Lepton Flavor Universality Violation Research Articles

Experimental Review on Lepton Universality and Lepton Flavour Violation tests inBdecays

Tests of lepton flavour universality inBdecays offer an excellent opportunity to test the Standard Model, and show hints of new physics in analyses performed by the LHCb, Belle and BaBar experiments. Several theoretical models proposed to explain possible violation of lepton flavour universality claim a connection with lepton flavour violation inBdecays. These proceedings review the experimental status of the tests of lepton flavour universality and the searches of lepton flavour violation inBdecays.

B-anomalies in U(2) flavor symmetry

We analyzed how to test flavor and helicity structures of the corresponding amplitudes in view of future data, motivated by the recent hints of lepton flavor universality violation observed in semileptonic B decays. The general assumption that non-standard effects are controlled by a U(2)5 flavor symmetry, minimally broken as in the Standard Model Yukawa sector, leads to stringent predictions on leptonic and semileptonic B decays. Future measurements will allow to prove or falsify this general hypothesis independently of its dynamical origin.

Further tests of lepton flavor universality from the charged lepton energy distribution in b→c semileptonic decays: The case of Λb→Λcℓν¯ℓ

In a general framework, valid for any $H\to H'\ell^-\bar\nu_\ell$ semileptonic decay, we analyze the $d^2\Gamma/(d\omega d\cos\theta_\ell)$ and $d^2\Gamma/ (d\omega dE_\ell)$ distributions, with $\omega$ being the product of the hadron four-velocities, $\theta_\ell$ the angle made by the three-momenta of the charged lepton and the final hadron in the $W^-$ center of mass frame and $E_\ell$ the charged lepton energy in the decaying hadron rest frame. Within the Standard Model (SM), $ d^2\Gamma/(d\omega dE_\ell)\propto \left(c_0(\omega)+c_1(\omega)E_\ell/M+c_2(\omega)E^2_\ell/M^2\right)$, with $M$ the initial hadron mass. We find that $c_2(\omega)$ is independent of the lepton flavor and thus it is an ideal candidate to look for lepton flavor universality (LFU) violations. We also find a correlation between the $a_2(\omega)$ structure function, that governs the $(\cos\theta_\ell)^2$ dependence of $d^2\Gamma/(d\omega d\cos\theta_\ell)$, and $c_2(\omega)$. Apart from trivial kinematical and mass factors, the ratio of $a_2(\omega)/c_2(\omega)$ is a universal function that can be measured in any semileptonic decay, involving not only $b\to c$ transitions. These two SM predictions can be used as new tests in the present search for signatures of LFU violations. We also generalize the formalism to account for some new physics (NP) terms. Finally, in order to illustrate our findings, we apply our general framework to the $\Lambda_b\to \Lambda_c \ell \bar\nu_\ell$ decay. We show that a measurement of $c_2$ (or $a_2$) for $\tau$ decay would not only be a direct measurement of the possible existence of NP, but it would also allow to distinguish from NP fits to $b\to c\tau\bar\nu_\tau$ anomalies in the meson sector, that otherwise give the same total and differential $d\Gamma/d\omega$ widths.

Strong CP problem with low-energy emergent QCD: The 4321 case

We analyze the strong CP problem and the implications for axion physics in the context of $U_1$ vector leptoquark models, recently put forward as an elegant solution to the hints of lepton flavor universality violation in B-meson decays. It is shown that in minimal gauge models containing the $U_1$ as a gauge boson, the Peccei-Quinn solution of the strong CP problem requires the introduction of two axions. Characteristic predictions for the associated axions can be deduced from the model parameter space hinted by B-physics, allowing the new axion sector to account for the dark matter of the Universe. We also provide a specific ultraviolet completion of the axion sector that connects the Peccei-Quinn mechanism to the generation of neutrino masses.

A vector leptoquark for the B-physics anomalies from a composite GUT

A vector leptoquark at the TeV scale, mostly coupled to the fermions of the third generation, is the preferred option to explain the hints of lepton flavor universality violation in the decays of B-mesons. It seems interesting to assume that this leptoquark belongs to the same beyond the Standard Model sector that solves the hierarchy prob- lem, since the third generation of fermions play the leading role in the instability of the Higgs potential. We present a composite Grand Unified Theory with resonances at the TeV that contains the required vector leptoquark and develops the Higgs as a pseudo Nambu-Goldstone boson. We show that anarchic partial compositeness of the Standard Model fermions can accommodate the couplings of Left-handed currents required by the B-anomalies, predicting very small couplings to the Right-handed currents without any additional hypothesis. By making use of an effective theory description of the strong dy- namics, in terms of weakly coupled resonances, we are able to compute the corrections to B-physics, as well as the one-loop potential for the pseudo Nambu-Goldstone bosons. The theory has a rich phenomenology and a candidate for dark matter.

Lepton flavor violating Lambda _brightarrow Lambda ell _1ell _2 decay

Inspired by the recent hints of lepton flavor universality violation in brightarrow sell ell and brightarrow cell nu transitions, we study lepton flavor violating exclusive Lambda _brightarrow Lambda ell _1^+ell _2^- (ell _1ne ell _2) decay, which is forbidden in the Standard Model. Starting from a general effective Hamiltonian for a brightarrow sell _1^+ell _2^- transition that includes vector and axial-vector operators, and scalar and pseudo-scalar operators, we derive a two-fold decay distribution of Lambda _brightarrow Lambda ell _1^+ell _2^-. The distribution helps us to construct the differential branching ratio and the lepton-side forward–backward asymmetry, which are studied in a vector leptoquark model. The parameter space of the vector leptoquark model is constrained by low energy observables.

With or without U(2)? Probing non-standard flavor and helicity structures in semileptonic B decays

Motivated by the recent hints of lepton flavor universality violation observed in semileptonic B decays, we analyze how to test flavor and helicity structures of the corresponding amplitudes in view of future data. We show that the general assumption that such non-standard effects are controlled by a U(2)q×U(2)ℓ flavor symmetry, minimally broken as in the Standard Model Yukawa sector, leads to stringent predictions on leptonic and semileptonic B decays. Future measurements of RD(⁎), RK(⁎), B(B¯c,u→ℓν¯), B(B¯→πℓν¯), B(B→πℓℓ¯), B(Bs,d→ℓℓ¯(′)), as well as various polarization asymmetries in B¯→D(⁎)τν¯ decays, will allow to prove or falsify this general hypothesis independently of its dynamical origin.

Search for an opposite sign muon-tau pair and a b -jet at the LHC in the context of flavor anomalies

Extant anomalies in several semileptonic $B$-meson decays argue for physics beyond the Standard Model. Measurements of both neutral-current decays (such as $R_K$, $R_{K^*}$ and $B_s\rightarrow \phi\mu\mu$) as well as charged-current ones- $R(D)$ and $R(D^*)$ -provide strong hints for the violation of lepton flavor universality. Recent studies have shown that a class of effective field theory (EFT) models may explain such anomalies in terms of only a few parameters which can be determined phenomenologically. In this literature, we examine such resolutions in the context of the requisite $(\bar{s}$ $b$)($\bar\tau\ \tau)$ operator, and look for its signals at the 13 TeV LHC, with a final state of one $b$-jet, and an oppositely charged $\mu$-$\tau$ pair, with the muon coming from the decay of one of the $\tau$ leptons. We obtain discovery and exclusion limits on the model parameters as a function of luminosity at the 13 TeV LHC.

Implication of RD(*) anomalies on semileptonic decays of Σb and Ωb baryons

The flavor changing decays of heavy bottom quarks to the corresponding lighter quarks ($u$, $c$ and $s$) in various $B$-meson decays via charged current and neutral current semileptonic transitions have emerged as promising candidates to explore the physics beyond the standard model. Experimentally the lepton flavor universality violation in $b \to (c,u)\,l\,\nu$ and $b \to s\,l^+ l^-$ transitions have been reported to a higher precision. The measurements of the lepton flavor violating ratios such as $R_{D^{(*)}}$, $R_{J/\Psi}$ and $R_{K^{(*)}}$ are observed to deviate from the standard model expectations at the level of $1.4\sigma$, $2.5\sigma$, $1.5\sigma$, $2.4\sigma$ and $2.2\sigma$ respectively. Motivated by these anomalies, we investigate the lepton flavor universality violation in $\Sigma_b \to \Sigma_c l \nu$ and $\Omega_b \to \Omega_c l \nu$ decays. We follow a model independent effective field theory formalism and study the implications of $R_{D^{(*)}}$ anomalies on $\Sigma_b \to \Sigma_c \tau \nu$ and $\Omega_b \to \Omega_c \tau \nu$ decay modes. We give predictions of various physical observables such as the ratio of branching ratios, total differential decay rate, forward-backward asymmetry, lepton side polarization fraction and convexity parameter within the standard model and within various new physics scenarios.

Tests of Lepton Flavour Universality at LHCb

In the Standard Model of particle physics the three charged leptons are identical copies of each other, apart from mass differences, and the electroweak coupling of the gauge bosons to leptons is independent of the lepton flavour. This prediction is called lepton flavour universality (LFU) and is well tested. In tree level decays, any violation of LFU would be a clear sign of physics beyond the Standard Model. Experimental tests of LFU in semileptonic decays of b-hadrons or rare b decays are highly sensitive to New Physics particles which preferentially couple to the 2nd and 3rd generations of leptons. Recent results from LHCb on lepton flavour universality in transitions and rare decays are discussed. The results are based on 3 fb−1 of proton-proton collisions collected at centre of mass energies and 8 TeV.

Quark masses, CKM angles and Lepton Flavour Universality violation

A properly defined and suitably broken U(2) flavour symmetry leads to successful quantitative relations between quark mass ratios and CKM angles. At the same time the intrinsic distinction introduced by U(2) between the third and the first two families of quarks and leptons may support anomalies in charged and neutral current semi-leptonic B-decays of the kind tentatively observed in current flavour experiments. We show how this is possible by the exchange of the (3, 1)2/3 vector leptoquark in two U(2)-models with significantly different values of Lepton Flavour Universality violation, observable in foreseen experiments.

Some results on lepton flavour universality violation

Motivated by recent experimental measurements on flavour physics, in tension with Standard Model predictions, we perform an updated analysis of new physics violating lepton flavour universality, by using the effective Lagrangian approach and in the Z^{'} and S_3 leptoquark models. We explicitly analyze the impact of considering complex Wilson coefficients in the analysis of B-anomalies, by performing a global fit of R_{K} and R_{K^{*0}} observables, together with varDelta M_s and A_{CP}^{mathrm{mix}}. The inclusion of complex couplings provides a slightly improved global fit, and a marginally improved varDelta M_s prediction.

Charged-lepton-flavor violation in |\u0394S| = 1 hyperon decays

Studies of lepton-flavor violation in strangeness-changing (|ΔS| = 1) transitions have a long tradition in the kaon sector where they provide some of the strongest limits on physics beyond the standard model. Recent hints of violation of lepton-flavor universality in B-meson decays have revived interest in lepton-flavor violation as the two phenomena appear simultaneously in many extensions of the standard model. At the same time, the LHCb experiment has produced new results for the hyperon process Σ+ → pμ+μ− and may be in a position to study other rare hyperon decay modes. With this in mind, we investigate |ΔS| = 1 hyperon decays into different-flavor lepton pairs e±μ∓ in a model-independent manner and contrast the coverage of parameter space that can be achieved with what is known from kaon modes. We include a comparison with selected two-body leptonic decays of charged mesons, with K → πν overline{v} modes, with μ → e conversion, and with lepton-flavor violating decays of other neutral mesons, all of which constrain the same parameter space in a complementary way.

Lepton flavor universality violation in semileptonic tree level weak transitions

The recent deviations of the experimental data on some parameters of the tree-level semileptonic $B$ and $B_c$ mesons decays from the standard model (SM) predictions indicate considerable violations of the lepton flavor universality, and as a result possible new physics (NP) effects. To better understand the possible NP effects it is necessary to study deeply the physical quantities defining these decays from many aspects. The calculations of the physical quantities require the determinations of the hadronic form factors entering the matrix elements of the considered transitions as the main inputs. We calculate the form factors governing the tree-level $B_c\rightarrow J/\psi l \nu$ and $B_c \rightarrow \eta_c l \nu$ transitions within the QCD sum rules method. The obtained form factors are used in the calculations of the branching ratios ($BR$s) of the $B_c \rightarrow J/\psi l \nu$ and $B_c \rightarrow \eta_cl \nu$ transitions as well as $R(J/\psi)$ and $R(\eta_c)$. Our result on $R(J/\psi)$ supports the present tension between the SM theory prediction and the experimental data. Our result on $R(\eta_c)$ can be checked in future experiments.

Phenomenology of decays in a scalar leptoquark model * * Supported by the National Natural Science Foundation of China (11775092, 11521064, 11435003, 11805077). XY is also supported in part by the startup research funding from CCNU

During the past few years, signs of lepton flavor universality (LFU) violation have been observed in and transitions. Recently, the and polarization fractions and in decay were likewise measured by the Belle collaboration. Motivated by these intriguing results, we revisit the and anomalies in a scalar leptoquark (LQ) model, where two scalar LQs, one of which is a singlet and the other a triplet, are introduced simultaneously. We consider five mediated decays, , , , and , and focus on the LQ effects on the distributions of the branching fractions, LFU ratios, and various angular observables in these decays. Under the combined constraints of the available data on , , , and , we perform scans for the LQ couplings and make predictions for a number of observables. Numerically it is found that both the differential branching fractions and LFU ratios are largely enhanced by the LQ effects, with the latter expected to provide testable signatures at the SuperKEKB and High-Luminosity LHC experiments.

Decay in supersymmetry with R-parity violation * * Supported by the National Natural Science Foundation of China (11775092, 11521064, 11435003, 11805077), XY is also supported in part by the startup research funding from CCNU

In past years, several hints of lepton flavor universality (LFU) violation have emerged from the and data. More recently, the Belle Collaboration has reported the first measurement of the longitudinal polarization fraction in the decay. Motivated by this intriguing result, along with the recent measurements of and polarization, we present the study of decays in supersymmetry (SUSY) with -parity violation (RPV). We consider , , and modes and focus on the branching ratios, LFU ratios, forward-backward asymmetries, polarizations of daughter hadrons, and the lepton. The RPV SUSY was capable of explaining the anomalies at the level, after taking into account various flavor constraints. In the allowed parameter space, the differential branching fractions and LFU ratios are largely enhanced by the SUSY effects, especially in the large dilepton invariant mass region. Moreover, a lower bound 10−6 is obtained. These observables could provide testable signatures at the high-luminosity LHC and SuperKEKB, and correlate with direct searches for SUSY.

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.

B anomalies and dark matter: a complex connection

We study an extension of the Standard Model that addresses the hints of lepton flavour universality violation observed in Brightarrow K^{(*)} l^+l^- decays at LHCb, while providing a viable candidate for dark matter. The model incorporates two new scalar fields and a Majorana fermion that induce one-loop contributions to B meson decays. We show that agreement with observational data requires the new couplings to be complex and that the Majorana fermion can reproduce the observed dark matter relic density. This combination of cosmological and flavour constraints sets an upper limit on the dark matter and mediator masses. We have studied LHC dijet and dilepton searches, finding that they rule out large regions of parameter space by setting lower bounds on the dark matter and mediator masses. In particular, dilepton bounds are much more constraining in a future high-luminosity phase. Finally, we have computed the scattering cross section of dark matter off nuclei and compared it to the sensitivity of current and future direct detection experiments, showing that parts of the parameter space could be accessible in the future to multi-ton experiments. Future collider and direct DM searches complement each other to probe large areas of the parameter space of this model.

Search for an Lμ − Lτ gauge boson using Z → 4μ events in proton-proton collisions at [formula omitted] TeV

A search for a narrow Z′ gauge boson with a mass between 5 and 70 GeV resulting from an Lμ−LτU(1) local gauge symmetry is reported. Theories that predict such a particle have been proposed as an explanation of various experimental discrepancies, including the lack of a dark matter signal in direct-detection experiments, tension in the measurement of the anomalous magnetic moment of the muon, and reports of possible lepton flavor universality violation in B meson decays. A data sample of proton-proton collisions at a center-of-mass energy of 13 TeV is used, corresponding to an integrated luminosity of 77.3 fb−1 recorded in 2016 and 2017 by the CMS detector at the LHC. Events containing four muons with an invariant mass near the standard model Z boson mass are analyzed, and the selection is further optimized to be sensitive to the events that may contain Z→Z′μμ→4μ decays. The event yields are consistent with the standard model predictions. Upper limits of 10−8–10−7 at 95% confidence level are set on the product of branching fractions B(Z→Z′μμ)B(Z′→μμ), depending on the Z′ mass, which excludes a Z′ boson coupling strength to muons above 0.004–0.3. These are the first dedicated limits on Lμ−Lτ models at the LHC and result in a significant increase in the excluded model parameter space. The results of this search may also be used to constrain the coupling strength of any light Z′ gauge boson to muons.

Self-interacting dark matter with a vector mediator: kinetic mixing with the mathrm{U}{(1)}_{{left(B-Lright)}_3} gauge boson

A spontaneously broken hidden U(1)h gauge symmetry can explain both the dark matter stability and the observed relic abundance. In this framework, the light gauge boson can mediate the strong dark matter self-interaction, which addresses astrophysical observations that are hard to explain in collisionless cold dark matter. Motivated by flavoured grand unified theories, we introduce right-handed neutrinos and a flavoured B − L gauge symmetry for the third family mathrm{U}{(1)}_{{left(B-Lright)}_3} . The unwanted relic of the U(1)h gauge boson decays into neutrinos via the kinetic mixing with the mathrm{U}{(1)}_{{left(B-Lright)}_3} gauge boson. Indirect detection bounds on dark matter are systematically weakened, since dark matter annihilation results in neutrinos. However, the kinetic mixing between mathrm{U}{(1)}_{{left(B-Lright)}_3} and U(1)Y gauge bosons are induced by quantum corrections and leads to an observable signal in direct and indirect detection experiments of dark matter. This model can also explain the baryon asymmetry of the Universe via the thermal leptogenesis. In addition, we discuss the possibility of explaining the lepton flavour universality violation in semi-leptonic B meson decays that is recently found in the LHCb experiment.