The effect of the location of the new Higgs doublet on the radiative lepton flavor violating decays in the split fermion scenario

We study the branching ratios of the lepton flavor violating processes μ→eγ, τ→eγ and τ→μγ in the split fermion scenario, with the assumption that the new Higgs doublet is restricted to the 4D brane (thin bulk) in one and two extra dimensions, in the framework of the two Higgs doublet model. We observe that the branching ratios are sensitive to the location of the 4D brane and, in the second case, the width of the thin bulk, especially for the μ→eγ decay.     

Limits on scalar-leptoquark masses from lepton-flavor-violating processes of the li → ljγ type

The contributions of scalar leptoquarks to lepton-flavor-violating processes of the l _i → l _j γ type are investigated on the basis of the minimal model relying on four-color symmetry and involving the Higgs mechanism of quark- and lepton-mass generation. It is shown that experimental data on the decay processes μ → eγ, τ → μγ, and τ → eγ are compatible with the existence of light scalar leptoquarks belonging to the type being considered and having masses around 1 TeV or below.     

Neutrino mixing and masses in a left–right model with mirror fermions

In the framework of a left–right model containing mirror fermions with gauge group SU(3) _C ⊗SU(2) _L ⊗SU(2) _R ⊗U(1) _Y′, we estimate the neutrino masses, which are found to be consistent with their experimental bounds and hierarchy. We evaluate the decay rates of the Lepton Flavor Violation (LFV) processes μ→eγ, τ→μγ and τ→eγ. We obtain upper limits for the flavor-changing branching ratios in agreement with their present experimental bounds. We also estimate the decay rates of heavy Majorana neutrinos in the channels N→W ^± l ^∓, N→Zν _l and N→Hν _l , which are roughly equal for large values of the heavy neutrino mass. Starting from the most general Majorana neutrino mass matrix, the smallness of active neutrino masses turns out from the interplay of the hierarchy of the involved scales and the double application of seesaw mechanism. An appropriate parameterization on the structure of the neutrino mass matrix imposing a symmetric mixing of electron neutrino with muon and tau neutrinos leads to tri-bimaximal mixing matrix for light neutrinos.     

Learning about flavour structure from τ→eβγ and μ→eγ?

Current and upcoming experiments should improve the sensitivity to e _α →e _β γ decays by an order of magnitude. This paper assumes that one of the τ→e _β γ decays is observed, and explores the structure and consequences of the required new flavoured couplings. In simple models (a low-scale seesaw, leptoquarks) it is shown that the dipole vertex function is proportional to a product of flavoured matrices from the Lagrangian (a “Jarlskog-like” invariant), provided that the loop particles are weakly coupled to the Higgs. Secondly, if the dipole vertex function has a hierarchical structure, this might imply that only some of the τ→e _β γ modes can be observed, due to the “approximate zero” implied by the bound on μ→eγ. The assumptions underlying this potential test of a hierarchical structure are discussed.     

FCNC top quark decays in extra dimensions

The flavor changing neutral top quark decay t→cX is computed, where X is a neutral standard model particle, in an extended model with a single extra dimension. The cases for the photon, X=γ, and a standard model Higgs boson, X=H, are analyzed in detail in a non-linear R_ξ gauge. We find that the branching ratios can be enhanced by the dynamics originating in the extra dimension. In the limit where 1/R≫m_t, we have found Br(t→cγ)≃10^-10 for 1/R = 0.5 TeV. For the decay t→cH, we have found Br(t→cH)≃10^-10 for a low Higgs mass value. The branching ratios go to zero when 1/R→∞.     

Unparticle physics and lepton flavor violating radion decays in the Randall–Sundrum scenario

We predict the branching ratios of the lepton flavor violating radion decays r→e^±μ^±, r→e^±τ^± and r→μ^±τ^± in the framework of the Randall-Sundrum scenario in which the lepton flavor violation is by scalar unparticle mediation. We observe that their BRs are strongly sensitive to the unparticle scaling dimension and, for small values, the branching ratios can reach values of the order of 10^-8, for the heavy lepton flavor case.     

Lepton flavor violating Z-boson decays induced by scalar unparticle

We predict the branching ratios of the lepton-flavor violating Z-boson decays Z→e^±μ^±, Z→e^±τ^± and Z→μ^±τ^± in the case that the lepton-flavor violation is carried by scalar unparticle mediation. We observe that their BRs are strongly sensitive to the unparticle scaling dimension, and the branching ratios can reach values of the order of 10^-8, for the heavy lepton-flavor case, for small values of the scaling dimension.     

μ → <Emphasis Type="Italic">e</Emphasis>γ decay versus the μ → <Emphasis Type="Italic">eee</Emphasis> bound and lepton flavor violating processes in supernova

Even tiny lepton flavor violation (LFV) due to some New Physics is able to alter the conditions inside a collapsing supernova core and probably to facilitate the explosion. LFV emerges naturally in a see-saw type-II model of neutrino mass generation. Experimentally, the LFV beyond the Standard Model is constrained by rare lepton decay searches. In particular, strong bounds are imposed on the μ → eee branching ratio and on the μ-e conversion in muonic gold. Currently, the μ→eγ is under investigation in the MEG experiment that aims at a dramatic increase in sensitivity in the next three years. We seek a see-saw type-II LFV pattern that fits all the experimental constraints, leads to Br(μ →eγ) ≳ Br(μμ →eee), and ensures a rate of LFV processes in supernova high enough to modify the supernova physics. These requirements are sufficient to eliminate almost all freedom in the model. In particular, they lead to the prediction 0.4 × 10⁻¹² ≲ Br(μ → eγ) ≲ 6 × 10⁻¹², which will be testable by MEG in the nearest future. The considered scenario also constrains the neutrino mass-mixing pattern and provides lower and upper bounds on τ-lepton LFV decays. We also briefly discuss a model with a single bilepton in which the μ → eee decay is absent at the tree level.     

Lepton flavor violating Z→l1 +l2 - decay in the split fermion scenario in the two Higgs doublet model

We predict the branching ratios of the Z→e^±μ^±, Z→e^±τ^± and Z→μ^±τ^± decays in the framework of the 2HDM, in the split fermion scenario. We observe that the branching ratios are not sensitive to a single extra dimension; however, this sensitivity is considerably large for two extra dimensions.     

Prospects for the measurement of the structure of the coupling of a Higgs boson to weak gauge bosons in weak boson fusion with the ATLAS detector

The prospects for the measurement of the tensor structure of the vertex between a standard model Higgs boson and two weak gauge bosons using the distribution of the azimuthal angles between the two tagging jets in the weak boson fusion channel are studied in a Monte Carlo analysis using the fast simulation of the ATLAS detector. The decay channels H→τ⁺τ^-→ll+4ν, H→τ⁺τ^-→lh+3ν at m_H=120 GeV and H→W⁺W^-→llνν at m_H=160 GeV are used in the analysis. For a standard model Higgs boson it is found that purely anomalous couplings are expected to be excluded at a confidence level corresponding to 2σ or more at m_H=120 GeV and more than 5σ at m_H=160 GeV from 30 fb^-1 of data. With a value of 1 roughly reproducing the standard model cross section for a purely anomalous coupling, the standard deviation in a measurement of a contribution of a CP even anomalous coupling in addition to the standard model coupling is estimated to be 0.20 at m_H=120 GeV and 0.09 at m_H=160 GeV.     

Search for the lepton flavour-violating decay μ→eγ

I introduce the upcoming MEG experiment, which will search for the rare decay μ→eγ down to the branching ratio of 10^-13. In order to suppress the background and achieve this unprecedented sensitivity, a great deal of thought went into designing this experiment. Here, I describe the essential components of this experiment, the beam line, the positron spectrometer, and the liquid xenon γ-ray detector. PACS 29.40.Gx; 29.40.Mc     

Bound states of the muon-antimuon system: Lifetimes and hyperfine splitting

We examine the properties of an atomic system consisting of a muon and antimuon. Expressions are derived for the probability of decay and the hyperfine splitting of the lower levels with allowance for the leading radiative corrections, which are of relative order α. The results for the lifetimes and the ground-state energy are τ(1³ S ₁)=1.7907(8)×10⁻¹² s, τ(1¹ S ₀)=0.59547(33)×10⁻¹² s, and E _hfs (1s)=4.23284(35)×10⁷ MHz. The relative probabilities for the various decay channels are calculated; in particular, for the 1 ³ S ₁ level it is found that Γ (μμ→eeγ)/Γ(μμ→ee)≈15%. Finally, possible applications are discussed. Zh. éksp. Teor. Fiz. 113, 409–431 (February 1998)     

Charged lepton electric dipole moments with localized leptons and new Higgs doublet in the two Higgs doublet model

We study the electric dipole moments of the leptons in the split fermion scenario, in the two Higgs doublet model, where the new Higgs scalars are localized around the origin in the extra dimension, with the help of the localizer field. We observe that the numerical value of the electric dipole moment of the electron (muon, tau) is of the order of the magnitude of 10^-31 (10^-24, 10^-22) (e cm), and this quantity is sensitive to the new Higgs localization in the extra dimension.     

Search for solar axions emitted in the M1-transition of 7Li* with Borexino CTF

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of ⁷Li^*. The Compton conversion of axion to a photon A+e→e+γ, axioelectric effect A+e+Z→e+Z, decay of axion in two photons A→2γ and Primakoff conversion on nuclei A+Z→γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons – g_Aeg_AN≤(1.0–2.4)×10^-10 at m_A≤450 keV and g_Aγg_AN≤5×10^-9 GeV^-1 at m_A≤10 keV are obtained (90%c.l.). For heavy axions with mass at 100<m_A<400 keV the limits g_Ae<(0.7–2.0)×10^-8 and g_Aγ<10^-9–10^-8 are obtained in assumption that g_AN depends on m_A as for KSVZ axion model. These limits are stronger than obtained in previous laboratory-based experiments using nuclear reactor and artificial radioactive sources. PACS 14.80.Mz; 29.40.Mc; 26.65.+t     

Probing CP-violating Higgs contributions in γγ → f$$ \bar f $$

We discuss the use of fermion polarization for studying neutral Higgs bosons at a photon collider. To this aim we construct polarization asymmetries which can isolate the contribution of a Higgs boson ϕ in γγ → f , f = τ/t, from that of the QED continuum. This can help in getting information on the γγϕ coupling in case ϕ is a CP eigenstate. We also construct CP-violating asymmetries which can probe CP mixing in case ϕ has indeterminate CP. Furthermore, we take the MSSM with CP violation as an example to demonstrate the potential of these asymmetries in a numerical analysis. We find that these asymmetries are sensitive to the presence of a Higgs boson as well as its CP properties over a wide range of MSSM parameters.      

The flavor of quantum gravity

We develop an effective field theory to describe the coupling of non-thermal quantum black holes to particles such as those of the Standard Model. The effective Lagrangian is determined by imposing that the production cross section of a non-thermal quantum black hole be given by the usual geometrical cross section. Having determined the effective Lagrangian, we estimate the contribution of a virtual hole to the anomalous magnetic moment of the muon, μ→eγ transition and to the electric dipole moment of the neutron. We obtain surprisingly weak bounds on the Planck mass due to a chiral suppression factor in the calculated low energy observables. The tightest bounds come from μ→eγ and the limit on the neutron electric dipole moment. These bounds are in the few TeV region.     

B- c→η’ℓ-ν̄ decay and lepton polarization asymmetry

In this paper we study the lepton polarization asymmetry for the semileptonic OZI-forbidden annihilation decay B^- _c→η’ℓ^-ν̄, where ℓ=μ,τ. Our results show that the branching ratios turn out to be of order 10^-4. Besides, we find that longitudinal, transversal and normal components of lepton polarizations can be measured for both μ and τ decay modes in the future experiments at the LHC. PACS 14.40.Nd; 13.20.He; 13.60.-r; 11.30.Er     

Production of the charged top-pions from TC2 model at the TeV energy e<Superscript>-</Superscript>γ colliders

The top-pions (Π_t ^0,±) and the top-Higgs (h_t ⁰) are the typical particles predicted by the topcolor-assisted technicolor (TC2) model and the observation of these particles can be regarded as direct evidence of the TC2 model. In this paper, we study three pair production processes of these new particles at the next generation eγ colliders, i.e., e^-γ→e^-Π_t ⁺Π_t ^-, e^-γ→ν_eΠ_t ^-Π_t ⁰ and e^-γ→ν_eΠ_t ^-h_t ⁰. The results show that the production rates can reach the level 10⁰–10¹ fb with reasonable parameter values. So one can expect that enough signals could be produced in future high- energy linear collider experiments. Furthermore, the flavor-changing (FC) decay modes Π_t ^-→bc̄, Π_t ⁰(h_t ⁰)→tc̄ can provide us with the typical signal to detect these new particles. PACS 12.60Nz; 14.80.Mz; 12.15.LK; 14.65.Ha     

Study of triple-gauge-boson couplings ZZZ, ZZγ and Zγγ at LEP

Neutral triple-gauge-boson couplings ZZZ, ZZγ and Zγγ have been studied with the DELPHI detector using data at energies between 183 and 208 GeV. Limits are derived on these couplings from an analysis of the reactions e⁺e^-→Zγ, using data from the final states γff̄, with f=q or ν, from e⁺e^-→ZZ, using data from the four-fermion final states qq̄qq̄, qq̄μ⁺μ^-, qq̄e⁺e^-, qq̄νν̄, μ⁺μ^-νν̄ and e⁺e^-νν̄, and from e⁺e^-→Zγ^*, in which the final state γ is off mass-shell, using data from the four-fermion final states qq̄e⁺e^- and qq̄μ⁺μ^-. No evidence for the presence of such couplings is observed, in agreement with the predictions of the Standard Model.