) and Imy_(L,R)~(tT)can be bounded optimally in the range(-0.4,0.4) at the HL-LHC with 2σ CL.For the case of m_T=800 GeV and s_L=0.1,Rey_(L,R)~(tT)and Imy_(L,R)~(tT) can be bounded optimally in the range(-0.5,0.5) at the HL-LHC with 2σ CL.The anomalous triple Higgs coupling δ_(hhh) can also affect the constraints on y_(L,R)~(tT).Finally,we determine that the top quark electric dipole moment can provide stronger bounds in the off-axis regions for some scenarios.