LFV Decays Research Articles

Flavorful vector-like leptons and the lepton-flavor-violating decays Z → lilj

The singlet and doublet VLS models are one kind of interesting new physics scenarios, which predict the existence of the SU(2) L singlet and doublet vector-like leptons (VLLs), respectively. Taking into account the constraints on these VLS models, we investigate their contributions to the LFV decays . Our numerical results show that, for the singlet VLS model, the values of the branching ratios and can be larger than in a wide range of the parameter space, which might be detected by CEPC/FCC-ee in the near future.

Reading the footprints of the B-meson flavor anomalies

Motivated by the recent LHCb announcement of a 3.1σ violation of lepton- flavor universality in the ratio RK = Γ(B → Kμ+μ−)/Γ(B → Ke+e−), we present an updated, comprehensive analysis of the flavor anomalies seen in both neutral-current (b → sℓ+ℓ−) and charged-current (b → ctau overline{nu} ) decays of B mesons. Our study starts from a model-independent effective field-theory approach and then considers both a simplified model and a UV-complete extension of the Standard Model featuring a vector leptoquark U1 as the main mediator of the anomalies. We show that the new LHCb data corroborate the emerging pattern of a new, predominantly left-handed, semileptonic current-current interaction with a flavor structure respecting a (minimally) broken U(2)5 flavor symmetry. New aspects of our analysis include a combined analysis of the semileptonic operators involving tau leptons, including in particular the important constraint from Bs- {overline{B}}_s mixing, a systematic study of the effects of right-handed leptoquark couplings and of deviations from minimal flavor-symmetry breaking, a detailed analysis of various rare B-decay modes which would provide smoking-gun signatures of this non-standard framework (LFV decays, di-tau modes, and B → K(*) nu overline{nu} ), and finally an updated analysis of collider bounds on the leptoquark mass and couplings.

High Scale Type-II Seesaw, Dominant Double Beta Decay within Cosmological Bound and LFV Decays in SU(‎5)

Very recently novel implementation of type-II seesaw mechanism for neutrino mass has been proposed in SU(5) grand unified theory with a number of desirable new physical phenomena beyond the standard model. Introducing heavy right-handed neutrinos and extra fermion singlets, in this work we show how the type-I seesaw cancellation mechanism works in this SU(5) framework. Besides predicting verifiable LFV decays, we further show that the model predicts dominant double beta decay with normal hierarchy or inverted hierarchy of active light neutrino masses in concordance with cosmological bound. In addition a novel right-handed neutrino mass generation mechanism, independent of type-II seesaw predicted mass hierarchy, is suggested in this work.

New mechanism for Type-II seesaw dominance in SO(10) with low-mass $$Z^{\prime }$$ Z ′ , RH neutrinos, and verifiable LFV, LNV and proton decay

Dominance of type-II seesaw mechanism for neutrino masses has attracted considerable attention because of a number of advantages. We show a novel approach to achieve Type-II seesaw dominance in non-supersymmetric $SO(10)$ grand unification where a low mass $Z^{\prime}$ boson and specific patterns of right-handed neutrino masses are predicted within the accessible energy range of the Large Hadron Collider. In spite of the high value of the seesaw scale, $M_{\Delta_L} \simeq 10^8-10^9$ GeV, the model predicts new dominant contributions to neutrino-less double beta decay in the $W_L-W_L$ channel close to the current experimental limits via exchanges of heavier singlet fermions used as essential ingredients of this model even when the light active neutrino masses are normally hierarchical or invertedly hierarchical. We obtain upper bounds on the lightest sterile neutrino mass $m_s\lesssim 3.0$ GeV, $2.0$ GeV, and $0.7$ GeV for normally hierarchical, invertedly hierarchical, and quasi-degenerate patterns of light neutrino masses, respectively. The underlying non-unitarity effects lead to lepton flavor violating decay branching ratios within the reach of ongoing or planned experiments and the leptonic CP-violation parameter nearly two orders larger than the quark sector. Some of the predicted values on proton lifetime for $p\to e^+\pi^0$ are found to be within the currently accessible search limits. Other aspects of model applications including leptogenesis etc. are briefly indicated.

A new track reconstruction algorithm for the Mu3e experiment based on a fast multiple scattering fit

A new fast track reconstruction algorithm developed for the high track multiplicity environment of the Mu3e experiment where track uncertainties are dominated by multiple scattering is presented.The goal of the Mu3e experiment is to search for the LFV decay μ+ → e+e−e+. To reach the sensitivity of 10-16 the experiment will be performed at a future high intensity beam line (HiMB) at the Paul-Scherrer Institute (Switzerland) providing more than 109 muons per second. Muons with a momentum of about 28 MeV/c are stopped on a target. Their decay at rest, in which mainly low momentum electrons with energies below 53 MeV are produced, is measured by the Mu3e tracking detector consisting of four cylindrical layers of thin silicon pixel sensors. The high granularity of the pixel detector with a pixel size of 80 × 80 μm2 allows for precise track reconstruction in the high occupancy environment of the Mu3e experiment reaching up to 100 tracks per readout frame of 50 ns. These tracks will be reconstructed online using a trigger-less readout scheme. The implementation of a fast 3-dimensional multiple scattering fit based on hit triplets, where spatial uncertainties are ignored, is described and performance results in the context of Mu3e experiment are presented. Also the implementation on Graphics Processor Units (GPUs) for fast online reconstruction is discussed.

LEPTON FLAVOR VIOLATION DECAYS OF VECTOR MESONS IN UNPARTICLE PHYSICS

We investigate the lepton flavor violation decays of vector mesons in the scenario of the unparticle physics by considering the constraint from μ-e conversion. In unparticle physics, the predictions of LFV decays of vector mesons depend strongly on the scale dimension [Formula: see text]. The predictions of LFV decays of vector mesons can reach the detective sensitivity in experiment in region of [Formula: see text], while the prediction of μ-e conversion rate can meet the experimental upper limit. For the searching of the lepton flavor violation processes of charged lepton sector in experiment, the process ϒ→eμ may be a promising one to be observed.

Lepton flavor violating decays of vector mesons

We estimate the rates of lepton flavor violating decays of the vector mesons $\rho, \omega, \phi \to e \mu$. The theoretical tools are based on an effective Lagrangian approach without referring to any specific realization of the physics beyond the standard model responsible for lepton flavor violation (\Lfv). The effective lepton-vector meson couplings are extracted from the existing experimental bounds on the nuclear $\mu^--e^-$ conversion. In particular, we derive an upper limit for the \Lfv branching ratio ${\rm Br}(\phi \to e \mu) \leq1.3 \times 10^{-21}$ which is much more stringent than the recent experimental result ${\rm Br}(\phi \to e \mu) < 2 \times 10^{-6}$ presented by the SND Collaboration. Very tiny limits on \Lfv decays of vector mesons derived in this letter make direct experimental observation of these processes unrealistic.