Lepton Universality Violation Research Articles

Phenomenology of dark tensor currents

In this work we suggest a dark mediator model which extends the usual U(1) Lagrangian by including new degrees of freedom coupling to Standard Model fermions. Such dark particles can contribute to the neutral pion decay. For interaction constant and the dark particle mass consistent with the observed anomalies in the nuclear decays by the ATOMKI group we show that the dark tensor particles naturally incorporate lepton universality violation which introduces sizable effect on the muon anomalous magnetic moment.

Lepton universality violation in the minimal flipped 331 model

Abstract We perform a detailed study of the b → cτν and b → sl+l− processes in a minimal flipped 331 model based on the SU(3)C × SU(3)L × U(1)X gauge symmetry. The non universal SU(3)L × U(1)X symmetry in the lepton sector gives rise to non universal neutral and charged currents involving heavy non SM gauge bosons and SM leptons that yield radiative contributions to the b → s, b → c, s → u and d → u transitions arising from one loop level penguin and box diagrams. We found that the observables related to these transitions agree with their experimental values in a region of parameter space that includes TeV scale exotic up type quarks, within the LHC’s reach.

Constraints on lepton universality violation from rare B decays

The LHCb Collaboration has recently released a new study of ${B}^{+}\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{*0}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ ($\ensuremath{\ell}=e$, $\ensuremath{\mu}$) decays, testing lepton universality with unprecedented accuracy using the whole Run 1 and 2 dataset. In addition, the CMS Collaboration has reported an improved analysis of the branching ratios ${B}_{(d,s)}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$. While these measurements offer, per se, a powerful probe of new physics, global analyses of $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ transitions also rely on the assumptions about nonperturbative contributions to the decay matrix elements. In this work, we perform a global Bayesian analysis of new physics in (semi)leptonic rare $B$ decays, paying attention to the role of charming penguins which are difficult to evaluate from first principles. We find data to be consistent with the Standard Model once rescattering from intermediate hadronic states is included. Consequently, we derive stringent bounds on lepton universality violation in $|\mathrm{\ensuremath{\Delta}}B|=|\mathrm{\ensuremath{\Delta}}S|=1$ (semi)leptonic processes.

Measurement of structure dependent radiative K+ → e+νγ decay at J-PARC

The J-PARC E36 experiment is aiming at searching for lepton universality violation by accurately measuring the ratio of branching ratio of the K + → e + ν(γ) (K e2(γ)) to K + → μ + ν(γ) (K μ2(γ)) decays. The experiment was performed at J-PARC employing a stopped K + method in conjunction with a 12-sector iron-core superconducting toroidal spectrometer. Charged particle momenta were determined by reconstructing charged particle trajectories in the spectrometer. Particle identification for e + and μ + was carried out using an aerogel Cherenkov counter and a lead-glass Cherenkov counter, as well as by measuring the time-of-flight between TOF counters. The peak structure due to K e2(γ) decay was successfully observed in the e + momentum spectrum after the e + requirement in the PID analysis. The structure dependentradiative K + → e + νγ () events were selected by imposing photon hits in the CsI(Tl) calorimeter or the GSC counter. The experimental spectra were accurately reproduced by the Monte Carlo simulation, indicating a correct understanding of the experimental conditions. The Br() value relative to Br(K e2(γ)) was determined from the and K e2(γ) yields corrected for the detector acceptances. The Br()/Br(K e2(γ)) ratio was derived to be 1.19 ± 0.07 stat ± 0.04 syst in the CsI(Tl) analysis and 1.25 ± 0.14 stat ± 0.04 syst in the GSC analysis.

Charming penguins and lepton universality violation in varvec{b rightarrow s ell ^+ ell ^-} decays

The LHCb experiment has recently presented new results on Lepton Universality Violation (LUV) in B rightarrow K^{(*)} ell ^+ ell ^- decays involving K_S in the final state, which strengthen the recent evidence of LUV obtained in B^+ rightarrow K^{+} ell ^+ ell ^- decays and the previous measurements of B rightarrow K^{*0} ell ^+ ell ^-. While LUV observables in the Standard Model are theoretically clean, their predictions in New Physics scenarios are sensitive to the details of the hadronic dynamics, and in particular of the charming penguin contribution. In this work, we show how a conservative treatment of hadronic uncertainties is crucial not only to assess the significance of deviations from the Standard Model but also to obtain a conservative picture of the New Physics responsible for LUV. Adopting a very general parameterization of charming penguins, we find that: (i) current data hint at a sizable q^2 and helicity dependence of charm loop amplitudes; (ii) conservative NP solutions to B anomalies favour a left-handed or an axial lepton coupling rather than a vector one.

Dark sector tensor currents contribution to lepton's anomalous magnetic moment

In this work we consider a model including dark sector bosons interacting through tensor currents with Standard Model leptons. We show that for certain values of the interaction constant this model has the potential of providing an explanation for the discrepancy between theory and experiment, regarding the anomalous magnetic moment of the muon. The effect on the already established measurements for the electron are small and the lepton universality violation is naturally incorporated. Possible experimental searches together with systematic approach to characteristic properties of the final states are discussed.

Discriminating 1D new physics solutions in b → sℓℓ decays * *Partly supported by the National Natural Science Foundation of China (11735003, 11975041, 11961141004), and the fundamental Research Funds for the Central Universities. Rui-Xiang Shi is supported by the National Natural Science Foundation of China (12147145) and Project funded by China Postdoctoral Science Foundation (2021M700343)

The recent measurements of , , , , a set of CP-averaged angular observables for the decay and its isospin partner by the LHCb Collaboration consistently hint at lepton universality violation in the transitions. In this work, we first perform global fits to the data and show that five one-dimensional scenarios, i.e, , , , , and can best explain the so-called B anamolies. Furthermore, we explore how these scenarios can be distinguished from each other. For this purpose, we first study the combinations of four angular asymmetries and find that they cannot distinguish the five new physics scenarios. We then show that a newly constructed ratio can uniquely discriminate the five new physics scenarios in proper intervals of if it can be measured with percent-level precision.

Test of lepton universality in beauty-quark decays

The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.

Lepton universality violation from neutral pion decays in RK(*) measurements

I show that the neutral pion decay in $B \to K^{(*)} \pi^0 \gamma$, with $\pi^0 \to ee \gamma$, might generate large sources of lepton flavor universality violation (LFUV) in measurements of the ratios, $R_{K^{(*)}}$: If the photons in the $K^{(*)} e e \gamma \gamma$ final state are reconstructed as Bremsstrahlung, the recovered electron-positron invariant mass can be pushed into the $1$-$6$ GeV$^{2}$ signal region, artificially enhancing the measured $B \to K^{(*)} e e$ branching ratio compared to $B \to K^{(*)} \mu\mu$. I present a conservative estimate and simulation of the $B \to K \pi^0 \gamma$ LFUV background at LHCb, that together suggest this effect could reduce the recovered $R_K$ up to several percent. A reliable assessment of the size of this effect will require dedicated simulations within experimental frameworks themselves.

Universal lepton universality violation in exclusive processes

In high energy exclusive processes involving leptons, QED corrections can be sensitive to infrared scales like the lepton mass and the soft photon energy cut, resulting in large logarithms that need to be resummed to all order in $\ensuremath{\alpha}$. When considering the ratio of the exclusive processes between two different lepton flavors, the ratio $R$ can be expressed in terms of factorized functions in the decoupled leptonic sectors. While some of the functional terms cancel, there remain the large logarithms due to the lepton mass difference and the energy cut. This factorization process can be universally applied to the exclusive processes such as $Z\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$ and ${B}^{\ensuremath{-}}\ensuremath{\rightarrow}{l}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{l}$, where the resummed result in the ratio gives significant deviations from the naive expectation from lepton universality.

Muon g−2 and semileptonic B decays in the Bélanger-Delaunay-Westhoff model with gauge kinetic mixing

In the model proposed by B\'elanger, Delaunay and Westhoff (BDW), a new sector consisted of vectorlike fermions and two complex scalars is charged under an extra Abelian symmetry $U(1)_X$. In this paper, we generalize the BDW model by introducing the kinetic mixing between the $U(1)_X$ and the standard model $U(1)_Y$ gauge fields. The new physics contributions to the muon anomalous magnetic moment and the Wilson coefficients $C_{9,10}^{(')}$ are obtained analytically. We have explored the free parameter space of the model, taking into account various constraints on the muon $g-2$ using recent data from the E989 experiment at Fermilab, the lepton universality violation in terms of $R_K$ and $R_{K^*}$, and the branching ratios of the semileptonic decays, $B^+ \rightarrow K^+ \mu^+ \mu^-$ and $B^0 \rightarrow K^{*0} \mu^+ \mu^-$, the LEP and LHC searches for sleptons and $Z'$ boson, as well as the perturbative requirement. The viable parameter regions of the model are identified. In the presence of the gauge kinetic mixing term, those regions are enlarged and significantly deformed in comparison to the case with vanishing kinetic mixing. In the near future, the E989 experiment with the projected sensitivity will be able to test significant parts of the currently allowed parameter regions.

Gravitational wave signatures of lepton universality violation

We analyze the prospects for using gravitational waves produced in early universe phase transitions as a complementary probe of the flavor anomalies in B meson decays. We focus on the Left-Right SU(4) Model, for which the strength of the observed lepton universality violation and consistency with other experiments impose a vast hierarchy between the symmetry breaking scales. This leads to a multipeaked gravitational wave signature within the reach of upcoming gravitational wave detectors.

An Explanation for the Violation of Lepton Universality in Beauty-Quark Decays: The Binary Isotope Mixture of Beauty-Quarks

This paper purposes an explanation for the recent evidence for the violation of lepton universality in beauty-quark decays at CERN’s Large Hadron Collider. A beauty meson (B+) transforms into a strange meson (K+) with the emission of either electron-positron (e+e-) or muon-antimuon (μ+μ-). The ratio (RK) of branching fractions for B+ → K+μ+μ- and B+→ K+e+e- decays is measured to be RK = 0.846 instead of 1 in the violation of lepton universality in the Standard Model. This paper proposes that the violation is derived from the binary isotope mixture of two beauty-quarks, b7 (4979 MeV mass) and b8 (143,258 MeV mass) whose masses are calculated from the periodic table of elementary particles. b7 is the observable B, while b8 is the hidden B to preserve the generation number symmetry between the three lepton family generations and the three quark family generations in the Standard Model. The preservation of the generation number symmetry forbids b8 to decay into K+μ+μ-. In the transition state involving the virtual particles (γ, W± and Z°) before the decay, b7 and b8 emerge to form the binary isotope mixture from B. The rates of emergence as the rates of diffuse in Graham’s law of diffusion are proportional to inverse square root of mass. The rate ratio between b8/b7 is (4979/143,258)1/2 = 0.1864. Since b7 decays into K+, e+e-, and μ+μ-, while b8 decays into K+, e+e-, and forbidden μ+μ-, the calculated ratio (RK) of branching fractions for B+→ K+μ+μ- and B+→ K+e+e- is 0.5/(0.1864 × 0.5+ 0.5) = 0.843 in excellent agreement with the observed 0.846. The agreement between the calculated RK and the observed RK confirms the validity of the periodic table of elementary particles which provides the answers for the dominance of matter over antimatter, dark-matter, and the mass hierarchy of elementary particles.

B anomalies under the lens of electroweak precision

The measurements carried out at LEP and SLC projected us into the precision era of electroweak physics. This has also been relevant in the theoretical interpretation of LHCb and Belle measurements of rare B semileptonic decays, paving the road for new physics with the inference of lepton universality violation in {R}_{K^{left(ast right)}} ratios. The simplest explanation of these flavour anomalies — sizeable one-loop contributions respecting Minimal Flavour Violation — is currently disfavoured by electroweak precision data. In this work, we discuss how to completely relieve the present tension between electroweak constraints and one-loop minimal flavour violating solutions to {R}_{K^{left(ast right)}} . We determine the correlations in the Standard Model Effective Field Theory that highlight the existence of such a possibility. Then, we consider minimal extensions of the Standard Model where our effective-field-theory picture can be realized. We discuss how these solutions to b → sℓℓ anomalies, respecting electroweak precision and without any new source of flavour violation, may point to the existence of a Z′ boson at around the TeV scale, within the discovery potential of LHC, or to leptoquark scenarios.

Bs→Dsℓν form factors for the full q2 range from lattice QCD with nonperturbatively normalized currents

We present a lattice QCD determination of the $B_s \to D_s \ell\nu$ scalar and vector form factors over the full physical range of momentum transfer. The result is derived from correlation functions computed using the Highly Improved Staggered Quark (HISQ) formalism, on the second generation MILC gluon ensembles accounting for up, down, strange and charm contributions from the sea. We calculate correlation functions for three lattice spacing values and an array of unphysically light $b$-quark masses, and extrapolate to the physical value. Using the HISQ formalism for all quarks means that the lattice current coupling to the $W$ can be renormalized non-perturbatively, giving a result free from perturbative matching errors for the first time. Our results are in agreement with, and more accurate than, previous determinations of these form factors. From the form factors we also determine the ratio of branching fractions that is sensitive to violation of lepton universality: $R(D_s) = \mathcal{B}(B_s\to D_s \tau \nu_{\tau})/\mathcal{B}(B_s\to D_s \ell \nu_{l})$, where $\ell$ is an electron or a muon. We find $R(D_s) = 0.2987(46)$, which is also more accurate than previous lattice QCD results. Combined with a future measurement of $R(D_s)$, this could supply a new test of the Standard Model. We also compare the dependence on heavy quark mass of our form factors to expectations from Heavy Quark Effective Theory.

A self-calibrating, double-ratio method to test tau lepton universality in W boson decays at the LHC

Measurements in W^+W^- events at LEP2 and in B hadron semileptonic decays at B factories and LHCb provide intriguing hints of a violation of lepton universality in the charged current coupling of tau leptons relative to those for electrons and muons. We propose a novel, self-calibrating method to test tau lepton universality in W boson decays at the LHC. We compare directly the ratio of the numbers of selected ell tau _{mathrm {,had}} and emu final states in di-leptonic top quark pair events with that in Z/gamma ^*rightarrow tau tau events. Here ell =e {mathrm {or}} mu and tau _{mathrm {,had}} is a candidate semi-hadronic tau decay. This “double-ratio” cancels to first order sensitivity to systematic uncertainties on the reconstruction of e, mu , and tau leptons, thus improving very significantly the precision to which tau lepton universality can be tested in W boson decay branching ratios at the LHC. Using particle-level Monte Carlo events, and a parameterised simulation of detector performance, we demonstrate the effectiveness of the method and estimate the most significant residual sources of uncertainty arising from experimental and phenomenological systematics. Our studies indicate that a single LHC experiment precision on the tau lepton universality test of around 1.4% is achievable with a data set of int !! L, hbox {,d}t = 140 mathrm {fb}^{-1} at {sqrt{s}}~=~13~hbox {TeV}. This would improve significantly upon the precision of 2.5% on the four-experiment combined LEP2 measurements. If the central value of the proposed new measurement were equal to the central value of the LEP2 measurement this would yield an observation of BSM physics at a significance level of around 5sigma .

Study of tau neutrino production in proton-nucleus interactions

Tau neutrino properties are not so well known in comparison to those of muon or electron neutrinos. The tau neutrino interaction cross-section was directly measured by the DONUT experiment in 2008 on a basis of 9 registered tau neutrinos, and with a large systematical error of 50% due to a poor knowledge of the tau neutrino flux in this beam dump experiment. However, more accurate measurement of the cross section is needed for future neutrino experiments. It would also allow testing the Lepton Universality (LU) of Standard Model in neutrino interactions. Recently several results for B-meson decays (LHCb, Babar) demonstrated hints of possible LU violation. The tau neutrinos are produced in the Ds meson decays to tau, Ds → τ vT, and the cascade decay of the tau τ → vT X. DsTau experiment has been proposed to study the tau neutrino production at CERN SPS. It will measure the Ds production differential cross-section in the proton-tungsten interactions. This will allow reducing of the uncertainty due to the tau neutrino flux in the DONUT result from 50% to 10%. The peculiar Ds cascade decay topology ("double kink") in a few mm range will be detected by nuclear emulsion tracker thanks to its excellent spatial resolution (∼50nm).In 2016 and 2017, we made test beam exposures of nuclear emulsion modules at CERN SPS 400GeV/c proton beam followed by the pilot run in August 2018. A physics run in 2021 is scheduled. The status and prospects of the DsTau project as well as a review of the modern nuclear emulsion technique and the methods of its data analysis are presented.

Hadronic τ Decays as New Physics Probes in the LHC Era.

We analyze the sensitivity of hadronic τ decays to nonstandard interactions within the model-independent framework of the standard model effective field theory. Both exclusive and inclusive decays are studied, using the latest lattice data and QCD dispersion relations. We show that there are enough theoretically clean channels to disentangle all the effective couplings contributing to these decays, with the τ→ππν_{τ} channel representing an unexpected powerful new physics probe. We find that the ratios of nonstandard couplings to the Fermi constant are bound at the subpercent level. These bounds are complementary to the ones from electroweak precision observables and pp→τν_{τ} measurements at the LHC. The combination of τ decay and LHC data puts tighter constraints on lepton universality violation in the gauge boson-lepton vertex corrections.

Probing new physics in semileptonic Λb decays

In recent times, several hints of lepton non-universality have been observed in semileptonic $B$ meson decays, both in the charged-current ($b \to c l \bar \nu_l$) and neutral-current ($b \to s ll $) transitions. Motivated by these intriguing results, we perform a model independent analysis of the semileptonic $\Lambda_b$ decays involving the quark level transitions $b \to (u,c) l \nu_l$, in order to scrutinize the nature of new physics. We constrain the new parameter space by using the measured branching ratios of $B_{c, u}^+ \to \tau^+ \nu_\tau$, $B \to \pi \tau \nu_\tau$ processes and the existing experimental results on $R_{D^{(*)}}$, $R_{J/\psi}$ and $R_{\pi}^l$ parameters. Using the constrained parameters, we estimate the branching ratios, forward backward asymmetries, hadron and lepton polarization asymmetries of the $\Lambda_b \to (\Lambda_c, p) l \nu_l$ processes. Moreover, we also examine whether there could be any lepton universality violation in these decay modes.

Measurements of lepton universality at LHCb

Lepton universality violation would represent a signal of physics beyond the Standard Model. Anomalies have been observed in the measurement of the branching ratios of semileptonic decays of beauty mesons in third-generation leptons. This contribution reports on the measurements of the observable $R(D^{\ast -}) = B(B^0 \rightarrow D^{\ast -} \tau^+ \nu _\tau)/B(B^0 \rightarrow D^\ast - \mu^+ \nu_\mu)$ and $R(J/\psi)$ = $B(B^+_c \rightarrow J/\psi \tau^+ \nu_\tau)/B(B^+_c \rightarrow J/\psi \mu^+ \nu_\mu)$ performed at LHCb with data collected during the Run 1 of the LHC.