Recently it was noticed that among many scenarios of new physics leptoquarks might compensate for the disagreement between the lattice and experimental results for the charmed strange meson decay constant. The leptoquarks might modify also the flavor changing neutral current charm decays. In this study we investigate impact of the scalar leptoquark with electric charge -1/3 on the dilepton invariant mass distribution in the D^+ --> pi^+ mu^+ mu^- decay and on the branching ratios of the D^0 --> mu^+ mu^- using the existing experimental results.
We show that FCNC processes are unavoidable for leptoquarks that couple to left handed quarks, and derive new FCNC bounds from neutral meson mixing. Despite being induced only at one loop, these processes lead to significant bounds since the leptoquark contributions do not suffer from GIM cancellations. Studying the implications of these bounds we find that (i) The D-Dbar mixing bound is the first significant FCNC bound from the up sector. Combining it with FCNC bounds from the down sector we arrive at a bound on the first generation couplings. (ii) The K-Kbar and D-Dbar mixings bound g^2/M wh…
Some interpretations of $R_{D^{(*)}}$ anomaly in $B$ meson decay using leptoquark (LQ) models can also generate top quark decays through Flavor Changing Neutral Current (FCNC). In this work we focus on two LQs, i.e. scalar $S_1$ and vector $U_1$ which are both singlet under the $SU(2)_L$ gauge group in the Standard Model (SM). We investigate their implications on the 3-body top FCNC decays $t\to c \ell_i \ell_j$ at tree level and the 2-body $t\to c V$ at one-loop level, with $\ell$ being the SM leptons and $V=γ, Z, g$ being the SM gauge bosons. We utilize the $2σ$ parameter fitting ranges of t…
Rare $B$-decays induced by flavour-changing neutral currents (FCNC) is one of the promising candidates for probing physics beyond the Standard model. However, for identifying potential new physics from the data, reliable control over QCD contributions is necessary. We focus on one of such QCD contributions - the charming loops - that potentially can lead to difficulties in disentangling new physics effects from the observables and discuss the possibility to gain control over theoretical predictions for charming loops.
We study the one-loop contribution of the effective flavor changing neutral couplings (FCNC) $tcZ$ to the charm quark electric dipole moment. Using the known limits on the top and charm quarks electric dipole moments, we place limits on these FCNC anomalous couplings.
We compare a non-factorizable charming-loop correction to an exclusive FCNC $B$-decay given in terms of the 3-particle Bethe-Salpeter amplitude (3BS) of the $B$-meson, $\langle 0|\bar q(x)G_{μν}(z)b(0)|B(p)\rangle$, with the corresponding correction to the $B$-meson semileptonic form factor. In spite of certain similarities, these two corrections are shown to have substantial differences: The form factor correction is dominated by the collinear light-cone configuration of 3BS: $z_μ= u x_μ$, $0 < u < 1$, $x^2=0$. In contrast, the FCNC amplitude is dominated by a different configuration with non…
We revisit the calculation of nonfactorizable corrections induced by charm-quark loops in exclusive FCNC $B$-decays. For the sake of clarity, we make use of a field theory with scalar particles: this allows us to focus on the conceptual issues and to avoid technical complications related to particle spins in QCD. We perform a straightforward calculation of the appropriate correlation function and show that it requires the knowledge of the full generic three-particle distribution amplitude with non-aligned arguments, $\langle 0|\bar s(y)G_{μν}(x)b(0)|B(p)\rangle$. Moreover, the dependence of th…
We study the invisible scalar of $S$ in charmed meson and baryon decays with flavor-changing neutral currents (FCNCs) based on the model-independent effective Lagrangian between the quarks and invisible scalar. From the bounds of the coupling constants extracted from the recent BES III experiment on the decay of $D^{0}\toπ^{0}\barνν$, we predict that the decay branching ratios of $D^{+}\toπ^{+}S$, $D_s^+\to K^+S$, $Λ_c\to pS$, $Ξ_c^{0(+)}\to Σ^{0(+)}S$, and $Ξ_c^0\toΛS$ can be as large as $10.6,~2.53,~2.39,~0.963,~5.77$, and $2.92\times10^{-4}$ with $m_{_S}\approx 1.1~{\rm GeV}$, respectively.…
Motivated by the disagreement between the experimental data and lattice calculations on the decay constant of the $D_{s}$ meson, we investigate leptoquark (LQ) contributions to the purely leptonic decays of a pseudoscalar (P). We concentrate on the LQs which only couple to the second-generation quarks before the electroweak symmetry breaking and we discuss in detail how flavor symmetry breaking effects are brought into the extension of the standard model after the spontaneous symmetry breaking. We find that the hermiticity of fermion mass matrices can not only reproduce the correct Cabibbo-Kob…
Rare flavour changing neutral current (FCNC) decays of top, beauty and charm quarks can provide a powerful probe for as yet unobserved particles. Recent results on FCNC $b \to s$, $c \to u$ and $t$ transitions from the LHC experiments are reviewed. Particular attention is paid to the angular distribution of the $B^{0} \to K^{*0} μ^{+} μ^{-}$ decay, where a measurement performed by LHCb shows a local discrepancy of 3.7 standard deviations with respect to the SM prediction. Using the decay $B^{+} \to K^{+}π^{-}π^{+}γ$, LHCb have also been able to demonstrate the polarisation of photons produced …
Low-energy experiments involving kaon, B-meson, D-meson, and hyperon flavor-changing neutral transitions have confirmed the loop-induced flavor-changing neutral current (FCNC) picture of the standard model (SM). The continuing study of these processes is essential to further refine this picture and ultimately understand the flavor dynamics. In this paper we consider deviations from the SM in the charm sector and their effect on FCNC processes. Specifically, we parameterize new physics in terms of left- and right-handed anomalous couplings of the W boson to the charm quark. We present a compreh…
We investigate the possibility that scalar leptoquarks generate consequential effects on the flavor-changing neutral-current decays of charmed hadrons into final states with missing energy ($\not\!\!E$) carried away by either standard model or sterile neutrinos. We focus on scenarios involving the $R_2$, $\tilde R_2$, and $\bar S_1$ leptoquarks and take into account various pertinent constraints, learning that meson-mixing ones and those inferred from collider searches can be of significance. We find in particular that the branching fractions of charmed meson decays $D\to M\!\not\!\!E$, $M=π,ρ…
In the Minimal Supersymmetric Model (MSSM) the hitherto unconstrained flavor mixing between top-squark and charm-squark will induce the flavor-changing neutral-current (FCNC) interaction between top quark and charm quark, which then give rise to various processes at the next generation linear collider (NLC), i.e., the top-charm associated productions via $e^+ e^-$, $e^- γ$ and $γγ$ collisions as well as the top quark rare decays $t \to c V$ ($V=g$, $γ$ or $Z$). All these processes involve the same part of the parameter space of the MSSM. Through a comparative analysis for all these processes a…
We revisit rare radiative leptonic decays $B_{s,d}\to γe^+e^-$ and $B_{s,d}\to γμ^+μ^-$ in the Standard Model and provide the updated estimates for various differential distributions (the branching ratios, the forward-backward asymmetry, and $R_{μ/e}$, the ratio of the differential distribution for muons over electrons in the final state). The new ingredients of this work compared to the existing theoretical analyses are the following: (i) we calculate all $B_d\toγ$ and $B_s\toγ$ form factors induced by the vector, axial-vector, tensor and pseudotensor quark currents within the relativistic di…
We present our recent results for long-distance QCD effects in the FCNC radiative leptonic decays $B\toγ\ell^+\ell^-$, $\ell=\{e,μ\}$. One encounters here two distinct types of long-distance effects: those encoded in the $B\toγ$ transition form factors induced by the $b\to q$ quark currents, and those related to the charm-loop effects. We calculate the $B\toγ$ form factors in a broad range of the momentum transfers making use of the relativisitc dispersion approach based on the constituent quark picture which has proven to provide reliable predictions for many weak-transition form factors. Con…
In supersymmetry (SUSY) the flavor mixing between top-squark (stop) and charm-squark (scharm) induces the flavor-changing neutral-current (FCNC) stop decay $\tilde t_1 \to c \tilde χ^0_1$. Searching for this decay serves as a probe of soft SUSY breaking parameters. Focusing on the stop pair production followed by the FCNC decay of one stop and the charge-current decay of the other stop, we investigate the potential of detecting this FCNC stop decay at the Fermilab Tevatron, the CERN Large Hadron Collider (LHC) and the next-generation $e^+e^-$ linear collider (LC). We find that this decay may n…
An analysis of charm mesons decaying semileptonically via Flavor Changing Neutral Currents is presented. We calculate the Wilson coefficients within the Standard Model. A window in the decay distribution, where physics beyond the Standard Model could be measured is identified. Exemplary, we study effects of leptoquark models.
This paper is a written version of a talk on rare (FCNC) D meson decays as presented at the Electroweak Moriond 2002. The presentation proceeds in two parts. We first consider Standard Model predictions, taking into account both short-distance and long distance effects. Then several New Physics options (e.g. supersymmatry, strong dynamics, extra large dimenions, etc) are considered.
The LHCb bounds on the branching ratio of rare decay $D^0 \to μ^+ μ^-$ and the constraints on the branching ratio of $D^+ \to π^+ μ^+ μ^-$ in the nonresonant regions enable us to improve constraints on new physics contributions. Using the effective Lagrangian approach we determine sizes of the Wilson coefficients allowed by the existing LHCb bounds on rare charm decays. Then we discuss contributions to rare charm meson decay observables in several models of new physics: a model with an additional spin-1 weak triplet, leptoquark models, Two Higgs doublets model of type III, and a $Z'$ model. He…
We briefly review the studies on the top quark FCNC processes at a next-generation linear collider. Such processes, including various FCNC top quark rare decays and top-charm associated productions, are extremely suppressed in the standard model (SM) but could be significantly enhanced in some extensions. We compared the predictions from different typical new physics models: the SM, the minimal supersymmetric model (MSSM), the general two-higgs-doublet model (2HDM) and the topcolor-assisted technicolor (TC2) model. Our conclusion is: (1) While all the new physics models can enhance the rates b…
Charm procceses are usually not considered to be favorable candidates in the search for new physics. Recent disagreement between experimental and lattice QCD results on the D_s decay constant has motivated us to systematically reinvestigate role of leptoquarks in rare charm meson decays. We include constrains coming from the light meson decays.
In this Thesis we have investigated some effects appearing in top quark and Higgs boson decays with flavor changing neutral currents (FCNC) in the framework of generic Two Higgs Doblet Models (2HDM) and the Minimal Supersymmetric Standard Model (MSSM). We have applied these two extensions to the SM to see whether they can produce new FCNC effects. Introductory chapters deal with the 2HDM and the MSSM. In the next chapters we have computed the following FCNC decays: (1) the branching ratios (B) of the top quark to Higgs bosons and charm quark in the 2HDM; (2) B and number of events at the Large…
Recently a lot of new experimental results on open charm hadrons have appeared. In particular many D meson resonances have been discovered. We discuss strong decays of positive and negative parity charmed mesons within heavy meson chiral perturbation theory and study the impact of excited charm states on the determination of the effective meson couplings. Motivated by recent experimental results we also reconsider semileptonic D to P l nu and D to V l nu decays within a model which combines heavy quark symmetry and properties of the chiral Lagrangian. Using limits of soft collinear effective t…
I would like to discuss $D^0-\bar{D}^0$ the mixing and rare D decays as manifestations of Flavor Changing Neutral Currents (FCNC) in the charm sector. I will first review the expectations in the Standard Model (SM) and then summarize some typical expectations in new physics scenarios. I would like to argue that the charm case offers a large window of opportunity and it may be possible to learn something about the origin of the fermion mass matrix.
We study (semi)leptonic rare charm decays and its opportunities in searches for physics beyond the Standard Model (BSM). In particular, we analyse the impact of potential BSM physics in $c\to ull'$ transitions, notably branching ratios, angular observables, asymmetries and Lepton Flavour Violating (LFV) decays. Testable effects are worked out model-independently and within Leptoquark models supplemented with flavour patterns to link $K$/$B$ decays.
Motivated by the disagreement between experimental and lattice QCD results on the D_s decay constant we systematically reinvestigate role of leptoquarks in charm meson decays. We consider scalar leptoquarks that transform as a weak interaction triplet, doublet, or singlet in a model independent approach, and also argue that in a particular SU(5) GUT model these leptoquark states, contained in the 45-dimensional Higgs representation, could be safe against proton decay bounds. Using the current experimental measurements in tau, kaon and charm sectors, we find that scalar leptoquarks cannot natur…
Motivated by the recent measurement of $D^0\toρ^0γ$ improved standard model (SM) predictions for branching ratios and CP asymmetries of radiative charm decay are given. Weak annihilation induced decays are probes of non-perturbative QCD approaches. Rare decays probe the SM and physics beyond the SM, e.g. leptoquark and supersymmetric models. Opportunities with $Λ_c\to pγ$ for future polarization measurements are presented.
Flavor symmetries that explain masses and mixings of the standard model fermions dictate flavor patterns for the couplings of scalar and vector leptoquarks to the standard model fermions. A generic feature is that couplings to $SU(2)$-doublet leptons are suppressed at least by one spurion of the discrete non-abelian symmetry breaking, responsible for neutrino mixing, while couplings to charged lepton singlets can be order one. We obtain testable patterns including those that predominantly couple to a single lepton flavor, or two, or in a skewed way. They induce lepton non-universality, which w…
Rare FCNC decay modes of charmed mesons are reviewed. The standard model predictions, including both short and long distance contributions, are summarized. Several new physics options that can give detectable signals are described.
The LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the curre…
Aspects of top quark physics can be studied at a future Circular Electron-Positron Collider (CEPC). In particular, Flavor-Changing Neutral Current interactions (FCNC) of the top quark can be studied even with a center-of-mass energy from $\sqrt{s}\gtrsim m_t$ to $\sqrt{s}=240$ GeV, through the production a single top quark in association with a charm quark, which can arise in several scenarios of new physics. We explore here this signal within the Left-Right Mirror Model, which is a Left-Right model that includes mirror fermions; this model is motivated as a possible solution to the strong CP …
We investigate the effect of scalar leptoquarks and family non-universal $Z^\prime$ boson on the rare leptonic decays of $B_{s, d}^*$ mesons, mediated by the FCNC transitions $b \to (d, s) l^+ l^-$. They are sensitive to a variety of new physics operators as opposed to the $B_{s,d}$ leptonic modes and hence, can provide an ideal testing ground to look for new physics beyond the standard model. We work out the constraint on new physics parameter space using the measured branching ratios of $B_{d, s} \to μ^+ μ^-$ processes and the $B_q - \bar B_q$ mixing data. Using the constrained parameters we…
Flavor changing neutral currents (FCNC) decays t to H + c, t to Z + c, and H to t + bar{c} are discussed in the context of Alternative Left-Right symmetric Models (ALRM) with extra isosinglet heavy fermions where FCNC decays may take place at tree-level and are only suppressed by the mixing between ordinary top and charm quarks, which is poorly constraint by current experimental values. The non-manifest case is also briefly discussed.
Electroweak baryogenesis in a two-Higgs doublet model is a well-motivated and testable scenario for physics beyond the Standard Model. An attractive way of providing $CP$ violation is through flavor-changing Higgs couplings, where the top-charm coupling is hardly constrained. This minimal scenario can be tested by searching for heavy charged and neutral Higgs bosons at the LHC. While the charged Higgs signature requires a dedicated analysis, the neutral Higgs signature will be covered by a general search for same-sign top pairs. Together, they provide a conclusive test of this kind of baryogen…
In this work, we calculate the form factors for $J/ψ\to \bar{D}^{(*)0}$ induced by the flavor changing neutral currents (FCNC) in terms of the QCD sum rules. Making use of these form factors, we further calculate the branching fractions of semileptonic decays $J/ψ\to \bar{D}^{(*)0} l ^+ l^-$ ($l=e, μ$). In particular, we formulate the matrix element $<J/ψ|T_{μν}| \bar{D}^{*0}>$ with $T_{μν}$ being a tensor current, which was not fully discussed in previous literature. Our analysis indicates that if only the standard model (SM) applies, the production of single charmed mesons at the present ele…
Top quark and Higgs boson decays induced by flavor-changing neutra currents (FCNC) are very much suppressed in the Standard Model (SM). Their detection in colliders like the Large Hadron Collider (LHC), Next Linear Collider (NLC) or Tevatron would be a signal of new physics. We evaluate the FCNC decays t \to H^0 + c, t \to Z + c, and H^0 \to t + {\bar c} in the context of Alternative Left-Right symmetric Models (ALRM) with extra isosinglet heavy fermions; in this case, FCNC decays occurs at tree-level and they are only suppressed by the mixing between ordinary top and charm quarks, which is po…
We investigate the potential for discovering new physics through the low-energy scattering processes $e^-p\to e^-(μ^-)Λ_c$, which could be accessible in the forthcoming $ep$ scattering experiments once adapted to our proposed setups. In the framework of a general low-energy effective Lagrangian, we demonstrate that, compared with the conventional flavor-changing neutral-current weak decays of charmed hadrons and the dilepton productions at high-energy colliders, the low-energy scattering processes can provide more competitive potentials for hunting the underlying new physics. In some specific …
Updated standard model predictions for $D_{(s)} \to P l^+ l^-$, $l=e,μ$, $P=π,K$ and inclusive decays are presented. Model-independent constraints on $|ΔC|=|ΔU|=1$ Wilson coefficients are worked out. New physics (NP) opportunities arise in semileptonic branching ratios for very large couplings only, however, are not excluded outside the resonance regions yet. The NP potential of resonance-assisted CP asymmetries and angular observables is worked out. Predictions are given for leptoquark models, and include lepton flavor violating and dineutrino decays. Whether NP can be seen depends on flavor …
We study the effect of scalar leptoquark and $Z^\prime$ boson on the rare decays of $D$ mesons involving flavour changing transitions $c \to u l^+ l^- (l^\mp_i l^\pm_j)$. We constrain the new physics parameter space using the branching ratio of the rare decay mode $D^0 \to μ^+ μ^-$ and the $D^0-\bar D^0$ oscillation data. We compute the branching ratios, forward-backward asymmetry parameters and flat terms in $D^{+(0)} \to π^{+(0)} μ^+ μ^-$ processes using the constrained parameters. The branching ratios of the lepton flavour violating $D$ meson decays, such as $D^0 \to μe, ~τe$ and $D^{+(0)} …
Flavor changing neutral current (FCNC) interactions for a top quark $t$ decays into $Xq$ with $X$ represents a neutral gauge or Higgs boson, and $q$ a up- or charm-quark are highly suppressed in the Standard Model (SM) due to the Glashow-Iliopoulos-Miami mechanism. Whilst current limits on the branching ratios of these processes have been established at the order of $10^{-4}$ from the Large Hadron Collider experiments, SM predictions are at least nine orders of magnitude below. In this work, we study some of these FCNC processes in the context of an extended mirror fermion model, originally pr…
We investigate all potentially viable scenarios that can produce the chiral enhancement required to simultaneously explain the $(g-2)_{e}$ and $(g-2)_μ$ data with either a single scalar leptoquark or a pair of scalar leptoquarks. We provide classification of these scenarios in terms of their ability to satisfy the existing limits on the branching ratio for the $μ\to e γ$ process. The simultaneous explanation of the $(g-2)_{e,μ}$ discrepancies, coupled with the current experimental data, implies that the $(g-2)_e$ loops are exclusively due to the charm quark propagation whereas the $(g-2)_μ$ lo…
We investigate the possibility of searching the lepton-flavor-violating (LFV) $τ\rightarrow μγ$ rare decay at the Super Charm-Tau Factory (CTF). By comparing the kinematic distributions of the LFV signal and the standard model (SM) background, we develop an optimized event selection criteria which can significantly reduce the background events. It is concluded that new $2 σ$ upper limit of about $1.9 \times 10^{-9}$ on $Br(τ\rightarrow μγ)$ can be obtained at the CTF, which is beyond the capability of Super-B factory in searching $τ$ lepton rare decay. Within the framework of the scalar leptoq…
We consider the feasibility of interpreting the $R_{K^{(*)}}$ anomaly in the colored Zee-Babu model. The model generates neutrino masses at two loops with the help of a scalar leptoquark $S\sim(3,3,-\frac{1}{3})$ and a scalar diquark $ω\sim(6,1,-\frac{2}{3})$, and contributes to the transition $b\to s\ell\ell$ via the exchange of a leptoquark $S$ at tree level. Under constraints from lepton flavor violating (LFV) and flavor changing neutral current (FCNC) processes, and direct collider searches for heavy particles, we acquire certain parameter space that can accommodate the $R_{K^{(*)}}$ anoma…
Motivated by the recent anomalies in $b \to s \ell^+ \ell^-$ transitions, we explore a minimal $Z^\prime$ scenario, in which the $Z^\prime$ boson has a flavour-changing coupling to charm and top quarks and a flavour-conserving coupling to muons. It is found that such a $Z^\prime$ boson can explain the current $b \to s \ell^+ \ell^-$ anomalies, while satisfying other flavour and collider constraints simultaneously. The $Z^\prime$ boson can be as light as few hundreds GeV. In this case, the $t \to c μ^+ μ^-$ decay and the $tZ^\prime$ associated production at the LHC could provide sensitive probe…
We analyze a new class of FCNC processes, the $f \to f^{\prime} \, \barγ$ decays of a fermion $f$ into a lighter (same-charge) fermion $f^{\prime}$ plus a {\it massless} neutral vector boson, a {\it dark photon} $\barγ$. A massless dark photon does not interact at tree level with observable fields, and the $f \!\to\! f^{\prime} \, \barγ$ decay presents a characteristic signature where the final fermion $f^{\prime}$ is balanced by a {\it massless invisible} system. Models recently proposed to explain the exponential spread in the standard-model Yukawa couplings can indeed foresee an extra unbro…
In the paper, we discuss the possibility of observation of heavy quarkoniums via the processes involving flavor changing neutral currents (FCNC). More explicitly, we systematically calculate the production of heavy charmonium and $(c\bar{b})$-quarkonium through the top quark semi-exclusive rare FCNC decays in the framework of the non-relativistic QCD (NRQCD) factorization theory. Our results show that the total decay widths $Γ_{t\to η_c} =1.20^{+1.04+1.14}_{-0.51-0.45}\times 10^{-16}$ GeV, $Γ_{t\to J/ψ} =1.37^{+1.03+1.30}_{-0.51-0.51}\times 10^{-16}$ GeV, $Γ_{t\to B_c}=2.06^{+0.17+0.91}_{-0.17…
There is no tree-level flavor changing neutral current (FCNC) in the standard model (SM) which contains only one Higgs doublet. If more Higgs doublets are introduced for various reasons, the tree level FCNC would be inevitable except extra symmetry was imposed. Therefore FCNC processes are the excellent probes for the physics beyond the SM (BSM). In this paper, we studied the lepton flavor violated (LFV) decay processes $h\rightarrowμτ$ and $τ\rightarrowμγ$ induced by Higgs-$μ$-$τ$ vertex. For $τ\rightarrowμγ$, its branching ratio is also related to the $ht\bar{t}$, $hτ^+τ^-$ and $hW^+W^-$ ver…
I explore the possibility of using heavy meson decays as probes for flavour changing neutral transitions (FCNC) $c\to uγ$ and $c\to ul^+l^-$. In the standard model, these are the most frequent FCN transitions among the quarks with charge $2/3e_0$. I predict the short and long distance contributions to the relevant heavy meson decays within the standard model and explore their sensitivity to different scenarios of physics beyond the standard model. The rare $B_c\to B_u^*γ$ decay is proposed as the most suitable case study of the $c\to uγ$ transition and is studied with the ISGW model. Its detec…
We present standard model (SM) estimates for exclusive $c \to u γ$ processes in heavy quark and hybrid frameworks. Measured branching ratios ${\cal{B}}(D^0 \to (ϕ, \bar K^{*0}) γ)$ are at or somewhat exceeding the upper range of the SM and suggest slow convergence of the $1/m_D, α_s$-expansion. Model-independent constraints on $|ΔC|=|ΔU|=1$ dipole operators from ${\cal{B}}(D^0 \to ρ^0 γ)$ data are obtained. Predictions and implications for leptoquark models are worked out. While branching ratios are SM-like CP asymmetries $\lesssim 10 \%$ can be induced. In SUSY deviations from the SM can be e…
We analyze rare charm baryon decays within the standard model and beyond. We identify all null test observables in unpolarized $Λ_c \to p \ell^+ \ell^-$, $\ell=e, μ$ decays, and study the new physics sensitivities. We find that the longitudinal dilepton polarization fraction $F_L$ is sensitive to electromagnetic dipole couplings $C_{7}^{(\prime)}$, and to the right-handed 4-fermion vector coupling $C_{9}^\prime$. The forward-backward asymmetry, $A_{\rm FB}$, due to the GIM-suppression a standard model null test already, probes the left-handed axialvector 4-fermion coupling $C_{10}$; its CP-asy…
