Lepton Flavor Universality Violation Research Articles

Anomalies in B mesons decays: a phenomenological approach

The experimental measurements on flavour physics, in tension with Standard Model predictions, exhibit large sources of lepton flavour universality violation. We perform an analysis of the effects of the global fits on the Wilson coefficients assuming the Standard Model Effective Field Theory with semileptonic dimension six operators at 1 TeV, and by including a set of different scenarios in which the New Physics contributions to the Wilson coefficients are present in one, two or three of the Wilson coefficients at a time. We compare the results of the global fit with respect to two cases: the Standard Model and the more general case in which new physics modifies three independent Wilson coefficients. The last mentioned scenario is the favoured one for explaining the tension between Standard Model predictions and B-physics anomalies, but a specific more restricted scenario can provide similar goodness with a smaller set of free parameters. A discussion of the implications of our analysis in leptoquark models is included.

A 331 model for Lepton Flavor Universality Violation in B decays

Lepton Flavor Universality Violation (LFUV) in [Formula: see text] decays in both neutral [Formula: see text] and charged [Formula: see text] processes observed in the ratios [Formula: see text] and [Formula: see text] is investigated in the framework of a model based on the extended symmetry gauge [Formula: see text] whose leptonic sector consists of five triplets. It is shown that in order to explain the experimentally observed deviations from the Standard Model in these flavor changing transitions, a nonminimal version of the model has to be considered. We investigate the ability of this model in accommodating the model-independent scenarios currently favored by global fits.

LFU violations in leptonic τ decays and B-physics anomalies

We present a complete analysis of Lepton Flavor Universality (LFU) violations in leptonic τ decays in motivated models addressing the B-physics anomalies, based on the SU(4)×SU(3)×SU(2)×U(1) gauge group. We show that the inclusion of vector-like fermions, required by B-physics data, leads to sizable modifications of the leading-log results derived within an Effective Field Theory approach. In the motivated parameter-space region relevant to the B-physics anomalies, the models predict a few per-mil decrease of the effective W-boson coupling to τ, within the reach of future experiments.

Correlating nonresonant di-electron searches at the LHC to the Cabibbo-angle anomaly and lepton flavor universality violation

In addition to the existing strong indications for lepton flavor universality violation in low-energy precision experiments, the CMS Collaboration at CERN recently released an analysis of nonresonant dilepton pairs which could constitute the first sign of lepton flavor universality violation in high-energy searches at the LHC. In this article, we show that the Cabibbo-angle anomaly, an (apparent) violation of first row and column Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity with $\ensuremath{\approx}3\ensuremath{\sigma}$ significance, and the CMS result can be correlated and commonly explained in a model-independent way by the operator $[{Q}_{\ensuremath{\ell}q}^{(3)}{]}_{1111}=({\overline{\ensuremath{\ell}}}_{1}{\ensuremath{\gamma}}^{\ensuremath{\mu}}{\ensuremath{\sigma}}^{I}{\ensuremath{\ell}}_{1})({\overline{q}}_{1}{\ensuremath{\gamma}}_{\ensuremath{\mu}}{\ensuremath{\sigma}}^{I}{q}_{1})$. This is possible without violating the bounds from the nonresonant dilepton search of ATLAS (which interestingly also observed slightly more events than expected in the electron channel) nor from $R(\ensuremath{\pi})=\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}/\ensuremath{\pi}\ensuremath{\rightarrow}e\ensuremath{\nu}$. We find a combined preference for the new physics hypothesis of $4.5\ensuremath{\sigma}$ and predict $1.0004<R(\ensuremath{\pi})<1.0009$ (95% C.L.) which can be tested in the near future with the forthcoming results of the PEN experiment.

Study of electroweak penguin B decays at Belle II experiment

The electroweak b → sll (l = e, µ) transition is a flavor-changing neutral current process that mediates through a one-loop penguin diagram. The decay is considered to be a good probe for the New Physics as particles predicted in the beyond Standard Model theories can enter into the loop. The exclusive decay B → K (*) l + l − was first observed by the Belle experiment and it provides many observables such as the branching fraction, CP asymmetry, forward-backward asymmetry, and other angular observables. Recently, the LHCb experiment has reported some clue of a lepton flavor universality violation from the branching fraction ratio of the B → Kµ + µ − and B → Ke + e − decays. In this presentation, we report the status of the B → Kl + l − decay analysis at the Belle II experiment which started the data taking in 2019. We also, present an activity at the Belle II Chulalongkorn University group where we study the B → KJ/ψ decay which has the same topology as the B → Kl + l − .

Hints of lepton flavor universality violations.

Mounting evidence shows where the standard model may be incomplete.

TeV scale resonant leptogenesis with Lμ−Lτ gauge symmetry in light of the muon g−2

Motivated by the growing evidence for the possible lepton flavour universality violation after the first results from Fermilab's muon $(g-2)$ measurement, we revisit one of the most widely studied anomaly free extensions of the standard model namely, gauged $L_{\mu}-L_{\tau}$ model, to find a common explanation for muon $(g-2)$ as well as baryon asymmetry of the universe via leptogenesis. The minimal setup allows TeV scale resonant leptogenesis satisfying light neutrino data while the existence of light $L_{\mu}-L_{\tau}$ gauge boson affects the scale of leptogenesis as the right handed neutrinos are charged under it. For $L_{\mu}-L_{\tau}$ gauge boson mass at GeV scale or above, the muon $(g-2)$ favoured parameter space is already ruled out by other experimental data while bringing down its mass to sub-GeV regime leads to vanishing lepton asymmetry due to highly restrictive structures of lepton mass matrices at the scale of leptogenesis. Extending the minimal model with two additional Higgs doublets can lead to a scenario consistent with successful resonant leptogenesis and muon $(g-2)$ while satisfying all relevant experimental data.

The role of right-handed neutrinos in b \u2192 c\u03c4 (\u03c0\u03bd\u03c4, \u03c1\u03bd\u03c4, \u03bc overline{nu} \u03bc\u03bd\u03c4) overline{nu} \u03c4 from visible final-state kinematics

In the context of lepton flavor universality violation (LFUV) studies, we fully derive a general tensor formalism to investigate the role that left- and right-handed neutrino new-physics (NP) terms may have in b → cτ overline{nu} τ transitions. We present, for several extensions of the Standard Model (SM), numerical results for the Λb → Λcτ overline{nu} τ semileptonic decay, which is expected to be measured with precision at the LHCb. This reaction can be a new source of experimental information that can help to confirm, or maybe rule out, LFUV presently seen in overline{B} meson decays. The present study analyzes observables that can help in distinguishing between different NP scenarios that otherwise provide very similar results for the branching ratios, which are our currently best hints for LFUV. Since the τ lepton is very short-lived, we consider three subsequent τ-decay modes, two hadronic πντ and ρντ and one leptonic μ overline{nu} μντ, which have been previously studied for overline{B} → D(*) decays. Within the tensor formalism that we have developed in previous works, we re-obtain the expressions for the differential decay width written in terms of visible (experimentally accessible) variables of the massive particle created in the τ decay. There are seven different τ angular and spin asymmetries that are defined in this way and that can be extracted from experiment. Those asymmetries provide observables that can help in constraining possible SM extensions.

Addendum to “Combined constraints on first generation leptoquarks”

In this addendum to the paper by Crivellin and Schnell [Phys. Rev. D 103, 115023 (2021)], we discuss the implications of the recent CMS analysis of lepton flavor universality violation in nonresonant di-lepton pairs for first generation leptoquarks. As CMS finds more electron events than expected from background, this analysis prefers the leptoquark representations ${\stackrel{\texttildelow{}}{S}}_{1},{S}_{2},{S}_{3},{\stackrel{\texttildelow{}}{V}}_{1},{V}_{2}({\ensuremath{\kappa}}_{2}^{\mathrm{RL}}\ensuremath{\ne}0)$ and ${V}_{3}$ which lead to constructive interference with the standard model (SM). In principle the excess could also be (partially) explained by the representations ${\stackrel{\texttildelow{}}{S}}_{2},{V}_{1}({\ensuremath{\kappa}}_{1}^{R}\ensuremath{\ne}0),{V}_{2}({\ensuremath{\kappa}}_{2}^{\mathrm{LR}}\ensuremath{\ne}0),{\stackrel{\texttildelow{}}{V}}_{2}$ which are interfering destructively, as this would still lead to the right effect in bins with high invariant mass where the new physics contribution dominates. However, in these cases large couplings would be required which are excluded by other observables. The representations ${S}_{1},{V}_{1}({\ensuremath{\kappa}}_{1}^{L}\ensuremath{\ne}0)$ cannot improve the fit to the CMS data compared to the SM.

Single leptoquark solutions to the B -physics anomalies

We revisit the possibilities of accommodating the experimental indications of the lepton flavor universality violation in $b$-hadron decays in the minimal scenarios in which the Standard Model is extended by the presence of a single $\mathcal{O}(1\,\mathrm{TeV})$ leptoquark state. To do so we combine the most recent low energy flavor physics constraints, including $R_{K^{(\ast)}}^\mathrm{exp}$ and $R_{D^{(\ast)}}^\mathrm{exp}$, and combine them with the bounds on the leptoquark masses and their couplings to quarks and leptons as inferred from the direct searches at the LHC and the studies of the large $p_T$ tails of the $pp\to \ell\ell$ differential cross section. We find that none of the scalar leptoquarks of $m_\mathrm{LQ} \simeq 1\div 2$ TeV can accommodate the $B$-anomalies alone. Only the vector leptoquark, known as $U_1$, can provide a viable solution which, in the minimal setup, provides an interesting prediction, i.e. a lower bound to the lepton flavor violating $b\to s\mu^\pm\tau^\mp$ decay modes, such as $\mathcal{B}(B\to K\mu\tau) \gtrsim 0.7\times 10^{-7}$.

The role of non-universal Z couplings in explaining the Vus anomaly

The tension among measurements of Vus from different channels, the so-called Cabibbo Angle Anomaly, can be interpreted as a signal of lepton flavor universality (LFU) violation in the W boson couplings. We investigate this issue in the framework of effective field theory, keeping the gauge structure of the Standard Model (SM) unchanged. We introduce gauge-invariant dimension-6 effective operators that couple the Higgs doublet to leptons, thereby giving non-universal tree-level contributions to the couplings of electroweak gauge bosons. Due to the SU(2)L gauge symmetry, a tension arises between the Vus measurements that are affected by new W couplings, and the electroweak precision measurements, which are also affected by the new Z couplings. We show that this tension can be alleviated by allowing additional sources of gauge-invariant couplings of Z boson to left- or right-handed leptons, and find the optimal regions indicated by the current data in the Wilson-coefficient space. We illustrate our model-independent results with the examples of minimal extensions of the SM involving the vector-like lepton (VLL) models. We point out that dimension-6 operators coupling the Higgs doublet to leptons can affect the rate of h→ττ decay significantly in general, however this effect is restricted to less than a per cent level for the minimal VLL models.

Improved Vcs determination using precise lattice QCD form factors for D→Kℓν

We provide a 0.8%-accurate determination of ${V}_{cs}$ from combining experimental results for the differential rate of $D\ensuremath{\rightarrow}K$ semileptonic decays with precise form factors that we determine from lattice QCD. This is the first time that ${V}_{cs}$ has been determined with an accuracy that allows its difference from 1 to be seen. Our lattice QCD calculation uses the highly improved staggered quark (HISQ) action for all valence quarks on gluon field configurations generated by the MILC Collaboration that include the effect of $u$, $d$, $s$, and $c$ HISQ quarks in the sea. We use eight gluon field ensembles with five values of the lattice spacing ranging from 0.15 fm to 0.045 fm and include results with physical $u/d$ quarks for the first time. Our calculated form factors cover the full ${q}^{2}$ range of the physical decay process and enable a Standard Model test of the shape of the differential decay rate as well as the determination of ${V}_{cs}$ from a correlated weighted average over ${q}^{2}$ bins. We obtain $|{V}_{cs}|=0.9663(53{)}_{\mathrm{latt}}(39{)}_{\mathrm{exp}}(19{)}_{{\ensuremath{\eta}}_{\mathrm{EW}}}(40{)}_{\mathrm{EM}}$, where the uncertainties come from lattice QCD, experiment, short-distance electroweak, and electromagnetic corrections, respectively. This last uncertainty, neglected for $D\ensuremath{\rightarrow}K\ensuremath{\ell}\ensuremath{\nu}$ hitherto, now needs attention if the uncertainty on ${V}_{cs}$ is to be reduced further. We also determine ${V}_{cs}$ values in good agreement using the measured total branching fraction and the rates extrapolated to ${q}^{2}=0$. Our form factors enable tests of lepton flavor universality violation. We find the ratio of branching fractions for ${D}^{0}\ensuremath{\rightarrow}{K}^{\ensuremath{-}}$ with $\ensuremath{\mu}$ and $e$ in the final state to be ${R}_{\ensuremath{\mu}/e}=0.9779(2{)}_{\mathrm{latt}}(50{)}_{\mathrm{EM}}$ in the Standard Model, with the uncertainty dominated by that from electromagnetic corrections.

Muon (g − 2) and XENON1T excess with boosted dark matter in Lμ − Lτ model

Motivated by the growing evidence for lepton flavour universality violation after the first results from Fermilab's muon (g−2) measurement, we revisit one of the most widely studied anomaly free extensions of the standard model namely, gauged Lμ−Lτ model, known to be providing a natural explanation for muon (g−2). We also incorporate the presence of dark matter (DM) in this model in order to explain the recently reported electron recoil excess by the XENON1T collaboration. We show that the same neutral gauge boson responsible for generating the required muon (g−2) can also mediate interactions between electron and dark fermions boosted by dark matter annihilation. The required DM annihilation rate into dark fermion require a hybrid setup of thermal and non-thermal mechanisms to generate DM relic density. The tightly constrained parameter space from all requirements remains sensitive to ongoing and near future experiments, keeping the scenario very predictive.

Correlating h→μ^{+}μ^{-} to the Anomalous Magnetic Moment of the Muon via Leptoquarks.

Recently, both ATLAS and CMS measured the decay h→μ^{+}μ^{-}, finding a signal strength with respect to the standard model expectation of 1.2±0.6 and 1.19_{-0.39-0.16}^{+0.41+0.17}, respectively. This provides, for the first time, evidence that the standard model Higgs couples to second generation fermions. This measurement is particularly interesting in the context of the intriguing hints for lepton flavor universality violation, accumulated within recent years, as new physics explanations could also be tested in the h→μ^{+}μ^{-} decay mode. Leptoquarks are prime candidates to account for the flavor anomalies. In particular, they can provide the necessary chiral enhancement (by a factor m_{t}/m_{μ}) to address a_{μ} with tera-electron-volt scale new physics. In this Letter we point out that such explanations of a_{μ} also lead to enhanced effects in h→μ^{+}μ^{-} and we examine the correlations between h→μ^{+}μ^{-} and a_{μ} within leptoquark models. We find that the effect in the branching ratio of h→μ^{+}μ^{-} ranges from several percent up to a factor of 3, if one aims at accounting for a_{μ} at the 2σ level. Hence, the new ATLAS and CMS Collaboration measurements already provide important constraints on the parameter space, rule out specific a_{μ} explanations, and will be very important to test the flavor anomalies in the future.

Mapping out the space for new physics with leptonic and semileptonic B_{(c)} decays

Decays of B mesons with leptons in the final state offer an interesting laboratory to search for possible effects of physics from beyond the Standard Model. In view of puzzling patterns in experimental data, the violation of lepton flavour universality is an interesting option. We present a strategy, utilising ratios of leptonic and semileptonic B decays, where the elements |V_{ub}| and |V_{cb}| of the Cabibbo–Kobayashi–Maskawa (CKM) matrix cancel, to constrain the short-distance coefficients of (pseudo)-scalar, vector and tensor operator contributions. The individual branching ratios allow us then to extract also the CKM matrix elements, even in the presence of new-physics contributions. Bounds on unmeasured leptonic and semileptonic decays offer important additional constraints. In our comprehensive analysis, we give also predictions for decays which have not yet been measured in a variety of scenarios.

Global analysis of leptophilic Z\u2032 bosons

New neutral heavy gauge bosons (Z′) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic Z′ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like (g − 2)μ) and generate flavour violating decays of charged leptons. In particular, mathrm{ell}to mathrm{ell}^{prime }voverline{v} decays, anomalous magnetic moments of charged leptons, ℓ → ℓ′γ and ℓ → 3ℓ′ decays place stringent limits on leptophilic Z′ bosons. Furthermore, in case of mixing Z′ with the SM Z, Z pole observables are affected. In light of these many observables we perform a global fit to leptophilic Z′ models with the main goal of finding the bounds for the Z′ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an Lμ− Lτ symmetry broken only by left-handed rotations, or considers the case of τ − μ couplings only. In the latter setup, on can explain the (g − 2)μ anomaly and the hint for lepton flavour universality violation in tau to mu voverline{v}/tau to evoverline{v} without violating bounds from electroweak precision observables.

New physics and the tau polarization vector in b \u2192 mathrm{c}tau {overline{nu}}_{tau } decays

For a general Hb → {H}_ctau {overline{nu}}_{tau } decay we analyze the role of the τ polarization vector {mathcal{P}}^{mu } in the context of lepton flavor universality violation studies. We use a general phenomenological approach that includes, in addition to the Standard Model (SM) contribution, vector, axial, scalar, pseudoscalar and tensor new physics (NP) terms which strength is governed by, complex in general, Wilson coefficients. We show that both in the laboratory frame, where the initial hadron is at rest, and in the center of mass of the two final leptons, a overrightarrow{mathcal{P}} component perpendicular to the plane defined by the three-momenta of the final hadron and the τ lepton is only possible for complex Wilson coefficients, being a clear signal for physics beyond the SM as well as time reversal (or CP-symmetry) violation. We make specific evaluations of the different polarization vector components for the Λb → Λc, {overline{B}}_c → ηc, J/ψ and overline{B} → D(*) semileptonic decays, and describe NP effects in the complete two-dimensional space associated with the independent kinematic variables on which the polarization vector depends. We find that the detailed study of {mathcal{P}}^{mu } has great potential to discriminate between different NP scenarios for 0− → 0− decays, but also for Λb → Λc transitions. For this latter reaction, we pay special attention to corrections to the SM predictions derived from complex Wilson coefficients contributions.

Angular analysis of Bs→f2′(1525)(→K+K−)μ+μ− decays as a probe to lepton flavor universality violation

The flavor anomalies reported in $R_K$, $R_{K^*}$, $P_{5}^{\prime}$ and $\mathcal{B}(B_s\, \to\, \phi\, \mu^+\, \mu^-)$ indicate lepton flavor universality violation in $b \to s\,l^+\,l^-$ quark level transition decays. The deviation from the SM prediction reported in the underlying flavor observables currently stand at the level of $2.5\sigma$, $2.4\sigma$, $3.3\sigma$ and $3.7\sigma$, respectively. In this context, we perform an angular analysis of the four-body differential decay of $B_s\, \to\, f_{2}^{\prime}\,(1525)\,(\to K^+\,K^-)\,\mu^+ \,\mu^-$ in a model independent effective field theory framework. The decay mode $B_s\, \to\, f_{2}^{\prime}(1525)\,l^+\,l^-$ undergoes similar $b \to s$ neutral current quark level transition and, in principle, can provide complementary information regarding lepton flavor universality violation in $b \to s\,l^+\,l^-$ quark level transition decays. We give predictions of various physical observables such as the branching ratio, the longitudinal polarization fraction, the forward-backward asymmetry, the angular observables $P_1$, $P_2$, $P_{4}^{\prime}$, $P_{5}^{\prime}$ and also the lepton flavor sensitive observables such as the ratio of branching ratio $R_{f_{2}^{\prime}}$, $Q_{F_L}$, $Q_{A_{FB}}$, $Q_{1}$, $Q_{2}$, $Q_{4}^{\prime}$, $Q_{5}^{\prime}$ for $B_s\, \to\, f_{2}^{\prime}\,(1525)\,(\to K^+\,K^-)\,\mu^+ \,\mu^-$ decays in the standard model and in the presence of several 1D and 2D new physics scenarios.

Investigation on effects of new physics in transitions * *Supported in part by the National Natural Science Foundation of China (11975195, 11705159); the Natural Science Foundation of Shandong province (ZR2019JQ004, ZR2018JL001) and the Project of Shandong Province Higher Educational Science and Technology Program (2019KJJ007)

Anomalies in decays induced by ( ) transitions may imply lepton flavor universality violations, which raises questions on such phenomena in the D decays induced by transitions. Current measurements of the pure leptonic and semi-leptonic D decays agree with the standard model (SM) predictions, and such agreements can be used to constrain the new physics (NP) contributions. In this work, we extend SM by assuming general effective Hamiltonians describing the transitions including the full set of the four-fermion operators. With the latest experimental data, we perform a least fit of the Wilson coefficient corresponding to each operator. The results indicate that the Wilson coefficients of tensor and scalar operators in the muon sector are in the order of while others are in the order of . The lepton flavor universality could be violated by interactions with the scalar operators. We also determine that the pure leptonic decays are significantly sensitive to scalar operators. The effects of NP on the semi-leptonic decays with electron final state are negligible; however, for the decays with the muon final state, the effects of scalar and tensor operators will appear in the forward-backward asymmetries and the muon helicity asymmetries of decays. The future measurements of these decays in the BESIII and Belle II experiments will facilitate the evaluation of NP effects.

Searching for lepton flavor universality violation and collider signals from a singly charged scalar singlet

In recent years, evidence for lepton flavour universality violation beyond the Standard Model has been accumulated. In this context, a singly charged $SU(2)_L$ singlet scalar ($\phi^\pm$) is very interesting, as it can only have flavour off-diagonal couplings to neutrinos and charged leptons, therefore necessarily violating lepton flavour (universality). In fact, it gives a (necessarily constructive) tree-level effect in $\ell\to\ell^\prime\nu\nu$ processes, while contributing to charged lepton flavour violating only at the loop-level. Therefore, it can provide a common explanation of the hints for new physics in $\tau\to\mu\nu\nu/\tau(\mu)\to e\nu\nu$ and of the Cabibbo Angle Anomaly. Such an explanation predicts ${\rm Br }[\tau\to e\gamma]$ to be of the order of a few times $10^{-11}$ while ${ \rm Br}[\tau\to e\mu\mu]$ can be of the order of $10^{-9}$ for order one couplings and therefore in the reach of forthcoming experiments. Furthermore, we derive a {novel} coupling-independent lower limit on the scalar mass of $\approx 200\,$GeV by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to $\approx300\,$GeV, showing the complementary between LHC searches and flavour observables. Furthermore, we point out that this model can be tested by reinterpreting DM mono-photon searches at future $e^+e^-$ colliders.