Mu Tau Research Articles

LFV Higgs and Z-boson decays: leptonic CPV phases and CP asymmetries

Heavy neutral leptons are motivated by several extensions of the Standard Model and their presence induces modifications in the lepton mixing matrix, including new Dirac and Majorana CP violating phases. It has been recently shown that these phases play an important role in lepton number and lepton flavour violating decays and transitions, with a striking impact for the predicted rates of certain observables. In this work, we now consider the potential role of the heavy neutral fermions and of the new CP violating phases in Higgs and Z-boson lepton flavour violating decays H,Zrightarrow ell _alpha ^pm ell _beta ^mp , as well as in the corresponding CP asymmetries. In order to allow for a thorough analysis, we derive the full analytical expressions of the latter observables, taking into account the effect of the (final state) charged lepton masses. A comprehensive exploration of lepton flavour violating Z and Higgs decays confirms that these are very sensitive to the presence of the additional heavy neutral leptons. Moreover, the new CPV phases have a clear impact on the decay rates, leading to both destructive and constructive interferences. Interestingly, in the mu tau sector, the Zrightarrow ell _alpha ^pm ell _beta ^mp rates are within reach of FCC-ee, with associated CP asymmetries that can potentially reach up to 20–30%.

RGE effects on the LFV scale from meson decays

We consider the lepton-flavor violating (LFV) lepton-quark dimension-6 operators and analyze their contributions to the LFV leptonic decays of vector, pseudoscalar, and scalar neutral mesons Mrightarrow ell _1 ell _2 as well as to mu (tau ) rightarrow ell ee, ell gamma gamma decays. These operators contribute to the purely leptonic processes via quark loop. On the basis of quark-hadron duality, we relate these loops to the appropriate meson-exchange contributions. In this way, we extract lower bounds on the individual scales of the studied LFV operators from the experimental and phenomenological limits on the leptonic decays of mesons and leptons. As a byproduct, we shall obtain new limits on the LFV leptonic decays of flavored mesons from the experimental bounds on the three-body lepton decays. We study the effects of QED and QCD radiative corrections to the LFV lepton-quark operators in question. We derive for them the one-loop matrix of the RGE evolution and examine its effect on the previously derived tree-level limits on these operators. We show that the QED corrections are particularly relevant due to operator mixing. Specifically, for some of them the limits on their individual LFV scales improve by up to 3 orders of magnitude.

Charged lepton flavor violation in light of muon g-2

The recent confirmation of the muon g-2 anomaly by the Fermilab g-2 experiment may harbinger a new era in mu and tau physics. In the context of general two Higgs doublet model, the discrepancy can be explained via one-loop exchange of sub-TeV exotic scalar and pseudoscalars, namely H and A, that have flavor changing neutral couplings rho _{tau mu } and rho _{mu tau } at sim 20 times the usual tau Yukawa coupling, lambda _tau . Taking rho _{ell ell ^prime }sim lambda _{ mathrm min(ell , ell ^prime )}, we show that the above solution to muon g-2 then predicts enhanced rates of various charged lepton flavor violating processes, which should be accessible at upcoming experiments. We cover muon related processes such as mu rightarrow e gamma , mu rightarrow eee and mu N rightarrow e N, and tau decays tau rightarrow mu gamma and tau rightarrow mu mu mu . A similar one-loop diagram with rho _{etau }= rho _{tau e} = mathcal{O}(lambda _e) induces mu rightarrow egamma , bringing the rate right into the sensitivity of the MEG II experiment. The mu egamma dipole can be probed further by mu rightarrow 3e and mu N rightarrow eN. With its promised sensitivity range and ability to use different nuclei, the mu N rightarrow eN conversion experiments can not only make discovery, but access the extra diagonal quark Yukawa couplings rho _{qq}. For the tau lepton, we find that tau rightarrow mu gamma would probe rho _{tau tau } down to lambda _tau or lower, while tau rightarrow 3mu would probe rho _{mu mu } to mathcal{O}(lambda _{mu }).

Enhancing sensitivity to non-standard neutrino interactions at INO combining muon and hadron information

In this paper, we explore the impact of flavor violating neutral current non-standard interaction (NSI) parameter varepsilon _{mu tau } in the oscillation of atmospheric neutrinos and antineutrinos separately using the 50 kt magnetized ICAL detector at INO. We find that due to non-zero varepsilon _{mu tau }, nu _mu rightarrow nu _mu and {bar{nu }}_mu rightarrow {bar{nu }}_mu transition probabilities get modified substantially at higher energies and longer baselines, where vacuum oscillation dominates. We demonstrate for the first time that by adding the hadron energy information along with the muon energy and muon direction in each event, the sensitivity of ICAL to the NSI parameter varepsilon _{mu tau } can be enhanced significantly. The most optimistic bound on varepsilon _{mu tau } that we obtain is -,,0.01< varepsilon _{mu tau } < 0.01 at 90% C.L. using 500 ktcdot yr exposure and considering E_mu , cos theta _mu , and E'_{mathrm{had}} as observables in their ranges of [1, 21] GeV, [− 1, 1], and [0, 25] GeV, respectively. We discuss for the first time the importance of the charge identification capability of the ICAL detector to have better constraints on varepsilon _{mu tau }. We also study the impact of non-zero varepsilon _{mu tau } on mass hierarchy determination and precision measurement of oscillation parameters.

Probing lepton flavor violating decays in MSSM with non-holomorphic soft terms

The Minimal Supersymmetric Standard Model (MSSM) can be extended to include non-holomorphic trilinear soft supersymmetry (SUSY) breaking interactions that may have distinct signatures. We consider non-vanishing off-diagonal entries of the coupling matrices associated with holomorphic (of MSSM) and non-holomorphic trilinear terms corresponding to sleptons with elements {A}_{ij}^l and {A}_{ij}^{prime l} . We first improve the MSSM charge breaking minima condition of the vacuum to include the off-diagonal entries {A}_{ij}^l (with i ≠ j). We further extend this analysis for non-holomorphic trilinear interactions. No other sources of lepton flavor violation like that from charged slepton matrices are considered. We constrain the interaction terms via the experimental limits of processes like charged leptons decaying with lepton flavor violation (LFV) and Higgs boson decaying to charged leptons with LFV. Apart from the leptonic decays we compute all the three neutral LFV Higgs boson decays of MSSM. We find that an analysis with non-vanishing {A}_{emu}^{prime } involving the first two generations of sleptons receives the dominant constraint from μ → eγ. On the other hand, {A}_{etau}^{prime } or {A}_{mu tau}^{prime } can be constrained from the CMS 13 TeV analysis giving limits to the respective Yukawa couplings via considering SM Higgs boson decaying into eτ or μτ final states. Contributions from Al is too little to have any significance compared to the large effect from {A}_{ij}^{prime l} .

Investigation of the Higgs boson anomalous FCNC interactions in the simple 3-3-1 model

We study phenomenological constraints on the simple 3-3-1 model with flavor-violating Yukawa couplings. Both Higgs triplets couple to leptons and quarks, which generates flavor-violating signals in both lepton and quark sectors. We have shown that this model allows for a large Higgs lepton flavor-violating rate decay h rightarrow mu tau and also may reach perfect agreement with other experimental constraints such as tau rightarrow mu gamma and (g-2)_mu . The contributions of flavor-changing neutral current couplings, Higgs–quark–quark couplings, mixing to the mesons are investigated. Br(h rightarrow q q^prime ) can be enhanced acknowledging the measurements of meson mixing. The branching ratio for t rightarrow q h can reach up to 10^{-3}, but it could be as low as 10^{-8}.

The signatures of the new particles h_{2} and Z^{}_{mu tau } at e-p colliders in the {U(1)}_{L^{}_{mu } - L^{}_{tau } } model

Considering the superior performances of the future e-p colliders, LHeC and FCC-eh, we discuss the feasibility of detecting the extra neutral scalar h_{2} and the light gauge boson Z^{}_{mu tau }, which are predicted by the {U(1)}_{L^{}_{mu } - L^{}_{tau }} model. Taking into account the experimental constraints on the relevant free parameters, we consider all possible production channels of h_{2} and Z^{}_{mu tau } at e-p colliders and further investigate their observability through the optimal channels in the case of the beam polarization hbox {P}(e^{-})= -0.8. We find that the signal significance above 5sigma of h_{2} as well as Z^{}_{mu tau } detecting can be achieved via process and a 5sigma sensitivity of Z^{}_{mu tau } detecting can be gained via process at e-p colliders with appropriate parameter values and a designed integrated luminosity. However, the signals of h_{2} decays into pair of SM particles are difficult to be detected.

Search for lepton flavour violating decays of the Higgs boson to \u03bc\u03c4 and e\u03c4 in proton-proton collisions at sqrt{s}=13 TeV

A search for lepton flavour violating decays of the Higgs boson in the μτ and eτ decay modes is presented. The search is based on a data set corresponding to an integrated luminosity of 35.9 fb−1 of proton-proton collisions collected with the CMS detector in 2016, at a centre-of-mass energy of 13 TeV. No significant excess over the standard model expectation is observed. The observed (expected) upper limits on the lepton flavour violating branching fractions of the Higgs boson are ℬ(H → μτ) < 0.25% (0.25%) and ℬ(H → eτ) < 0.61% (0.37%), at 95% confidence level. These results are used to derive upper limits on the off-diagonal μτ and eτ Yukawa couplings sqrt{{left|{Y}_{mu tau}right|}^2+{left|{Y}_{tau mu}right|}^2}<1.43times {10}^{-3} and sqrt{{left|{Y}_{mathrm{e}tau}right|}^2+{left|{Y}_{tau mathrm{e}}right|}^2}<2.26times {10}^{-3} at 95% confidence level. The limits on the lepton flavour violating branching fractions of the Higgs boson and on the associated Yukawa couplings are the most stringent to date.

A leptoquark model to accommodate RKexp\u2009

We show that a modification of the model with a low energy scalar leptoquark state carrying hypercharge Y = 7/6 allows to accommodate both RK< 1 and RK∗ < 1, through loop effects, consistent with recent observations made at LHCb. We describe details of the model, compute the relevant Wilson coefficient and, after discussing a number of constraints, we examine the phenomenological consequences of the model. The bounds on the lepton flavor violating decay rates, induced by this model, include mathrm{mathcal{B}}left(Zto mu tau right) lesssim mathcal{O}left(1{0^{-}}^7right) , and mathrm{mathcal{B}}left(Bto Kmu tau right) lesssim mathcal{O}left(1{0^{-}}^9right) . We also comment on the interpretation of the bounds on the leptoquark mass obtained from the direct searches at LHC.

Leptophilic neutral Higgs bosons in two Higgs doublet model at a linear collider

This paper addresses the question of the observability of neutral Higgs bosons through the leptonic decay in a two Higgs doublet model (2HDM). Both scalar and pseudo-scalar Higgs bosons (H, A) are considered. The model is set to type IV to enhance the leptonic decay. In such a scenario, a signal production process like e^+e^- rightarrow A^0H^0 rightarrow tau tau mu mu or mu mu tau tau would provide a clear signal on top of the background in a di-muon invariant mass distribution far from the Z boson pole mass. The analysis is based on a tau -id algorithm which preselects events if they have two tau jets by requiring a hadronic tau decay. Several benchmark points are defined for the search, requiring a linear collider operating at sqrt{s} = 0.5 and 1 TeV. It is shown that the signal can be observed on top of the background in all benchmark points at an integrated luminosity of 1000 mathrm{fb}^{-1}.

Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: Hrightarrow etau , Hrightarrow mu tau , and Zrightarrow mu tau . The searches are based on the data sample of proton–proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 mathrm{fb}^{-1} at a centre-of-mass energy of sqrt{s}=8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(Hrightarrow etau )<1.04%, Br(Hrightarrow mu tau )<1.43%, and Br(Zrightarrow mu tau )<1.69times 10^{-5}.

Simultaneous explanation of the RK and {R}_{D^{left(*right)}} puzzles: a model analysis

RK and {R}_{D^{left(*right)}} are two B-decay measurements that presently exhibit discrepancies with the SM. Recently, using an effective field theory approach, it was demonstrated that a new-physics model can simultaneously explain both the RK and {R}_{D^{left(*right)}} puzzles. There are two UV completions that can give rise to the effective Lagrangian: (i) V B: a vector boson that transforms as an SU(2)L triplet, as in the SM, (ii) U1: an SU(2)L-singlet vector leptoquark. In this paper, we examine these models individually. A key point is that V B contributes to {B}_s^0hbox{-} {overline{B}}_s^0 mixing and τ → 3μ, while U1 does not. We show that, when constraints from these processes are taken into account, the V B model is just barely viable. It predicts mathrm{mathcal{B}}left({tau}^{-}to {mu}^{-}{mu}^{+}{mu}^{-}right)simeq 2.1times {10}^{-8} . This is measurable at Belle II and LHCb, and therefore constitutes a smoking-gun signal of V B. For U1, there are several observables that may point to this model. Perhaps the most interesting is the lepton-flavor-violating decay Y(3S) → μτ, which has previously been overlooked in the literature. U1 predicts {left.mathrm{mathcal{B}}left(varUpsilon (3S)to mu tau right)right|}_{max }=8.0times 1{0}^{-7} . Thus, if a large value of mathrm{mathcal{B}}left(varUpsilon (3S)to mu tau right) is observed — and this should be measurable at Belle II — the U1 model would be indicated.

Search for new phenomena in different-flavour high-mass dilepton final states in pp collisions at [Formula: see text]Tev with the ATLAS detector.

A search is performed for a heavy particle decaying into different flavour dilepton pairs (emu , etau or mu tau ), using 3.2 fb^{-1} of proton–proton collision data at sqrt{s}=13 TeV collected in 2015 by the ATLAS detector at the Large Hadron Collider. No excess over the Standard Model prediction is observed. Limits at the 95 % credibility level are set on the mass of a Z^prime boson with lepton-flavour-violating couplings at 3.0, 2.7 and 2.6 TeV, and on the mass of a supersymmetric tau sneutrino with R-parity-violating couplings at 2.3, 2.2 and 1.9 TeV, for emu , etau and mu tau final states, respectively. The results are also interpreted as limits on the threshold mass for quantum black hole production.

Search for a charged Higgs boson in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A search for a charged Higgs boson is performed with a data sample corresponding to an integrated luminosity of 19.7 +/- 0.5 inverse femtobarns collected with the CMS detector in proton-proton collisions at sqrt(s) = 8 TeV. The charged Higgs boson is searched for in top quark decays for m(H+/-) < m(t) - m(b), and in the direct production pp -> t (b) H+/- for m(H+/-) > m(t) - m(b). The H+/- -> tau+/- nu[tau] and H+/- -> t b decay modes in the final states tau[h]+jets, mu tau[h], l+jets, and ll' (l = e, mu) are considered in the search. No signal is observed and 95% confidence level upper limits are set on the charged Higgs boson production. A model-independent upper limit on the product branching fraction B( t -> H+/- b ) B( H+/- -> tau+/- nu[tau] )= 1.2-0.15% is obtained in the mass range m(H+/-) = 80-160 GeV, while the upper limit on the cross section times branching fraction sigma( pp -> t (b) H+/- ) B( H+/- -> tau+/- nu[tau] ) = 0.38-0.025 pb is set in the mass range m(H+/-) = 180-600 GeV. Here, cross section sigma( pp -> t (b) H+/- ) stands for the sum sigma( pp -> t-bar (b) H+ ) + sigma( pp -> t (b-bar) H- ). Assuming B( H+/- -> t b ) = 1, an upper limit on sigma ( pp -> t (b) H+/- ) of 2.0-0.13 pb is set for m(H+/-) = 180-600 GeV. The combination of all considered decay modes and final states is used to set exclusion limits in the m(H+/-)-tan(beta) parameter space in different MSSM benchmark scenarios.

Search for new phenomena in events with three or more charged leptons in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of $pp$ collisions at $\sqrt{s}=8$ TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb$^{-1}$. Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to $e\tau$ or $\mu\tau$, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale $\Lambda$, with the strongest mass constraints arising in regions where the mass equals $\Lambda$. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.

Lepton-flavored scalar dark matter with minimal flavor violation

We explore scalar dark matter that is part of a lepton flavor triplet satisfying symmetry requirements under the hypothesis of minimal flavor violation. Beyond the standard model, the theory contains in addition three right-handed neutrinos that participate in the seesaw mechanism for light neutrino mass generation. The dark-matter candidate couples to standard-model particles via Higgs-portal renormalizable interactions as well as to leptons through dimension-six operators, all of which have minimal flavor violation built-in. We consider restrictions on the new scalars from the Higgs boson measurements, observed relic density, dark-matter direct detection experiments, LEP II measurements on e+e- scattering into a photon plus missing energy, and searches for flavor-violating lepton decays. The viable parameter space can be tested further with future data. Also, we investigate the possibility of the new scalars' couplings accounting for the tentative hint of Higgs flavor-violating decay h -> mu tau recently detected in the CMS experiment. They are allowed by constraints from other Higgs data to produce a rate of this decay roughly compatible with the CMS finding.

Lepton flavor violation in B decays?

The LHCb Collaboration's measurement of R_{K}=B(B^{+}→K^{+}μ^{+}μ^{-})/B(B^{+}→K^{+}e^{+}e^{-}) lies 2.6σ below the Standard Model prediction. Several groups suggest this deficit to result from new lepton nonuniversal interactions of muons. But nonuniversal leptonic interactions imply lepton flavor violation in B decays at rates much larger than are expected in the Standard Model. A simple model shows that these rates could lie just below current limits. An interesting consequence of our model, that B(B_{s}→μ^{+}μ^{-})_{exp}/B(B_{s}→μ^{+}μ^{-})_{SM}≅R_{K}≅0.75, is compatible with recent measurements of these rates. We stress the importance of searches for lepton flavor violations, especially for B→Kμe, Kμτ, and B_{s}→μe, μτ.

Non-standard neutrino interactions in the mu–tau sector

We discuss neutrino mass hierarchy implications arising from the effects of non-standard neutrino interactions on muon rates in high statistics atmospheric neutrino oscillation experiments like IceCube DeepCore. We concentrate on the mu–tau sector, which is presently the least constrained. It is shown that the magnitude of the effects depends strongly on the sign of the ϵμτ parameter describing this non-standard interaction. A simple analytic model is used to understand the parameter space where differences between the two signs are maximized. We discuss how this effect is partially degenerate with changing the neutrino mass hierarchy, as well as how this degeneracy could be lifted.

Resolving the octant ofθ23via radiativeμ−τsymmetry breaking

We point out that the observed neutrino mixing pattern at low energies is very likely to originate from the 3 times 3 lepton flavor mixing matrix U which possesses the exact mu-tau permutation symmetry |U_{mu i}| = |U_{tau i}| (for i=1,2,3) at a superhigh energy scale Lambda_{mu tau} \sim 10^{14} GeV. The deviation of theta_{23} from 45^\circ and that of delta from 270^\circ in the standard parametrization of U are therefore a natural consequence of small mu-tau symmetry breaking via the renormalization-group equations (RGEs) running from Lambda_{mu tau} down to the electroweak scale Lambda_{EW} \sim 10^2 GeV. In fitting current experimental data we find that the RGE-corrected value of theta_{23} is uniquely correlated with the neutrino mass ordering: theta_{23} \simeq 42.4^\circ reported by Capozzi et al (or theta_{23} \simeq 48.9^\circ reported by Forero et al) at Lambda_{EW} can arise from theta_{23} = 45^\circ at Lambda_{mu tau} in the minimal supersymmetric standard model if the neutrino mass ordering is inverted (or normal). Accordingly, the preliminary best-fit results of delta at Lambda_{EW} can also evolve from delta = 270^\circ at Lambda_{mu tau} no matter whether the massive neutrinos are Dirac or Majorana particles.

Scalar spin of elementary fermions

We show that, using the experimentally observed values of CKM and PMNS mixing matrices, all known elementary fermions can be assigned a new quantum number, the scalar spin, in a unique way. This is achieved without introduction of new degrees of freedom. The assignment implies that tau-neutrino should be an anti-Dirac spinor, while mu–tau leptons and charm–top, strange–bottom quarks form Dirac–anti-Dirac scalar spin doublets. The electron and its neutrino remain as originally described by Dirac.