Lepton Flavor Universality Violation Research Articles

Lepton flavor violation in a Z′ model for the b→s anomalies

In recent years, several observables associated to semileptonic $b \to s$ processes have been found to depart from their predicted values in the Standard Model, including a few tantalizing hints of lepton flavor universality violation. In this work we consider an existing model with a massive $Z^\prime$ boson that addresses the anomalies in $b \to s$ transitions and extend it with a non-trivial embedding of neutrino masses. We analyze lepton flavor violating effects, induced by the non-universal interaction associated to the $b \to s$ anomalies and by the new physics associated to the neutrino mass generation, and determine the expected ranges for the most relevant observables.

On the amplitudes for the CP-conserving K\xb1(KS) \u2192 \u03c0\xb1(\u03c00)\u2113+\u2113\u2212 rare decay modes

The amplitudes for the rare decay modes K± → π±ℓ+ℓ− and KS → π0ℓ+ℓ− are studied with the aim of obtaining predictions for them, such as to enable the possibility to search for violations of lepton-flavour universality in the kaon sector. The issue is first addressed from the perspective of the low-energy expansion, and a two-loop representation of the corresponding form factors is constructed, leaving as unknown quantities their values and slopes at vanishing momentum transfer. In a second step a phenomenological determination of the latter is proposed. It consists of the contribution of the resonant two-pion state in the P wave, and of the leading short-distance contribution determined by the operator-product expansion. The interpolation between the two energy regimes is described by an infinite tower of zero-width resonances matching the QCD short-distance behaviour. Finally, perspectives for future improvements in the theoretical understanding of these amplitudes are discussed.

Study of the D^{0}→K^{-}μ^{+}ν_{μ} Dynamics and Test of Lepton Flavor Universality with D^{0}→K^{-}ℓ^{+}ν_{ℓ} Decays.

Using e^{+}e^{-} annihilation data of 2.93 fb^{-1} collected at center-of-mass energy sqrt[s]=3.773 GeV with the BESIII detector, we measure the absolute branching fraction of D^{0}→K^{-}μ^{+}ν_{μ} with significantly improved precision: B_{D^{0}→K^{-}μ^{+}ν_{μ}}=(3.413±0.019_{stat}±0.035_{syst})%. Combining with our previous measurement of B_{D^{0}→K^{-}e^{+}ν_{e}}, the ratio of the two branching fractions is determined to be B_{D^{0}→K^{-}μ^{+}ν_{μ}}/B_{D^{0}→K^{-}e^{+}ν_{e}}=0.974±0.007_{stat}±0.012_{syst}, which agrees with the theoretical expectation of lepton flavor universality within the uncertainty. A study of the ratio of the two branching fractions in different four-momentum transfer regions is also performed, and no evidence for lepton flavor universality violation is found with current statistics. Taking inputs from global fit in the standard model and lattice quantum chromodynamics separately, we determine f_{+}^{K}(0)=0.7327±0.0039_{stat}±0.0030_{syst} and |V_{cs}|=0.955±0.005_{stat}±0.004_{syst}±0.024_{LQCD}.

On lepton flavor universality in top quark decays

We propose a novel strategy to test lepton flavor universality (LFU) in top decays, applicable to top pair production at colliders. Our proposal exploits information in kinematic distributions and mostly hinges on data-driven techniques, thus having very little dependence on our theoretical understanding of top pair production. Based on simplified models accommodating recent hints of LFU violation in charged current B meson decays, we show that existing LHC measurements already provide non-trivial information on the flavor structure and the mass scale of such new physics (NP). We also project that the measurements of LFU in top decays at the high-luminosity LHC could reach a precision at the percent level or below, improving the sensitivity to LFU violating NP in the top sector by more than an order of magnitude compared to existing approaches.

Building models for B-physics anomalies

Several different experiments show evidences of Lepton Flavour Universality violation in semi-leptonic B meson decays, both in charged and neutral currents. These anomalies can be interpreted in terms of new short-distance interactions that involve mainly the third generation of fermions. I will first discuss the present status of the anomalies, and their connection to other low- and high-energy observables, in a model-independent way, adopting an Effective Field Theory approach based on a CKM-like flavour structure. I will then extend the analysis to a few simple UV completions with different types of mediator, discussing the main issues that have to be faced in each case. A few models emerge, which look capable of accommodating both charged- and neutral-current anomalies consistently with all the experimental bounds, with associated flavour and high-pT signatures within the reach of present and future experiments.

Constraining new physics in b \u2192 c\u2113\u03bd transitions

B decays proceeding via b → cℓν transitions with ℓ = e or μ are tree-level processes in the Standard Model. They are used to measure the CKM element Vcb, as such forming an important ingredient in the determination of e.g. the unitarity triangle; hence the question to which extent they can be affected by new physics contributions is important, specifically given the long-standing tension between Vcb determinations from inclusive and exclusive decays and the significant hints for lepton flavour universality violation in b → cτ ν and b → sℓℓ decays. We perform a comprehensive model-independent analysis of new physics in b → cℓν, considering all combinations of scalar, vector and tensor interactions occuring in single-mediator scenarios. We include for the first time differential distributions of B → D∗ℓν angular observables for this purpose. We show that these are valuable in constraining non-standard interactions. Specifically, the zero-recoil endpoint of the B → Dℓν spectrum is extremely sensitive to scalar currents, while the maximum-recoil endpoint of the B → D∗ℓν spectrum with transversely polarized D∗ is extremely sensitive to tensor currents. We also quantify the room for e-μ universality violation in b → cℓν transitions, predicted by some models suggested to solve the b → cτ ν anomalies, from a global fit to B → Dℓν and B → D∗ℓν for the first time. Specific new physics models, corresponding to all possible tree-level mediators, are also discussed. As a side effect, we present Vcb determinations from exclusive B decays, both with frequentist and Bayesian statistics, leading to compatible results. The entire numerical analysis is based on open source code, allowing it to be easily adapted once new data or new form factors become available.

Flavour anomalies: a review

The concept of lepton flavour universality (LFU), according to which the three lepton families are equivalent except for their masses, is a cornerstone prediction of the Standard Model (SM). LFU can be violated in models beyond the SM by new physics particles that couple preferentially to certain generations of leptons. In the last few years, hints of LFU violation have been observed in both tree-level b → cℓν and loop-level b → sℓℓ transitions. These measurements, combined with the tensions observed in angular observables and branching fractions of rare semileptonic b decays, point to a coherent pattern of anomalies that could soon turn into the first observation of physics beyond the SM. These proceedings review the anomalies seen by collider experiments, and give an outlook for the near future.

Model of vector leptoquarks in view of the B -physics anomalies

Lepton number as a fourth color is an intriguing theoretical idea which is combined with a possible left-right symmetry within the famous Pati-Salam (PS) model. In the conventional PS model, a spontaneous breaking of the PS gauge group down to the SM one can only take place at very high scales (above the PeV scale) due to the stringent bounds from $K_L\to\mu e$ and $K\to\pi\mu e$ induced by the resulting vector leptoquarks. In this paper, we show that these constraints can be avoided once additional vector-like fermions are introduced and thus a breaking at the TeV scale is possible. We consider the flavor phenomenology of this model in the context of the intriguing hints for new physics in semileptonic $B$ decays. The necessary violation of lepton flavor universality is induced by mixing SM and vector-like fermions. Concerning $R(D)$ and $R(D^*)$ we find that sizable effects are possible while respecting the bounds from other flavor observables but predicting a large enhancement of $B_s\to\tau^+\tau^-$. Furthermore, also in $b\to s\ell^+\ell^-$ transitions the observed deviations from the SM predictions (including $R(K)$ and $R(K^*)$) can be explained with natural values for the free parameters of the model without any fine-tuning, predicting sizable decay rates for $b\to s\tau\mu$. Finally, the anomaly in anomalous magnetic moment of the muon can be accounted for by a loop-contribution involving the vector leptoquark and vector-like leptons.

A composite pNGB leptoquark at the LHC

The measurements of RK(∗) and RD(∗) by BaBar, Belle and the LHCb collaborations could be showing a hint of lepton flavor universality violation that can be accommodated by the presence of suitable leptoquarks at the TeV scale. We consider an effective description, with leptoquarks arising as composite pseudo Nambu-Goldstone bosons, as well as anarchic partial compositeness of the SM fermions. Considering the RK(∗) anomaly within this framework, we study pair production of {S}_3sim {left(overline{3},3right)}_{1/3} at the LHC. We focus on the component S31/3 of the triplet, which decays predominantly into tτ and bν, and study the bounds from existing searches at sqrt{s}=13 TeV at the LHC. We find that sbottom searches in the boverline{b}+{E}_{mathrm{T}}^{mathrm{miss}} final state best explore the region in parameter space preferred by our model and currently exclude S31/3 masses up to ∼1 TeV. Additional searches, considering the tτ and tμ decay modes, are required to probe the full physical parameter space. In this paper we also recast existing studies on direct leptoquark searches in the tτ tτ channel and SM toverline{t}toverline{t} searches, and obtain the regions in parameter space currently excluded. Practically the whole physical parameter space is currently excluded for masses up to ∼0.8 TeV, which could be extended up to ∼1 TeV with the full Run 3 dataset. We conclude that pair production searches for this leptoquark can benefit from considering the final state tτ b + ETmiss, where the largest branching ratio is expected. We appraise that future explorations of leptoquarks explaining the B-anomalies with masses beyond the TeV should also consider single and non-resonant production in order to extend the mass reach.

Simultaneous interpretation of K and B anomalies in terms of chiral-flavorful vectors

We address the presently reported significant flavor anomalies in the Kaon and B meson systems such as the CP violating Kaon decay (ϵ′/ϵ) and lepton-flavor universality violation in B meson decays left({R}_{K^{left(*right)}}, {R}_{D^{left(*right)}}right) , by proposing flavorful and chiral vector bosons as the new physics constitution at around TeV scale. The chiral-flavorful vectors (CFVs) are introduced as a 63-plet of the global SU(8) symmetry, identified as the one-family symmetry for left-handed quarks and leptons in the standard model (SM) forming the 8-dimensional vector. Thus the CFVs include massive gluons, vector leptoquarks, and W′, Z′-type bosons, which are allowed to have flavorful couplings with left-handed quarks and leptons, and flavor-universal couplings to right-handed ones, where the latter arises from mixing with the SM gauge bosons. The flavor texture is assumed to possess a “minimal” structure to be consistent with the current flavor measurements on the K and B systems. Among the presently reported significant flavor anomalies in the Kaon and B meson systems (ϵ′/ϵ, {R}_{K^{left(*right)}}, {R}_{D^{left(*right)}} ), the first two ϵ′/ϵ and {R}_{K^{left(*right)}} anomalies can simultaneously be interpreted by the presence of CFVs; the {R}_{D^{left(*right)}} anomaly is predicted not to survive, due to the approximate SU(8) flavor symmetry. Remarkably, we find that as long as both of the ϵ′/ϵ and {R}_{K^{left(*right)}} anomalies persist beyond the SM, the CFVs predict the enhanced {K}^{+}to {pi}^{+}nu overline{nu} and {K}_Lto {pi}^0nu overline{nu} decay rates compared to the SM values, which will readily be explored by the NA62 and KOTO experiments, and they will also be explored in new resonance searches at the Large Hadron Collider.

Low-energy effects of Lepton Flavour Universality Violation

The persisting anomalous data in semileptonic B-decays point towards New Physics models exhibiting large sources of Lepton Flavour Universality Violation. In this work we generalise previous studies by considering frameworks which include an enlarged set of semileptonic four-fermion operators invariant under the SM gauge group, with New Physics affecting mainly the third generation. We derive the low-energy effective Lagrangian including the leading electroweak corrections, mandatory to obtain reliable predictions. As a particularly interesting case, we analyse the scenario where the dominant New Physics effects are encoded in the Wilson coefficient C9, as favoured by global fit analyses of b → s data. We find that also in this case the stringent experimental bounds on Z-pole observables and τ decays challenge a simultaneous explanation of charged and neutral-current non-standard data.

Implications of scalar and tensor explanations of {mathrm{R}}_{{mathrm{D}}^{left(ast right)}}

We investigate the implications of scalar and tensor operators proposed to accommodate the hints of lepton flavor universality violation in charged-current B-meson decays. We show that these scenarios unavoidably induce chirality enhanced contributions to charged lepton magnetic moments and Yukawa couplings. By using an effective field theory approach we quantify in a model independent way the connection of the {R}_{D^{left(ast right)}} anomaly with the tau g − 2 and the Higgs decay h → τ τ , which can offer an alternative to test these scenarios. Concrete New Physics models giving rise to our setup will be illustrated.

Revisiting R-parity violating interactions as an explanation of the B-physics anomalies

In the last few years, the ratios R_{D^{(*)}} and of R_{K^{(*)}} have reportedly exhibited significant deviations from the relevant standard model predictions, hinting towards a possible violation of Lepton Flavor Universality and a window to New Physics. We investigate to what extent the inclusion of R-parity violating couplings in the minimal supersymmetric standard model can provide a better fit to the anomalies simultaneously. We perform this analysis employing an approximate, non-abelian mathcal {G}_f=U(2)_q times U(2)_ell flavour symmetry, which features a natural explanation of the appropriate hierarchy of the R-parity violating couplings. We show that, under the requirement of a supersymmetric spectrum with much heavier left-handed doublet superpartners, our assumption favors a considerable enhancement in the tree-level charged-current B rightarrow D^{(*)} tau overline{nu }, while the the anomalies induced by b rightarrow sell ^+ell ^- receive up to an approximate 30 % improvement. The consistency with all relevant low-energy constraints is assessed.

B Meson Anomalies in a Pati-Salam Model within the Randall-Sundrum Background

Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by K_{L}→μe and K→πμe, the scale can be lowered to a few TeV by adding vectorlike fermions. Furthermore, in this case, the intriguing hints for lepton flavor universality violation in b→sμ^{+}μ^{-} and b→cτν processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension, and the resulting massive vector leptoquark automatically has the same mass scale as the vectorlike fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavor universality violation in semileptonic B decays. Assuming flavor alignment in the down sector, we find that in b→sℓ^{+}ℓ^{-} transitions, the observed deviations from the standard model predictions [including R(K) and R(K^{*})] can be explained with natural values for the free parameters of the model. Even though we find sizable effects in R(D), R(D^{*}), and R(J/Ψ), one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from D-D[over ¯] mixing and τ→3μ.

Flavor physics and flavor anomalies in minimal fundamental partial compositeness

Partial compositeness is a key ingredient of models where the electroweak symmetry is broken by a composite Higgs state. Recently, a UV completion of partial compositeness was proposed, featuring a new strongly coupled gauge interaction as well as new fundamental fermions and scalars. We work out the full flavor structure of the minimal realization of this idea and investigate in detail the consequences for flavor physics. While CP violation in kaon mixing represents a significant constraint on the model, we find many viable parameter points passing all precision tests. We also demonstrate that the recently observed hints for a violation of lepton flavor universality in $B\to K^{(*)}\ell\ell$ decays can be accommodated by the model, while the anomalies in $B\to D^{(*)}\tau\nu$ cannot be explained while satisfying LEP constraints on $Z$ couplings.

Anomalies in b→s Transitions and Dark Matter

Since 2013, the LHCb collaboration has reported on the measurement of several observables associated with b→s transitions, finding various deviations from their predicted values in the Standard Model. These include a set of deviations in branching ratios and angular observables, as well as in the observables RK and RK⁎, specially built to test the possible violation of Lepton Flavor Universality. Even though these tantalizing hints are not conclusive yet, the b→s anomalies have gained considerable attention in the flavor community. Here we review new physics models that address these anomalies and explore their possible connection to the dark matter of the Universe. After discussing some of the ideas introduced in these works and classifying the proposed models, two selected examples are presented in detail in order to illustrate the potential interplay between these two areas of current particle physics.

A light Higgs at the LHC and the B-anomalies

After the Higgs discovery, the LHC has been looking for new resonances, decaying into pairs of Standard Model (SM) particles. Recently, the CMS experimen observed an excess in the di-photon channel, with a di-photon invariant mass of about 96 GeV. This mass range is similar to the one of an excess observed in the search for the associated production of Higgs bosons with the Z neutral gauge boson at LEP, with the Higgs bosons decaying to bottom quark pairs. On the other hand, the LHCb experiment observed a discrepancy with respect to the SM expectations of the ratio of the decay of B-mesons to K-mesons and a pair of leptons, {R}_{K^{left(ast right)}}=BRleft(Bto {K}^{left(ast right)}{mu}^{+}{mu}^{-}right)/BRleft(Bto {K}^{left(ast right)}{e}^{+}{e}^{-}right) . This observation provides a hint of the violation of lepton-flavor universality in the charged lepton sector and may be explained by the existence of a vector boson originating form a U{(1)_{L_{mu}}}_{-{L}_{{}_{tau }}} symmetry and heavy quarks that mix with the left-handed down quarks. Since the coupling to heavy quarks could lead to sizable Higgs di-photon rates in the gluon fusion channel, in this article we propose a common origin of these anomalies identifying a Higgs associated with the breakdown of the U{(1)_{L_{mu}}}_{-{L}_{{}_{tau }}} symmetry and at the same time responsible to the quark mixing, with the one observed at the LHC. We also discuss the constraints on the identification of the same Higgs with the one associated with the bottom quark pair excess observed at LEP.

Searching for New Physics with b→sτ^{+}τ^{-} Processes.

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b→cℓ^{-}ν[over ¯]_{ℓ} (i.e., R_{D}, R_{D^{*}}, and R_{J/ψ}) and the neutral-current transitions b→sℓ^{+}ℓ^{-} (i.e., R_{K} and R_{K^{*}}). Hints for LFU violation in R_{D^{(*)}} and R_{J/ψ} point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b→sμ^{+}μ^{-} processes as well as in the ratios R_{K} and R_{K^{*}}, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b→sτ^{+}τ^{-} transitions. In this Letter, we predict the branching ratios of B→Kτ^{+}τ^{-}, B→K^{*}τ^{+}τ^{-}, and B_{s}→ϕτ^{+}τ^{-}, taking into account NP effects in the Wilson coefficients C_{9(^{'})}^{ττ} and C_{10(^{'})}^{ττ}. Assuming a common NP explanation of R_{D}, R_{D^{(*)}}, and R_{J/ψ}, we show that a very large enhancement of b→sτ^{+}τ^{-} processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of B_{s}→τ^{+}τ^{-}, B_{s}→ϕτ^{+}τ^{-}, and B→K^{(*)}τ^{+}τ^{-} under these assumptions are in the observable range for LHCb and Belle II.

LHCb anomaly in optimised observables and potential of model

Over the last few years LHCb found some discrepancies in FCNC transitions, including anomalies in the angular observables of , particularly in , in the low dimuon mass region. Recently, these anomalies have been confirmed by Belle, CMS and ATLAS. As direct evidence of physics beyond the Standard Model is absent so far, these anomalies are being interpreted as indirect hints of new physics. In this context, we study the implications of the family non-universal model for the angular observables , and newly proposed lepton flavor universality violation observables, Q4,5, in the decay channel in the low dimuon mass region. To see the variation in the values of these observables from their Standard Model values, we have chosen different scenarios for the model. It is found that these angular observables are sensitive to the values of the parameters of the model. We have also found that with the present parametric space of the model, the -anomaly could be accommodated. However, more statistics on the anomalies in the angular observables are helpful to reveal the status of the considered model and, in general, the nature of new physics.

Footprints of leptoquarks: from R_{K^{(*)}} to K rightarrow pi nu bar{nu }

Rare K rightarrow pi nu bar{nu } decays, being dominated by short distance contributions within the standard model (SM), open a window for new physics (NP) searches at low energies. The K rightarrow pi nu bar{nu } branching ratios are expected to be measured with sim 10% accuracies by NA62/CERN and KOTO/JPARC. The theoretical uncertainties of branching ratios within the SM are well under control. In the B sector, it is tentative to explain the B-meson anomalies R_{D^{(*)}} and/or R_{K^{(*)}} by effects of physics beyond the SM. Although NP seems to be present in the third fermion generation it might also manifest in the flavor changing neutral current transition s rightarrow d . Together with the anticipated good experimental sensitivities and accurate theoretical predictions for Krightarrow pi nu bar{nu }, this motivates studies of correlated effects of NP in rare Krightarrow pi nu bar{nu } and B rightarrow K^{(*)} mu ^+ mu ^- decays. Here we consider the loop induced effects in K rightarrow pi nu bar{nu } in two leptoquark models designed to address lepton-flavor universality violation in the R_{K^{(*)}} anomalies.