Test Of Lepton Universality Research Articles

Lepton Flavour Universality tests in b → clν decays at LHCb

The Standard Model predicts that the electroweak couplings to the three charged leptons are identical. However, in the last decade, experimental measurements have suggested that semileptonic processes involving taus could have a slightly enhanced decay rate compared to their muonic counterparts. If confirmed, this would be an unambiguous sign of New Physics, with various scenarios introducing additional interactions that couple preferentially to the third generation. Two recent lepton universality tests performed at LHCb are presented in these proceedings of the first edition of the New Frontiers in Lepton Flavour workshop.

Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic B-Meson Decays at Belle II.

We present the first measurement of the ratio of branching fractions of inclusive semileptonic B-meson decays, R(X_{e/μ})=B(B→Xeν)/B(B→Xμν), a precision test of electron-muon universality, using data corresponding to 189 fb^{-1} from electron-positron collisions collected with the Belle II detector. In events where the partner B meson is fully reconstructed, we use fits to the lepton momentum spectra above 1.3 GeV/c to obtain R(X_{e/μ})=1.007±0.009(stat)±0.019(syst), which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.

Lepton Universality Test with MUSE at PSI

Lepton universality (LU) typically refers to the lepton coupling, which is considered to be the same for e, μ, and τ leptons, if the interaction is electroweak according to the Standard Model, and it is hence a compelling probe for New Physics. The same principle of universal electroweak lepton interaction leads to the expectation that lepton scattering yields are equal for e and μ beams under the same kinematic condition. The mere mass difference between e and μ affects kinematic quantities (such as the relation between scattering angle and Q 2), and the lepton mass dependence of elastic cross sections for leptons scattered from structured and pointlike objects are taken into account. By comparing e +, e −, μ +, and μ − scattering yields, two-photon exchange (TPE) effects, universal or not, can be separated from the general LU test of the e/μ yield ratio. With its separable mixed beams of e +/μ + and e −/μ −, respectively, the MUSE experiment at PSI is not only designed to measure the proton charge radius with four lepton species, but is also uniquely suited to probe TPE and LU, while benefitting from partial cancellations of certain shared systematics. An overview will be given of the MUSE experiment, the sensitivity, and the present status.

Heavy + heavy and heavy + light pseudoscalar to vector semileptonic transitions

Using a symmetry-preserving regularisation of a vector times vector contact interaction (SCI), we complete a systematic treatment of twelve semileptonic transitions with vector meson final states: Drightarrow rho , D_{(s)}rightarrow K^*, D_srightarrow phi , Brightarrow rho , B_srightarrow K^*, B_{(s)}rightarrow D_{(s)}^*, B_c rightarrow B_{(s)}^*, J/psi , D^*; and thereby finalise a unified analysis of semileptonic decays of heavy + heavy and heavy + light pseudoscalar mesons to both pseudoscalar and vector meson final states. The analysis is marked by algebraic simplicity, few parameters, and the ability to consistently describe systems from Nambu-Goldstone modes to heavy + heavy mesons. Regarding the behaviour of the transition form factors, the SCI results compare well wherever sound experimental or independent theory analyses are available; hence, the SCI branching fraction predictions should be a reasonable guide. Considering the ratios R(D_{(s)}^{(*)}), R(J/psi ), R(eta _c), whose values are key tests of lepton universality in weak interactions, the SCI values agree with Standard Model predictions. The B_{(s)}rightarrow D_{(s)}^* transitions are used to predict the precursor functions that evolve into the universal Isgur–Wise function in the heavy-quark limit, with results that conform with those from other sources where such are available. The study also exposes effects on the transition form factors that flow from interference between emergent hadron mass from the strong interaction and Higgs boson couplings via current-quark masses, including flavour symmetry violation.

Compatibility of muon g − 2, W mass anomaly in type-X 2HDM

Recently CDF II Collaboration reported that they measured W boson mass precisely. The measurement is deviated from the Standard Model (SM) Prediction at 7σ. Also, the recent FNAL measurement of the muon magnetic moment shows a 4.2σ deviation. To resolve the W boson as well as the muon (g−2) anomalies, we explore the type-X two Higgs doublet model (2HDM). We analyze the (mA,tan⁡β) parameter space of the type-X 2HDM consistent with the muon (g−2), lepton universality test and recent W boson mass measurements from the CDF II collaboration. We find that the measurement of the h→AA branching ratio gives a strong lower mass bound on mH.

Tests of Lepton Universality Using B^{0}→K_{S}^{0}ℓ^{+}ℓ^{-} and B^{+}→K^{*+}ℓ^{+}ℓ^{-} Decays.

Tests of lepton universality in B^{0}→K_{S}^{0}ℓ^{+}ℓ^{-} and B^{+}→K^{*+}ℓ^{+}ℓ^{-} decays where ℓ is either an electron or a muon are presented. The differential branching fractions of B^{0}→K_{S}^{0}e^{+}e^{-} and B^{+}→K^{*+}e^{+}e^{-} decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb^{-1} . The results are consistent with the standard model and previous tests of lepton universality in related decay modes. The first observation of B^{0}→K_{S}^{0}e^{+}e^{-} and B^{+}→K^{*+}e^{+}e^{-} decays is reported.

Lepton universality and lepton flavor conservation tests with dineutrino modes

SU(2)_L-invariance links charged dilepton and dineutrino couplings. Phenomenological implications are worked out for flavor changing neutral currents involving strange, charm, beauty and top quark transitions in a model-independent way. We put forward novel tests of lepton universality and lepton flavor conservation in |varDelta c|= |varDelta u|=1 and |varDelta b|= |varDelta q|=1, q=d,s, transitions suitable for the experiments Belle II and BES III. Single top production plus dileptons uniquely probes semileptonic four-fermion |varDelta t|= |varDelta q'|=1, q'=u,c, couplings and further study at the LHC is encouraged. Tests with single top production associated with dileptons and missing energy are suitable for study at future e^+ e^- or muon colliders.

Interplay of dineutrino modes with semileptonic rare B-decays

We present a systematic global analysis of dineutrino modes b → qν overline{nu} , q = d, s, and charged dilepton b → qℓ+ℓ− transitions. We derive improved or even entirely new limits on dineutrino branching ratios including decays B0→ (K0, Xs)ν overline{nu} , Bs→ ϕν overline{nu} and B0→ (π0, ρ0)ν overline{nu} from dineutrino modes which presently are best constrained: B+→ (K+, π+, ρ+)ν overline{nu} and B0→ K*0ν overline{nu} . Using SMEFT we obtain new flavor constraints from the dineutrino modes, which are stronger than the corresponding ones from charged dilepton rare b-decay or Drell-Yan data, for eτ and ττ final states, as well as for μτ ones in b → s processes. The method also allows to put novel constraints on semileptonic four-fermion operators with top quarks. Implications for ditau modes b → sτ+τ− and b → dτ+τ− are worked out. Even stronger constraints are obtained in simplified BSM frameworks such as leptoquarks and Z′-models. Furthermore, the interplay between dineutrinos and charged dileptons allows for concrete, novel tests of lepton universality in rare B-decays. Performing a global fit to b → sμ+μ−, sγ transitions we find that lepton universality predicts the ratio of the B0→ K*0ν overline{nu} to B0→ K0ν overline{nu} (B+→ K+ν overline{nu} ) branching fractions to be within 1.7 to 2.6 (1.6 to 2.4) at 1σ, a region that includes the standard model, and that can be narrowed with improved charged dilepton data. There is sizable room outside this region where universality is broken and that can be probed with the Belle II experiment. Using results of a fit to B0→ μ+μ−, {B}_s^0 → overline{K} *0μ+μ− and B+→ π+μ+μ− data we obtain an analogous relation for |∆b| = |∆d| = 1 transitions: if lepton universality holds the ratio of the B0→ ρ0ν overline{nu} to B0→ π0ν overline{nu} (B+→ π+ν overline{nu} ) branching fractions is within 2.5 to 5.7 (1.2 to 2.6) at 1 σ. Putting upper limits on mathcal{B} (Bs→ ν overline{nu} ) at the level of 10−5 and mathcal{B} (B0→ ν overline{nu} ) below 10−6 would allow to control backgrounds from (pseudo-)scalar operators such as those induced by light right-handed neutrinos.

Radiative corrections to τ→π(K)ντ[γ] : A reliable new physics test

The ratios $R_{\tau/P}\equiv \Gamma(\tau \to P \nu_\tau [\gamma]) / \Gamma(P \to\mu \nu_\mu[\gamma])$ ($P=\pi, K$) provide sensitive tests of lepton universality $\left|g_\tau/g_\mu\right|=1$ and are a useful tool for new physics searches. The radiative corrections to $R_{\tau/P}$ are computed following a large-$N_C$ expansion to deal with hadronic effects: Chiral Perturbation Theory is enlarged by including the lightest multiplets of spin-one heavy states such that the relevant Green functions are well-behaved at high energies. We find $\delta R_{\tau/\pi}=(0.18\pm 0.57 )\%$ and $\delta R_{\tau/K}=(0.97\pm 0.58 )\%$, which imply $\left|g_\tau/g_\mu\right|_\pi=0.9964\pm 0.0038$ and $\left|g_\tau/g_\mu\right|_K=0.9857\pm 0.0078$, compatible with and at $1.8\sigma$ of lepton universality, respectively. We test unitarity and bind non-standard effective interactions with the $\tau \to P \nu_\tau [\gamma]$ decays.

Form-factor-independent test of lepton universality in semileptonic heavy meson and baryon decays

In the semileptonic decays of heavy mesons and baryons, the lepton-mass dependence factors out in the quadratic ${\mathrm{cos}}^{2}\ensuremath{\theta}$ coefficient of the differential $\mathrm{cos}\ensuremath{\theta}$ distribution. We call the corresponding normalized coefficient the convexity parameter. This observation opens the path to a test of lepton universality in semileptonic heavy meson and baryon decays that is independent of form-factor effects. By projecting out the quadratic rate coefficient, dividing out the lepton-mass-dependent factor, and restricting the phase space integration to the $\ensuremath{\tau}$ lepton phase space, one can define optimized partial rates which, in the Standard Model, are the same for all three $(e,\ensuremath{\mu},\ensuremath{\tau})$ modes in a given semileptonic decay process. We discuss how the identity is spoiled by new physics effects. We discuss semileptonic heavy meson decays such as ${\overline{B}}^{0}\ensuremath{\rightarrow}{D}^{(*)+}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ and ${B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}J/\ensuremath{\psi}({\ensuremath{\eta}}_{c}){\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ and semileptonic heavy baryon decays such as ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Lambda}}}_{c}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}$ for each $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, $\ensuremath{\tau}$.

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

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

Test of lepton universality with {Lambda}_b^0to {pK}^{-}{mathrm{ell}}^{+}{mathrm{ell}}^{-} decays

The ratio of branching fractions of the decays {Lambda}_b^0 → pK−e+e− and {Lambda}_b^0 → pK−μ+μ−, {R}_{pK}^{-1} , is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb−1 recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range 0.1 < q2< 6.0 GeV2/c4 and the pK− mass range m(pK−) < 2600 MeV/c2, the ratio of branching fractions is measured to be {R}_{pK}^{-1}={1.17}_{-0.16}^{+0.18}pm 0.07 , where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay {Lambda}_b^0 → pK−e+e−.

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 .

Effective-field theory analysis of the τ−→(Kπ)−ντ decays

We analyze the $\tau^-\to(K\pi)^-\nu_\tau$ decays within an effective field theory description of heavy new physics (NP) modifying the SM left-handed weak charged current and include refined SM input for the participant meson form factors exploiting chiral symmetry, dispersion relations and (lattice) data. We include the leading dimension six operators and work at linear order in the effective couplings. Within this setting we: i) follow the derivation in Phys. Rev. Lett. 120 (2018) no.14, 141803 where it was proved unambiguously that it is impossible to understand the BaBar anomaly in the CP asymmetry measurement within this framework. We allow for reasonable variations of the hadronic input involved and study the associated uncertainty; ii) first show that the anomalous bump present in the published Belle data for the $K_S\pi^-$ invariant mass distribution close to threshold cannot be due to heavy NP; iii) first bind the heavy NP effective couplings using $\tau^-\to(K\pi)^-\nu_\tau$ decays and show that they are competitive with those found in hyperon semileptonic decays (but clearly not with those obtained in Kaon (semi)leptonic decays for NP scalar currents). We put forward that the comparison of the considered tau decays with (semi)leptonic kaon and hyperon decays provide with meaningful tests of lepton universality for (NP) tensor interactions. We also compare the SM predictions with the possible deviations caused by NP in a couple of Dalitz plot distributions, in the forward-backward asymmetry and in the di-meson invariant mass distribution and discuss the most interesting measurements to be performed at Belle-II using these decays data.

Tau-lepton Physics at the FCC-ee circular e$^+$e$^-$ Collider

The future FCC-ee collider is designed to deliver \boldsymbol{\mathrm{e^+e^-}}e+e− collisions to study with ultimate precision the Z, W, and Higgs bosons, and the top quark. In a high-statistics scan around the Z pole, \boldsymbol{1.3\times 10^{11}}1.3×1011 events \boldsymbol{\mathrm{Z}\to\tau\tau}Z→𝛕𝛕 will be produced, the largest sample of \boldsymbol{\tau\tau}𝛕𝛕 events foreseen at any lepton collider. With their large boost, \boldsymbol{\tau}𝛕 leptons from Z decays are particularly well suited for precision measurements. The focus of this report is on tests of lepton universality from precision measurement of \boldsymbol{\tau}𝛕 properties and on tests of charged lepton flavour violation in Z decays and in \boldsymbol{\tau}𝛕 decays. In both of these areas, FCC-ee promises sensitivities well beyond present experimental limits.

Searching for flavored gauge bosons

Standard Model may allow an extended gauge sector with anomaly-free flavored gauge symmetries, such as Li −Lj , Bi −Lj , and B −3Li, where i, j = 1, 2, 3 are flavor indices. We investigate phenomenological implications of the new flavored gauge boson Z′ in the above three classes of gauge symmetries. Focusing on the gauge boson mass above 5 GeV, we use the lepton universality test in the Z and τ /μ decays, LEP searches, LHC searches, neutrino trident production bound, and LHC Z → 4μ searches to put constraints on the g′ −MZ′ plane. When L1 is involved, the LEP bounds on the e−e+ → ℓ−ℓ+ processes give the most stringent bounds, while the LHC bound becomes the strongest constraints in the large {M}_{Z^{prime }} region when Bi is involved. The bound from Z → 4μ productions, which is applicable for L2-involved scenarios, provides stringent bounds in the small {M}_{Z^{prime }} region. One exception is the B − 3L2 scenario, in which case only a small region is favored due to the lepton universality.

Status and progress of the HFLAV-Tau group activities

We report the status and progress of the Heavy Flavour Averaging Group work on the global fit of τ tau lepton branching fractions, on the lepton universality tests and on the measurement of the Cabibbo-Kobayashi-Maskawa matrix element |Vus| using τ lepton measurements. We also review the prospects for improving the precision of the |Vus| measurement.

Tests of the standard model in B→Dℓνℓ , B→D*ℓνℓ and Bc→J/ψℓνℓ

A number of recent experimental measurements suggest the possibility of a breakdown of lepton ($\ell$) universality in exclusive $b \to c \ell \nu_\ell$ semileptonic meson decays. We analyze the full differential decay rates for several such processes, and show how to extract combinations of the underlying helicity amplitudes that are completely independent of $m_\ell$. Ratios of these combinations for different $\ell$ (as well as some combinations for a single value of $\ell$) therefore equal unity in the Standard Model and provide stringent tests of lepton universality. Furthermore, the extractions assume the form of weighted integrals over the differential decay rates and therefore are useful even in situations where data in some regions of allowed phase space may be sparse.

Extraction of |V_{cd}| and |V_{cs}| from experimental decay rates using lattice QCD D rightarrow pi (K) ell nu form factors

We present a determination of the Cabibbo–Kobayashi–Maskawa matrix elements |V_{cd}| and |V_{cs}| obtained by combining the momentum dependence of the semileptonic vector form factors f_+^{D rightarrow pi }(q^2) and f_+^{D rightarrow K}(q^2), recently determined from lattice QCD simulations, with the differential rates measured for the semileptonic D rightarrow pi ell nu and D rightarrow K ell nu decays. Our analysis is based on the results for the semileptonic form factors produced by the European Twisted Mass Collaboration with N_f = 2 + 1 + 1 flavors of dynamical quarks in the whole range of values of the squared 4-momentum transfer accessible in the experiments. The statistical and systematic correlations between the lattice data as well as those present in the experimental data are properly taken into account. With respect to the standard procedure based on the use of only the vector form factor at zero 4-momentum transfer, we obtain more precise and consistent results: |V_{cd} |= 0.2341 ~ (74) and |V_{cs} |= 0.970 ~ (33). The second-row CKM unitarity is fulfilled within the current uncertainties: |V_{cd}|^2 + |V_{cs}|^2 + |V_{cb}|^2 = 0.996 ~ (64). Moreover, using for the first time hadronic inputs determined from first principles, we have calculated the ratio of the semileptonic D rightarrow pi (K) decay rates into muons and electrons, which represent a test of lepton universality within the SM, obtaining in the isospin-symmetric limit of QCD: mathcal{{R}}_{LU}^{Dpi } = 0.985~(2) and mathcal{{R}}_{LU}^{DK} = 0.975~(1).

Soft-photon corrections to the Bethe-Heitler process in the γp→l+l−p reaction

We report on the calculation of first-order QED corrections for the $\gamma p\rightarrow l^+l^-p^{\prime}$ process. An upcoming experiment at MAMI (Mainz) aims to compare the cross sections of muon- and electron-pair production in this reaction to test lepton universality. Precise knowledge of the electromagnetic radiative corrections is needed for these measurements. As a first step, we present the leading QED radiative corrections in the soft-photon approximation when accounting for the finite lepton mass. For the kinematics at MAMI, we find corrections of the percent level for muons, and of order $10\%$ for electrons.