Lepton Flavor Violation Signal Research Articles

Complementary signals of lepton flavor violation at a high-energy muon collider

A muon collider would be a powerful probe of flavor violation in new physics. There is a strong complementary case for collider measurements and precision low-energy probes of lepton flavor violation (as well as CP violation). We illustrate this by studying the collider reach in a supersymmetric scenario with flavor-violating slepton mixing. We find that the collider could discover sleptons and measure the slepton and neutralino masses with high precision, enabling event reconstruction that could cleanly separate flavor-violating new physics signals from Standard Model backgrounds. The discovery reach of a high-energy muon collider would cover a comparably large, and overlapping, range of parameter space to future μ → e conversion and electron EDM experiments, and unlike precision experiments could immediately shed light on the nature of new physics responsible for flavor violation. This complementarity strengthens the case that a muon collider could be an ideal energy-frontier laboratory in the search for physics beyond the Standard Model.

Lepton flavor violation induced by dark matter

Guided by gauge principles we discuss a predictive and falsifiable UV complete model where the Dirac fermion that accounts for the cold dark matter abundance in our universe induces the lepton flavor violation (LFV) decays $\mu \rightarrow e\gamma$ and $\mu \rightarrow e e e$ as well as $\mu-e$ conversion. We explore the interplay between direct dark matter detection, relic density, collider probes and lepton flavor violation to conclusively show that one may have a viable dark matter candidate yielding flavor violation signatures expected to be fully probed in the upcoming of experiments. Interestingly, keeping the dark matter mass not far from the TeV, our model has an approximate prediction for the maximum LFV signal one could have while reproducing the correct dark matter relic density.

Probing lepton flavor violation signal via [formula omitted] in the littlest Higgs model with T-parity at the ILC

In the littlest Higgs model with T-parity, the new interactions between the mirror leptons and the Standard Model leptons can induce some lepton flavor violation (LFV) processes at loop level. We study the possibility of the ILC to probe the LFV production processes e + e − ( γ γ ) → l i l ¯ j . Our results show that the rates of γ γ → l i l ¯ j can reach 1 fb in optimal cases after reasonable kinematical cuts, which implies that these processes may be observed at the ILC.

Neutrino mass, sneutrino dark matter and signals of lepton flavor violation in the MRSSM

We study the phenomenology of mixed-sneutrino dark matter in the Minimal R-Symmetric Supersymmetric Standard Model (MRSSM). Mixed sneutrinos fit naturally within the MRSSM, as the smallness (or absence) of neutrino Yukawa couplings singles out sneutrino A-terms as the only ones not automatically forbidden by R-symmetry. We perform a study of randomly generated sneutrino mass matrices and find that (i) the measured value of $\Omega_{DM}$ is well within the range of typical values obtained for the relic abundance of the lightest sneutrino, (ii) with small lepton-number-violating mass terms $m_{nn}^{2} {\tilde n} {\tilde n}$ for the right-handed sneutrinos, random matrices satisfying the $\Omega_{DM}$ constraint have a decent probability of satisfying direct detection constraints, and much of the remaining parameter space will be probed by upcoming experiments, (iii) the $m_{nn}^{2} {\tilde n} {\tilde n}$ terms radiatively generate appropriately small Majorana neutrino masses, with neutrino oscillation data favoring a mostly sterile lightest sneutrino with a dominantly mu/tau-flavored active component, and (iv) a sneutrino LSP with a significant mu component can lead to striking signals of e-mu flavor violation in dilepton invariant-mass distributions at the LHC.

Waiting for μ →eγ from the MEG experiment

The Standard Model (SM) predictions for the lepton flavor-violating (LFV) processes like μ → eγ are well far from any realistic experimental resolution, thus, the appearance of μ → eγ at the running MEG experiment would unambiguously point towards a New Physics (NP) signal. In this article, we discuss the phenomenological implications in case of observation/improved upper bound on μ → eγ at the running MEG experiment for supersymmetric (SUSY) scenarios with a see-saw mechanism accounting for the neutrino masses. We outline the role of related observables to μ → eγ in shedding light on the nature of the SUSY LFV sources providing useful tools i) to reconstruct some fundamental parameters of the neutrino physics and ii) to test whether an underlying SUSY Grand Unified Theory (GUT) is at work. The perspectives for the detection of LFV signals in τ decays are also discussed.

Effective Lagrangian description of Higgs mediated flavor violating electromagnetic transitions: Implications on lepton flavor violation

Higgs mediated flavor violating electromagnetic interactions, induced at the one-loop level by a nondiagonal $H{f}_{i}{f}_{j}$ vertex, with ${f}_{i}$ and ${f}_{j}$ charged leptons or quarks, are studied within the context of a completely general effective Yukawa sector that comprises $S{U}_{L}(2)\ifmmode\times\else\texttimes\fi{}{U}_{Y}(1)$-invariant operators of up to dimension-six. Exact formulae for the one-loop $\ensuremath{\gamma}{f}_{i}{f}_{j}$ and $\ensuremath{\gamma}\ensuremath{\gamma}{f}_{i}{f}_{j}$ couplings are presented and their related processes used to study the phenomena of Higgs mediated lepton flavor violation. The experimental limit on the $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ decay is used to derive a bound on the branching ratio of the $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}\ensuremath{\gamma}$ transition, which is 6 orders of magnitude stronger than the current experimental limit. Previous results on the $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$ and $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}\ensuremath{\gamma}$ decays are reproduced. The possibility of detecting signals of lepton flavor violation at $\ensuremath{\gamma}\ensuremath{\gamma}$ colliders is explored through the $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{l}_{i}{l}_{j}$ reaction, putting special emphasis on the $\ensuremath{\tau}\ensuremath{\mu}$ final state. Using the bound imposed on the $H\ensuremath{\tau}\ensuremath{\mu}$ vertex by the current experimental data on the muon anomalous magnetic moment, it is found that about half a hundred events may be produced in the International Linear Collider.

Supersymmetric Higgs mediated lepton flavor violation at a photon collider

We study a new signature of lepton flavor violation (LFV) at the Photon Collider (PC) within Supersymmetric (SUSY) theories. We consider the minimal supersymmetric standard model within a large $\tan\beta$ scenario with all superpartner masses in the ${\cal O}$(TeV) while the heavy Higgs bosons masses lie below the TeV and develop sizable loop induced LFV couplings to the leptons. We consider a photon collider based on an $e^+e^-$ linear collider with $\sqrt{s}=800$ GeV with the parameters of the TESLA proposal and show that, with the expected integrated $\gamma\gamma$-luminosity $L_{\gamma\gamma}=200\div 500$ fb$^{-1}$, the "$\mu\tau$ fusion" mechanism is the dominant channel for the process $\gaga\to\mu\tau b\bar{b}$ providing detailed analytical and numerical studies of the signal and backgrounds. We impose on the parameter space present direct and indirect constraints from $B$ physics and rare LFV $\tau$-decays and find that the LFV signal can be probed for masses of the heavy neutral Higgs bosons $A,H$ from 300 GeV up to the kinematical limit $\simeq 600$ GeV for 30$\leq\tan\beta\leq$60.

Lepton flavor violation and supersymmetric Dirac leptogenesis

Dirac leptogenesis (or Dirac neutrinogenesis), in which neutrinos are purely Dirac particles, is an interesting alternative to the standard leptogenesis scenario. In its supersymmetric version, the modified form of the superpotential required for successful baryogenesis contributes new, generically nonflavor-diagonal terms to the slepton and sneutrino mass matrices. In this work, we examine how current experimental bounds on flavor-changing effects in the lepton sector (and particularly the bound on {mu}{yields}e{gamma}) constrain Dirac leptogenesis and we find that it is capable of succeeding with superpartner masses as low as {approx}100 GeV. For such light scalars and electroweakinos, upcoming experiments such as MEG are generically expected to observe signals of lepton flavor violation.

LEPTON FLAVOR VIOLATING SIGNALS OF THE NEUTRAL SCALARS FROM TOPCOLOR SCENARIO AT HADRON COLLIDERS

In the context of topcolor-assisted techicolor (TC2) models, we study the lepton flavor violating (LFV) signals of the neutral scalars (the top-pion and the top-Higgs boson) at hadron colliders via considering their contribution to the processes [Formula: see text] (l = μ or e). We find that the LFV signals of these new particles cannot be detected at the Tevatron with [Formula: see text], while might be observable in the future LHC with [Formula: see text].

Lepton Flavor Violation: — Experimental —

Lepton flavor violation (LFV) in the charged sector has been gaining great interests these days. Experimental researches looking for muon LFV such as MEG and MECO are in preparation, and aiming to discover the muon LFV signal within this decade. There is also another activity called PRISM/PRIME project underway, which aims to expand muon LFV research furthermore. The status of those experimental studies will be described. The idea of building Muon Factory and its relevance to the future of neutrino physics is also commented upon.

Probing Lepton Flavor Violation Signal Induced by R-violating Minimal Supersymmetric Standard Model at a Linear Collider

The lepton-flavor violation (LFV) effect at an e+e− linear collider (LC), in the frame of R-parity violating minimal supersymmetric standard model is studied. We take the R-parity violating processes e+e−→eμ± as signal, and define the summation of the two processes as ``experiment'' observable. We find that the cross-section summation can reach (101)fb in the parameter space without sneutrino resonance effect(s1/2 ~ m). The summation treatment manifests uniform differential distribution on cos θ, where θ denotes the polar angles of both outgoing e+/e− respectively to incoming electron beam in two signal processes. The uniform feature together with eμ collinearity would help to reduce the SM background dramatically. Consequently we conclude that at a 500 GeV LC with 480 fb−1 annual luminosity, it's either possible to detect the distinctive R-violating LFV eμ signal, or exclude sneutrino to m > 1.1 TeV at 95% CL in the machine's biennial runtime interval.

Rare muon decays at the neutrino factories

New experiments (MEG, MECO) searching for the lepton-flavor violation signal in the muon decay are shown in relation to their ancestor experiments (MEGA, SINDRUM II). It will be also shown tha the neutrino factories and their technologies could be very useful for persuading the rare muon decay experiments further more. As an example, a project PRISM will be explained and the prospect to the rare muon decays and the neutrino factories are shown.

Neutrino oscillations, SUSY see-saw mechanism and charged lepton flavor violation

Neutrino oscillations give clear evidence for non-vanishing neutrino masses and lepton-flavor violation (LFV) in the neutrino sector. This provides strong motivation to search for signals of LFV also in the charged lepton sector, and to probe the SUSY see-saw mechanism. We compare the sensitivity of rare radiative decays on the right-handed Majorana mass scale M R with the reach in slepton-pair production at a future linear collider.

A search for leptoquark bosons and lepton flavor violation in $e^+ p$ collisions at HERA

A search for new bosons possessing couplings to lepton-quark pairs is performed in the H1 experiment at HERA using 1994 to 1997 data corresponding to an integrated luminosity of \(37\) pb\(^{-1}\). First generation leptoquarks (LQs) are searched in very high \(Q^2\) neutral (NC) and charged (CC) current data samples. The measurements are compared to Standard Model (SM) expectations from deep-inelastic scattering (DIS). A deviation in the \(Q^2\) spectrum previously observed in the 1994 to 1996 dataset at \(Q^2 \gtrsim 15000\) GeV\(^2\) remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at \(M \simeq 200\) GeV which is not observed with the 1997 dataset alone. The NC DIS data is used to constrain the Yukawa couplings \(\lambda\) of first generation scalar and vector LQs in the Buchmuller–Ruckl–Wyler effective model. Scalar LQs are excluded for masses up to 275 GeV for a coupling of electromagnetic strength, \(\lambda = 0.3\). A sensitivity to coupling values \(\lesssim 1\) is established for masses up to 400 GeV for any LQ type. The NC and CC DIS data are combined to constrain \(\lambda\) for arbitrary branching ratios of the LQ into eq in a generic model. For a decay branching ratio into \(e^+ u\) pairs as small as 10%, LQ masses up to 260 GeV are ruled out for \(\lambda = 0.3\). LQs possessing couplings to mixed fermion generations, which could lead to signals of lepton flavor violation (LFV), are searched in events with a high transverse momentum \(\mu\) or \(\tau\). No \(\mu + X\) or \(\tau + X\) event candidate is found that is compatible with LQ kinematics. Constraints are set on the Yukawa coupling involving the \(\mu\) and \(\tau\) lepton in a yet unexplored mass range.