Search For Lepton Flavor Violation Research Articles

Mu2e: A high-sensitivity search for charged lepton flavor violation at Fermilab

The Mu2e collaboration proposes to search for the coherent, neutrinoless conversion of muons into electrons in the field of a nucleus, μ − N → e − N , with a single-event sensitivity of ∼ 10 − 16 , an improvement of a factor of 10,000 over existing limits. This lepton flavor-violating reaction probes mass scales unavailable by direct searches at either present or planned high energy colliders. The apparatus is described as well as a scheme by which the experiment can be mounted in the present Fermilab accelerator complex. Prospects for increased sensitivity using the Project X accelerator upgrades that are being proposed by Fermilab are briefly discussed.

Neutrino masses from loop-induced Dirac Yukawa couplings

We consider a possibility to naturally explain tiny neutrino masses without the lepton number violation. We study a simple model with SU(2)L singlet charged scalars (s1±,s2±) as well as singlet right-handed neutrino (νR). Yukawa interactions for Dirac neutrinos, which are forbidden at the tree level by a softly-broken Z2 symmetry, are induced at the one-loop level via the soft-breaking term in the scalar potential. Consequently neutrinos obtain small Dirac masses after the electroweak symmetry breaking. It is found that constrains from neutrino oscillation measurements and lepton flavor violation search results (especially for μ→eγ) can be satisfied. We study the decay pattern of the singlet charged scalars, which could be tested at the LHC and the ILC. We discuss possible extensions also, e.g. to introduce dark matter candidate.

TeV SCALE MODELS OF NEUTRINO MASSES AND THEIR PHENOMENOLOGY

We review various TeV scale models for neutrino masses utilizing different types of seesaw mechanisms, higher dimensional operators, expanded gauge symmetries, or low scale extra dimensions. In addition, we discuss the implications of these models for the collider experiments and low energy lepton flavor violation searches.

Search for lepton flavour violation at HERA

A search for second and third generation scalar and vector leptoquarks produced in ep collisions via the lepton flavour violating processes ep→μX and ep→τX is performed by the H1 experiment at HERA. The full data sample taken at a centre-of-mass energy s=319 GeV is used for the analysis, corresponding to an integrated luminosity of 245 pb−1 of e+p and 166 pb−1 of e−p collision data. No evidence for the production of such leptoquarks is observed in the H1 data. Leptoquarks produced in e±p collisions with a coupling strength of λ=0.3 and decaying with the same coupling strength to a muon–quark pair or a tau–quark pair are excluded at 95% confidence level up to leptoquark masses of 712 GeV and 479 GeV, respectively.

Low-energy neutrino scattering measurements at future spallation source facilities

In the future several spallation source facilities will be available worldwide. spallation sources produce large amounts of neutrinos from decay-at-rest muons and thus can be well adapted to accommodate state-of-the-art neutrino experiments. In this paper low-energy neutrino scattering experiments that can be performed at such facilities are reviewed. Estimation of expected event rates is given for several nuclei, electrons and protons at a detector located close to the source. A neutrino program at spallation sources comprises neutrino–nucleus cross section measurements relevant for neutrino and core-collapse supernova physics, electroweak tests and lepton-flavor violation searches.

Rare decays of the tau lepton at Belle

We report recent results of searches for lepton flavor violation and second class current in decays of τ leptons using the data sample collected in the Belle experiment at the KEKB electron-positron asymmetric-energy collider.

Search for Charged Lepton Flavor Violation in NarrowΥDecays

Charged lepton flavor violating processes are unobservable in the standard model, but they are predicted to be enhanced in several extensions to the standard model, including supersymmetry and models with leptoquarks or compositeness. We present a search for such processes in a sample of 99M Y(2S) decays and 117M Y(3S) decays collected with the BABAR detector. We place upper limits on the branching fractions BF(Y(nS) -> e tau) and BF(Y(nS) -> mu tau) (n = 2, 3) at the 10^(-6) level and use these results to place lower limits of order 1 TeV on the mass scale of charged lepton flavor violating effective operators.

Non-standard neutrino interactions in the Zee–Babu model

We investigate non-standard neutrino interactions (NSIs) in the Zee–Babu model. The size of NSIs predicted by this model is obtained from a full scan over the parameter space, taking into account constraints from low-energy experiments such as searches for lepton flavor violation (LFV) and the requirement to obtain a viable neutrino mass matrix. The dependence on the scale of new physics as well as on the type of the neutrino mass hierarchy is discussed. We find that NSIs at the source of a future neutrino factory may be at an observable level in the νe→ντ and/or νμ→ντ channels. In particular, if the doubly charged scalar of the model has a mass in reach of the LHC and if the neutrino mass hierarchy is inverted, a highly predictive scenario is obtained with observable signals at the LHC, in upcoming neutrino oscillation experiments, in LFV processes, and for NSIs at a neutrino factory.

Probing neutrino masses and tri-bimaximality with lepton flavor violation searches

We examine relation between neutrino oscillation parameters and prediction of lepton flavor violation, in light of deviations from tri-bimaximal mixing. Our study shows that upcoming experimental searches for lepton flavor violation process can provide useful implications for neutrino mass spectrum and mixing angles. With simple structure of heavy right-handed neutrino and supersymmetry breaking sectors, the discovery of τ→μγ decay determines neutrino mass hierarchy if large (order 0.1) reactor angle is established.

Flavour violation at the LHC: type-I versus type-II seesaw in minimal supergravity

We reconsider the role that the possible detection of lepton flavour violating (LFV) decays of supersymmetric particles at the Large Hadron Collider (LHC) can play in helping reconstruct the underlying neutrino mass generation mechanism within the simplest high-scale minimal supergravity (mSUGRA) seesaw schemes. We study in detail the LFV scalar tau decays at the LHC, assuming that the observed neutrino masses arise either through the pure type-I or the simpler type-II seesaw mechanism. We perform a scan over the mSUGRA parameter space in order to identify regions where lepton flavour violating decays of ?02 can be maximized, while respecting current low-energy constraints, such as those coming from the bounds on Br(? ? e?). We estimate the cross section for ?02 ? ?01+?+?. Though insufficient for a full reconstruction of the seesaw, the search for LFV decays of supersymmetric states at the LHC brings complementary information to that coming from low energy neutrino oscillation experiments and LFV searches.

Leptoquarks and neutrino masses at the LHC

The properties of light leptoquarks predicted in the context of a simple grand unified theory and their observability at the LHC are investigated. The SU ( 5 ) symmetry of the theory implies that the leptoquark couplings to matter are related to the neutrino mass matrix. We study the resulting connection between neutrino masses and mixing parameters and the leptoquark decays, and show that different light neutrino hierarchies imply distinctive leptoquark decay signatures. We also discuss low-energy constraints implied by searches for charged lepton flavour violation, studies of meson decays, and electroweak precision data. We perform a detailed parton-level study of the leptoquark signals and the Standard Model backgrounds at the LHC. With the clean final states containing a di-lepton plus two jets, the QCD production of the leptoquark pair can be observed for a leptoquark mass of one TeV and beyond. By examining the lepton flavor structure of the observed events, one could further test the model predictions related to the neutrino mass spectrum. In particular, b-flavor tagging will be useful in distinguishing the neutrino mass pattern and possibly probing an unknown Majorana phase in the Inverted Hierarchy or the Quasi-Degenerate scenario. Electroweak associated production of the leptoquark doublet can also be useful in identifying the quantum numbers of the leptoquarks and distinguishing between the neutrino mass spectra, even though the corresponding event rates are smaller than for QCD production. We find that with only the clean channel of μ + E̸ T + jets , one could expect an observable signal for a leptoquark masses of about 600 GeV or higher.

Search for Lepton Flavor Violating τ Decay at Belle

We review recent searches for lepton flavor violation in τ − decays based on > 500 fb − 1 of data collected with the Belle detector at the KEKB asymmetric-energy e + e − collider. No evidence for these decays is observed and we set 90% confidence level upper limits on the branching fractions at the O ( 10 − 8 ) level.

Lepton Flavor violation searches with kaons at NA62

The ratio RK=Γ(K±→e±ν)/Γ(K±→μ±ν) between the two leptonic decay rates of charged kaons is precisely predicted within the Standard Model (SM). A recent theoretical work noticed that SUSY extensions of the SM can induce mu-electron universality violation in such a way to introduce a shift of the RK value up to few percent respect to SM prediction. Presently the experimental relative error of RK, that includes also the last two recent measurements from NA48 and KLOE, is around 1.5%. A new precise measurement of RK can be of extreme interest in order to have a more stringent test of the SM. During summer 2007 the NA62 collaboration performed a dedicated period of measurements at the CERN SPS in order to achieve both the statistical and systematic accuracy for a 0.5% relative error. A sample of about 110000K+→e+ν decays has been collected and special runs dedicated to the study of the main sources of systematic effects have been performed. The experimental set-up and the analysis strategy will be described.

Search for Lepton Flavor Violation in Upsilon Decays

In this Letter, we describe a search for lepton flavor violation (LFV) in the bottomonium system. We search for leptonic decays Upsilon(nS)-->mutau (n=1, 2, and 3) using the data collected with the CLEO III detector. We identify the tau lepton using its leptonic decay nu_{tau}nu[over ]_{e}e and utilize multidimensional likelihood fitting with probability density function shapes measured from independent data samples. We report our estimates of 95% C.L. upper limits on LFV branching fractions of Upsilon mesons. We interpret our results in terms of the exclusion plot for the energy scale of a hypothetical new interaction versus its effective LFV coupling in the framework of effective field theory.

Charged Lepton Flavor Violation Experiments

We provide a review of the status of experimental searches for lepton flavor violation involving electrons, muons, and tau leptons. Future experimental programs are discussed and placed in the context of theories beyond the standard model.

Search for lepton flavor violating τ decay at Belle experiment

We review recent searches for lepton flavor violation in τ− decays, ℓ− ℓ+ ℓ− (ℓ = e μ and ϕ, K0 (892) and K̄0 (892)), based on using > 500 fb-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e− collider. No evidence for these decays is observed and we set 90% confidence level upper limits on the branching fractions as the O(10-8) level.

Search for lepton flavour violation in ep collisions at HERA

A search for the lepton flavour violating processes ep->mu X and ep -> tau X is performed with the H1 experiment at HERA. Final states with a muon or tau and a hadronic jet are searched for in a data sample corresponding to an integrated luminosity of 66.5 pb-1 for e^+ p collisions and 13.7 pb^-1 for e^- p collisions at a centre-of-mass energy of 319 GeV. No evidence for lepton flavour violation is found. Limits are derived on the mass and the couplings of leptoquarks inducing lepton flavour violation in an extension of the Buchm"uller-R"uckl-Wyler effective model. Leptoquarks produced in ep collisions with a coupling strength of lambda=0.3 and decaying with the same coupling strength to a muon-quark pair or a tau-quark pair are excluded at 95% confidence level up to masses of 459 GeV and 379 GeV, respectively.

The Super Flavor Factory

The main physics goals of a high luminosity e+e- flavor factory are discussed, including the possibilities to perform detailed studies of the CKM mechanism of quark mixing, and constrain virtual Higgs and non-standard model particle contributions to the dynamics of rare B_u,d,s decays. The large samples of $D$ mesons and tau leptons produced at a flavor factory will result in improved sensitivities on D mixing and lepton flavor violation searches, respectively. One can also test fundamental concepts such as lepton universality to much greater precision than existing constraints and improve the precision on tests of CPT from B meson decays. Recent developments in accelerator physics have demonstrated the feasibility to build an accelerator that can achieve luminosities of O(10^36) cm^-2 s^-1.

Concluding talk at NOW 2006

This writeup summarizes the status of neutrino oscillations, including recent fluxes and experimental data, as of summer 2006. A discussion is given on the current status of absolute scale of neutrino mass from tritium, neutrinoless double beta decay and cosmological observations, as well as the prospects for the next generation of experiments, including lepton flavor violation searches, and their theoretical significance.

TAU PHYSICS FROM B FACTORIES

Some recent τ-physics results are presented from the BaBar and Belle experiments at the SLAC and KEK B factories, which produce copious numbers of τ-lepton pairs. Measurements of the tau mass and lifetime allow to test lepton universality and CPT invariance, while searches for lepton-flavour violation in tau decays are powerful ways to look for physics beyond the Standard Model. In semihadronic, non-strange tau decays, the vector hadronic final state is particularly important in helping determine the hadronic corrections to the anomalous magnetic moment of the muon, while studies of strange final states are the best available ways to measure the CKM matrix element V us and the mass of the strange quark.