Lepton Flavor Non-universality Research Articles

Flipped Quartification: Product Group Unification with Leptoquarks.

The quartification model is an SU(3)4 extension with a bi-fundamental fermion sector of the well-known SU(3)3 bi-fundamentalfication model. An alternative "flipped" version of the quartification model is obtained by rearrangement of the particle assignments. The flipped model has two standard (bi-fundamentalfication) families and one flipped quartification family. In contrast to traditional product group unification models, flipped quartification stands out by featuring leptoquarks and thus allows for new mechanisms to explain the generation of neutrino masses and possible hints of lepton-flavor non-universality.

Standard model anomalies: lepton flavour non-universality, g − 2 and W-mass

We critically analyze the body of results that hints to the existence of New Physics from possible violations of lepton universality observed by the LHCb experiment in the μ/e ratios RK and {R}_{K^{ast }} to the g − 2 lepton anomalies. The analysis begins with a theoretical, in depth, study of the μ/e ratios RK and {R}_{K^{ast }} as well as the process Bs→ μ+μ−. Here we consider the impact of complex Wilson coefficients and derive constraints on their imaginary and real parts. We then move to a comprehensive comparison with experimental results. We show that, by fitting a single Wilson coefficient, the deviations from the Standard Model are at the 4.7σ level when including only the hadronic insensitive observables while it increases to 6.1σ when including also the hadronic sensitive ones. When switching on all relevant Wilson coefficients and combining both hadronic sensitive and insensitive data into the fit, the deviation from the Standard Model peaks at 7.2σ and decreases at the 4.9σ level if we assume that the central values of RK and {R}_K^{ast } are taken to be unity. We further estimate other unaccounted for SM contributions and show that their inclusion still requires New Physics to fit the data. We then introduce the g − 2 lepton anomalies as well as the most recent W-mass results. Different theoretical models are considered that can explain the discrepancies from the Standard Model. In the final part of our work we estimate the impact of the forthcoming data from LHCb (coming from LHC Run3) and Belle II, when it will have accumulated about 5 ab−1.

More indications for lepton nonuniversality in b → sℓ+ℓ−

Recently the LHCb collaboration has confirmed the evidence for lepton flavour nonuniversality at the 3.1σ level via an updated measurement of RK. In this work we analyse this evidence within a model-independent approach. We make projections for future measurements which indicate that LHCb will be in the position to discover lepton nonuniversality with the Run 3 data in a single observable. We analyse other ratios based on our analysis of the present measurements of the ratios RK(⁎) and analyse if they are able to differentiate between various new physics options within the effective field theory at present or in the near future. We also compare the present deviations in the ratios with NP indications in the angular observables of exclusive b→sℓℓ transitions. Finally, we update our global analysis considering all b→sℓℓ observables altogether, including a 20-parameter fit in connection of a Wilks' test.

Lepton-flavour non-universality of {bar{B}}rightarrow D^*ell {{bar{nu }}} angular distributions in and beyond the Standard Model

We analyze in detail the angular distributions in {bar{B}}rightarrow D^*ell {{bar{nu }}} decays, with a focus on lepton-flavour non-universality. We investigate the minimal number of angular observables that fully describes current and upcoming datasets, and explore their sensitivity to physics beyond the Standard Model (BSM) in the most general weak effective theory. We apply our findings to the current datasets, extract the non-redundant set of angular observables from the data, and compare to precise SM predictions that include lepton-flavour universality violating mass effects. Our analysis shows that the number of independent angular observables that can be inferred from current experimental data is limited to only four. These are insufficient to extract the full set of relevant BSM parameters. We uncover a sim 4sigma tension between data and predictions that is hidden in the redundant presentation of the Belle 2018 data on {bar{B}}rightarrow D^*ell {{bar{nu }}} decays. This tension specifically involves observables that probe e-mu lepton-flavour universality. However, we find inconsistencies in these data, which renders results based on it suspicious. Nevertheless, we discuss which generic BSM scenarios could explain the tension, in the case that the inconsistencies do not affect the data materially. Our findings highlight that e-mu non-universality in the SM, introduced by the finite muon mass, is already significant in a subset of angular observables with respect to the experimental precision.

Exploring the role of new physics in $b \to u \tau \bar \nu$ decays

Abstract The recent measurements on $R_D$, $R_{D^*}$, and $R_{J/\psi}$ by three pioneering experiments, BaBar, Belle, and LHCb, indicate that the notion of lepton flavor universality is violated in weak charged-current processes, mediated through $b \to c \ell \bar \nu_\ell$ transitions. These intriguing results, which delineate a tension with their Standard Model predictions at the level of (2–3)$\sigma$, have triggered many new physics propositions in recent times, and are generally attributed to the possible implication of new physics in $ b \to c \tau \bar \nu$ transitions. This, in turn, opens up another avenue, i.e., $ b \to u \tau \bar \nu$ processes, to look for new physics. Since these processes are doubly Cabibbo suppressed, the impact of new physics could be significant enough, leading to sizable effects in some of the observables. In this work, we investigate in detail the role of new physics in $B \to (\pi,\rho,\omega)\tau \bar \nu$ and $B_s \to (K,K^*) \tau \bar \nu$ processes considering a model-independent approach. In particular, we focus on standard observables like branching fraction, the lepton flavor non-universality (LNU) parameter, forward–backward asymmetry, and polarization asymmetries. We find significant deviations in some of these observables, which can be explored by the currently running experiments, LHCb and Belle-II. We also briefly comment on the impact of the scalar leptoquark $R_2(3,2,7/6)$ and vector leptoquark $U_1(3,1,2/3)$ on these decay modes.

Investigating the effects of U 1 vector leptoquark on mediated B decays

The recent measurements of lepton flavor universality (LFU) violating observables in semileptonic and b → sℓ + ℓ − transitions by various experiments hint towards the possible role of new physics (NP) in and b → sμ + μ − decay channels. This in turn suggests that the same class of NP as appeared in , might also show up in other tree level processes involving transition. In this paper, we study the implications of the vector leptoquark (LQ) U 1(3, 1, 2/3), which is one of the few scenarios that can simultaneously explain all the observed LFU violating signals, on the semileptonic decays and . In particular, we pay our attention to the branching fractions, lepton flavor non-universality (LNU) observables, forward–backward asymmetry and the polarization asymmetry parameters of these decay modes. We find that substantial deviations in the branching fractions as well as LNU observables could be possible due to the additional contributions from U 1 LQ, which can be probed by the currently running experiments LHCb and Belle-II.

High pT correlated tests of lepton universality in lepton(s) + jet(s) processes; An EFT analysis

We suggest a new class of tests for searching for lepton flavor non-universality (LFNU) using ratio observables and based on correlations among the underlying LFNU new physics (NP) effects in several (seemingly independent) di-lepton and single lepton + jet(s) processes. This is demonstrated by studying the effects generated by LFNU 4-Fermi interactions involving 3rd generation quarks. We find that the sensitivity to the scale (Λ) of the LFNU 4-Fermi operators significantly improves when the correlations among the various di-lepton +jets and single-lepton + jets processes are used, reaching Λ∼O(10) TeV at the HL-LHC.

Confronting B anomalies with low-energy parity violation

Indirect searches have the potential to probe scales beyond the realm of direct searches. In this paper, we consider the implications of two parity violating experiments: weak charge of proton ${Q}_{W}^{p}$ and the Caesium atom ${Q}_{W}^{Cs}$ on the solutions to lepton flavor nonuniversality violations in the decay of $B$ mesons. Working in a generic implementation of a minimal ${Z}^{\ensuremath{'}}$ model, we assume the primary contribution being due to the electron to facilitate comparison with the low ${q}^{2}$ parity violating experiments. We demonstrate that the conclusion is characterized by different limiting behavior depending on the chirality of the lepton current. The correlation developed in this study demonstrates the effectiveness in studying the synergy between different experiments leading to a deeper understanding of the interpretation of the existing data. It is shown that a possible future improvement in the parity violating experiments can have far reaching implications in the context of direct searches. We also comment on the prospect of addition of the muon to the fits and the role it plays in ameliorating the constraints on models of ${Z}^{\ensuremath{'}}$. This offers a complimentary understanding of the pattern of the coupling of the new physics to the leptons, strongly suggesting either a muon only or a combination of solutions to the anomalies.

Sensitivity of future lepton colliders and low-energy experiments to charged lepton flavor violation from bileptons

The observation of charged lepton flavor violation is a clear sign of physics beyond the Standard Model (SM). In this work, we investigate the sensitivity of future lepton colliders to charged lepton flavor violation via on-shell production of bileptons, and compare their sensitivity with current constraints and future sensitivities of low-energy experiments. Bileptons couple to two charged leptons with possibly different flavors and are obtained by expanding the general SM gauge invariant Lagrangians with or without lepton number conservation. We find that future lepton colliders will provide complementary sensitivity to the charged-lepton-flavor-violating couplings of bileptons compared with low-energy experiments. The future improvements of muonium-antimuonium conversion, lepton flavor non-universality in leptonic {\tau} decays, electroweak precision observables and the anomalous magnetic moments of charged leptons will also be able to probe similar parameter space.

A nonunitary interpretation for a single vector leptoquark combined explanation to the B-decay anomalies

In order to simultaneously account for both RD(∗) and RK (∗) anomalies in B-decays, we consider an extension of the Standard Model by a single vector leptoquark field, and study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary quark-lepton mixing induced by SU(2)L breaking is insufficient, we find that effectively nonunitary mixings hold the key to simultaneously address the RK (∗) and RD(∗) anomalies. As an intermediate step towards various UV-complete models, we show that the mixings of charged leptons with additional vector-like heavy leptons successfully provide a nonunitary framework to explain RK (∗) and RD(∗) . These realisations have a strong impact for electroweak precision observables and for flavour violating ones: isosinglet heavy lepton realisations are already excluded due to excessive contributions to lepton flavour violating Z -decays. Furthermore, in the near future, the expected progress in the sensitivity of charged lepton flavour violation experiments should allow to fully probe this class of vector leptoquark models.

Testing leptoquark and Z′ models via B→K1(1270,1400)μ+μ− decays

The measurements of ${R}_{{K}^{(*)}}=\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})/\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}{e}^{+}{e}^{\ensuremath{-}})$ in recent years have hinted lepton flavor nonuniversality and thus drawn widespread attentions. If these anomalies are induced by new physics (NP), deviations from the SM predictions may also be found in other channels via the same process at the quark level. In this work, we study in $B\ensuremath{\rightarrow}{K}_{1}(1270,1400){\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decays the effects of two popular classes of NP models which can address the $b\ensuremath{\rightarrow}s$ anomalies, i.e., the leptoquark models and the ${Z}^{\ensuremath{'}}$ models. By assuming that NP only affects the $b\ensuremath{\rightarrow}s{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ transition, we find that the unpolarized and polarized lepton flavor universality (LFU) ratios ${R}_{{K}_{1}^{(L,T)}(1270)}$ are useful to distinguish among the NP models (scenarios) and the SM because they are sensitive to the NP effects and insensitive to the mixing angle ${\ensuremath{\theta}}_{{K}_{1}}$, while the ${R}_{{K}_{1}^{(L,T)}(1400)}$ are sensitive to both NP and ${\ensuremath{\theta}}_{{K}_{1}}$. Another ratio ${R}_{\ensuremath{\mu}}({K}_{1})=\mathcal{B}(B\ensuremath{\rightarrow}{K}_{1}(1400){\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})/\mathcal{B}(B\ensuremath{\rightarrow}{K}_{1}(1270){\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$ is shown to depend weekly on the effects from the NP models (scenarios) under consideration, and thus can be used to determine the ${\ensuremath{\theta}}_{{K}_{1}}$ and complement the ${R}_{{K}_{1}^{(L,T)}}$ in the probe for NP.

Lepton flavor non-universality in the B-sector: a global analyses of various new physics models

The measurements of the ratios R_{K^{(*)}} along with R_{D^{(*)}} hint towards lepton flavor non universality which is in disagreement with the standard model. In this work, we reanalyze the four new physics models, which are widely studied in the literature as a candidates for the simultaneous explanations of these measurements. These are, standard model like vector boson (VB), SU(2)_L-singlet vector leptoquark (U_1), SU(2)_L-triplet scalar leptoquark (S_3) and SU(2)_L triplet vector leptoquark (U_3) models. We assume a coupling only to the third generation in the weak basis, so that the b rightarrow s mu ^+ mu ^- transition is generated only via mixing effects. Preforming a global fit to all relevant data, we show that the vector boson model violates the current upper bound on {Br}(tau rightarrow 3mu ) and hence is inconsistent with the present data. Further, we show that within this framework, the U_1 leptoquark model cannot simultaneously accommodate R_{K^{(*)}} and R_{D^{(*)}} measurements. We emphasize that this conclusion is independent of the additional constraints coming from renormaliztion group running effects and high-p_T searches. In addition, we show that the S_3 and U_3 models are highly disfavored by the constraints coming from brightarrow s nu bar{nu } data. Finally, we find a that hypothesis of two LQ particles is also challenged by brightarrow s bar{nu } nu data.

Search for Lepton Flavor Non-universality with B→ (K, K*) τ+τ- Decays

The decay modes of B meson consisting tau leptons as final state particle are seemed to be the key feature for searching new physics. As the experimental measurements for b → sτ+τ− processes are still not confirmed, the theoretical study of these decays is playing vital role in recent times. The lepton flavor universality violation gains much curiosity after the observation of RK and $$ {R}_{K^{\ast }} $$ anomalies in semileptonic $$ B\to \left(K,{K}^{\ast}\right)\mu \overline{\mu} $$ channel which provides a hint towards μ − e non-universality in flavor sector. In this paper, we have devoted our concern to the decay mode B → (K, K∗)τ+τ− to predict the branching ratio. We have also searched for the τ − l (where l = e, μ) lepton flavor non-universality for neutral current transitions. We have estimated our results in the standard model as well as in Z′ model.

Constraints on the charged Higgs bosons from the 221 LFNU model

The model with lepton flavor non-universality can generate the active neutrino masses via the Zee mechanism, which predicts the existence of the charged Higgs bosons by introducing new free parameters and Mh where i, j denote e, μ, or τ. We calculate the contributions of to the , , and decays, and extract individual constraints on the factor /Mh by combining the current experimental measurements. We find that can provide more stringent constraints on /Mh than , while the obtained constraints on /Mh are comparable from these decays. The contributions of to the muon anomalous magnetic moment aμ and non-standard interactions of neutrinos are also discussed.

Hadron collider sensitivity to fat flavourful Z\u2032s for {R}_{K^{left(ast right)}}

We further investigate the case where new physics in the form of a massive Z′ particle explains apparent measurements of lepton flavour non-universality in B → K(*)l+l− decays. Hadron collider sensitivities for direct production of such Z′s have been previously studied in the narrow width limit for a μ+μ− final state. Here, we extend the analysis to sizeable decay widths and improve the sensitivity estimate for the narrow width case. We estimate the sensitivities of the high luminosity 14 TeV Large Hadron Collider (HL-LHC), a high energy 27 TeV LHC (HE-LHC), as well as a potential 100 TeV future circular collider (FCC). The HL-LHC has sensitivity to narrow Z′ resonances consistent with the anomalies. In one of our simplified models the FCC could probe 23 TeV Z′ particles with widths of up to 0.35 of their mass at 95% confidence level (CL). In another model, the HL-LHC and HE-LHC cover sizeable portions of parameter space, but the whole of perturbative parameter space can be covered by the FCC.

R -parity violating solutions to the RD(*) anomaly and their GUT-scale unifications

Recently, several $B$-physics experiments report interesting anomalies in the semi-leptonic decays of $B$-mesons, such as the excess in the $R_{D^{(\ast)}}$ measurements. These anomalies seem to suggest intriguing hints of lepton flavor non-universality, and the R-parity violating (RPV) interactions are candidates for explaining this non-universality. In this paper, we discuss the RPV interactions for resolving the $R_{D^{(\ast)}}$ anomaly with the Grand Unified Theory (GUT) assumption. To solve the $R_{D^{(\ast)}}$ anomaly, it is known that large RPV couplings and around $1~{\rm TeV}$ sfermion masses are required. At the same time, large RPV couplings are conducive to realize the bottom-tau Yukawa unification which appears in the GUT models. On the other hand, there are problems for realizing favorable sfermion masses in the constrained minimal supersymetric standard model. To resolve these problems, we show that two non-universalities, the non-universal sfermion masses and the non-universal gaugino masses, are favorable.

B anomalies and leptoquarks at the LHC: Beyond the lepton-quark final state

Leptoquarks provide some of the simplest explanations to the hints of lepton flavor non-universality in $B$ decays. In particular, a new confining gauge group can provide a natural and appealing origin for the leptoquarks. So far, direct collider searches have been based on two body decays, namely to a quark and a lepton. We study how composite dynamics can give rise to additional states resulting in modified collider signatures of leptoquarks, as well as new production modes in cascade decays of heavier states. Instead of the standard signature, each leptoquark can result in as many as four jets and a lepton. We reinterpret relevant ATLAS and CMS searches to set limits on this scenario, show how this can relax the current bounds, and propose ways to better constrain this class of models in the future. For example, we show that a leptoquark related to the $R_{D^{(*)}}$ anomaly could still be as light as 500 GeV.

New global fits to b → s data with all relevant parameters

The LHCb experiment has made several measurements in b→s transitions which indicate tensions with the Standard Model predictions. Assuming the source of these tensions to be new physics, we present new global fits to all Wilson coefficients which can effectively receive beyond the Standard Model contributions. While the theoretically clean ratios RK(⁎) which are sensitive to lepton flavour non-universality may unambiguously establish lepton non-universal new physics in the near future, most of the other tensions with the SM in the b→s data, in particular in the angular observables of the B→K⁎μμ decays and in the branching ratios of the Bs→ϕμμ decays, depend on the estimates of non-factorisable power corrections. Therefore, we also analyze the dependence of the new global fit on these corrections.

The lepton flavor violating exclusive [formula omitted] decays in SUSY without R-parity

Inspired by the recent anomaly measurements of the lepton-flavor violating decays h→μτ and the lepton flavor non-universality in decays b¯→s¯ℓ−ℓ+, we investigate the lepton flavor violating exclusive b¯→s¯ℓi−ℓj+(i≠j and ℓ=e,μ,τ) decays within supersymmetry. Relevant R-parity violating couplings are constrained by using the latest experimental upper limits on the branching ratios of Bs→ℓi−ℓj+ and B→K(⁎)ℓi−ℓj+ flavor changing neutral current processes, and we find that all relevant branching ratios are very sensitive to the moduli of the squark and sneutrino exchange coupling products. In addition, the constrained lepton number violating effects on the dilepton invariant mass spectra, the single lepton polarization asymmetries and the differential forward–backward asymmetries are also studied. These lepton-flavor violating B decays could be used for the search of lepton flavor violation at the running LHC and the forthcoming Belle-II.

Implications of the and anomalies**Supported by National Natural Science Foundation of China (11575110, 11655002, 11735010), Natural Science Foundation of Shanghai (15DZ2272100, 15ZR1423100), Young Thousand Talents Plan and Key Laboratory for Particle Physics, Astrophysics and Cosmology, Ministry of Education

We discuss the implications of the recently reported and anomalies, the lepton flavor non-universality in the and decay channels. Using two sets of hadronic inputs of form factors, we perform a fit of new physics to the and data, and significant new physics contributions are found. We suggest the study of lepton flavor universality in a number of related rare and decay channels, and in particular we give predictions for the -to-e ratios of decay widths with different polarizations of the final state particles, and of the processes, which are presumably more sensitive to the structure of the underlying new physics. With the new physics contributions embedded in the Wilson coefficients, we present theoretical predictions for lepton flavor non-universality in these processes.