Test Of Lepton Flavor Universality Research Articles

Recent highlights from the BELLE and BELLE II experiments

The BELLE experiment at the KEKB collider in Tsukuba operated as a B-factory until 2018, making significant contributions to flavor physics. Building on this legacy, the BELLE II experiment, an upgraded version, now operates at the SuperKEKB energy-asymmetric e+e− collider. BELLE II has collected 424fb−1 of data, including 365fb−1 at the ϒ(4S) resonance. A wide range of physics topics can be studies with this data. In this proceeding, we review our latest results, which include tests of lepton flavor universality, Cabibbo-Kobayashi-Maskawa physics, searches for rare B-meson decays, and spectroscopy studies. We also discuss contributions to the gμ−2 prediction.

First lattice QCD calculation of J/ψ semileptonic decay containing D and Ds particles

We perform the first lattice calculation on the semileptonic decay of J/ψ using the (2+1)-flavor Wilson-clover gauge ensembles generated by CLQCD collaboration. Three gauge ensembles with different lattice spacings, from 0.0519 to 0.1053 fm, and pion masses, mπ∼300 MeV, are utilized. After a naive continuum extrapolation using three lattice spacings, we obtain Br(J/ψ→Dseνe)=1.90(6)(5)Vcs×10−10 and Br(J/ψ→Deνe)=1.21(6)(9)Vcd×10−11, where the first errors are statistical, and the second come from the uncertainties of Cabibbo-Kobayashi-Maskawa matrix element Vcs(d). The ratios of the branching fractions between lepton μ and e are also calculated as RJ/ψ(Ds)=0.97002(8) and RJ/ψ(D)=0.97423(15) after performing a continuum limit including only a2 term. The ratios provide necessary theoretical support for the future experimental test of lepton flavor universality. Published by the American Physical Society 2024

Global fit to b→cτν anomalies as of Spring 2024

Recently, several new experimental results of the test of lepton flavor universality (LFU) in B→D(*) semileptonic decays were announced: the first result of RD from the LHCb run 1 data, the first results of RD and RD* from the LHCb run 2 data, and the first result of RD* from the Belle II collaboration. Including these new data, a global analysis still prefers the violation of the LFU between the tau and light leptons. A new world average of the data from the , LHCb, Belle, and Belle II collaborations is RD=0.342±0.026 and RD*=0.287±0.012. Including this new data, we update a circumstance of the b→cτν¯ measurements and their implications for new physics (NP). Incorporating recent developments for the B→D(*) form factors in the Standard Model, we observe a 4.4σ deviation from the Standard Model prediction. Our updates also include model-independent NP formulas for the related observables and the global fittings of parameters for leptoquark scenarios as well as single NP operator scenarios. Furthermore, we show future potential to indirectly distinguish different NP scenarios with the use of the precise measurements of the polarization observables in B→D(*)τν¯ at the Belle II and the high-pT flavored-tail searches at the LHC. We also discuss an impact on the LFU violation in ϒ→l+l−. Published by the American Physical Society 2024

Measurement of Angular Coefficients of B[over ¯]→D^{*}ℓν[over ¯]_{ℓ}: Implications for |V_{cb}| and Tests of Lepton Flavor Universality.

We measure the complete set of angular coefficients J_{i} for exclusive B[over ¯]→D^{*}ℓν[over ¯]_{ℓ} decays (ℓ=e, μ). Our analysis uses the full 711 fb^{-1} Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the B[over ¯]→D^{*} transition and the Cabibbo-Kobayashi-Maskawa matrix element |V_{cb}|. Using recent lattice QCD calculations for the hadronic form factors, we find |V_{cb}|=(40.7±0.7)×10^{-3} using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter w and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations.

Test of lepton flavor universality in B±→K±μ+μ− and B±→K±e+e− decays in proton-proton collisions at s=13TeV

A test of lepton flavor universality in B±→K±μ+μ− and B±→K±e+e− decays, as well as a measurement of differential and integrated branching fractions of a nonresonant B±→K±μ+μ− decay are presented. The analysis is made possible by a dedicated data set of proton-proton collisions at s=13TeV recorded in 2018, by the CMS experiment at the LHC, using a special high-rate data stream designed for collecting about 10 billion unbiased b hadron decays. The ratio of the branching fractions B(B±→K±μ+μ−) to B(B±→K±e+e−) is determined from the measured double ratio R(K) of these decays to the respective branching fractions of the B±→J/ψK± with J/ψ→μ+μ− and e+e− decays, which allow for significant cancellation of systematic uncertainties. The ratio R(K) is measured in the range 1.1<q2<6.0GeV2 , where q is the invariant mass of the lepton pair, and is found to be R(K)=0.78−0.23+0.47 , in agreement with the standard model expectation R(K)≈1 . This measurement is limited by the statistical precision of the electron channel. The integrated branching fraction in the same q 2 range, B(B±→K±μ+μ−)=(12.42±0.68)×10−8 , is consistent with the present world-average value and has a comparable precision.

Indirect constraints on top quark operators from a global SMEFT analysis

We perform a model-independent analysis of top-philic New Physics scenarios, under the assumption that only effective operators involving top quarks are generated at tree level. Within the SMEFT framework, we derive indirect constraints on Wilson Coefficients by combining a large set of low-energy observables: B-meson and kaon decays, meson mixing observables, precision electroweak and Higgs measurements, anomalous magnetic moments, lepton flavour violating processes, lepton flavour universality tests, and measurements of the Cabibbo angle. We consider the renormalization group evolution of the operators and use the one-loop matching of the SMEFT onto the LEFT. The global analysis is then used to perform one-parameter, two-parameter, and global fits, as well as applications to explicit ultraviolet models. We find that the inclusion of measurements from different physics sectors reveals a strong interplay and complementarity among the observables. The resulting constraints are also compared to direct bounds provided by top quark productions at the LHC.

Third-family quark-lepton Unification and electroweak precision tests

We analyze the compatibility of the hypothesis of third-family quark-lepton unification at the TeV scale with electroweak precision data, lepton flavor universality tests, and high-pT constraints. We work within the framework of the UV complete flavor non-universal 4321 gauge model, which is matched at one loop to the Standard Model Effective Field Theory. For consistency, all electroweak precision observables are also computed at one loop within the effective field theory. At tree level, the most sizeable corrections are to W → τντ and Z → ντντ due to integrating out a pseudo-Dirac singlet fermion required by the model for neutrino mass generation. At loop level, the new colored states of the model generate large flavor-universal contributions to the electroweak precision observables via leading- and next-to-leading log running effects, yielding a significant improvement in the electroweak fit (including an increase in the W-boson mass). These effects cannot be decoupled if the model addresses the charged-current B-meson anomalies. Overall, we find good compatibility between the data sets, while simultaneously satisfying all low- and high-energy constraints.

HIKE: High Intensity Kaon Experiments at the CERN SPS

The availability of high intensity kaon beams at the CERN SPS North Area gives rise to unique possibilities for sensitive tests of the Standard Model in the quark flavor sector. Precise measurements of the branching ratios for the flavor-changing neutral current decays can provide unique constraints on CKM unitarity and, potentially, evidence for new physics. Building on the success of the NA62 experiment, plans are taking shape at CERN for a comprehensive program that will include experimental phases to measure the branching ratio for to ∼5% and to to ∼20% precision. These planned experiments would also carry out lepton flavor universality tests, lepton number and flavor conservation tests, and perform other precision measurements in the kaon sector, as well as searches for exotic particles in kaon decays. We overview the physics goals, detector requirements, and project status for the next generation of kaon physics experiments at CERN.

Lepton Flavor Universality at Belle and Belle II

We report the test of lepton flavor universality at Belle and Belle II experiments considering b → c and b → s transitions. We use 711 fb−1 and 189 fb−1 of data collected at 𝒴(4S ) resonance from electron-positron collisions for Belle and Belle II, respectively. For the b → c transition, we present the measurement of lepton-flavor-universality in B0 → D*−ℓ+νℓ decay, exclusive measurement of ℛ(D) &amp; ℛ(D*) in Belle data, and inclusive ℛ(Xe/µ) in Belle II. For the b → s transitions, we provide measurements of RK and RK* using Belle data, measurement of the branching fraction for the decay B → K*ℓℓ, and test of lepton-flavor-universality in B → J/ψK using Belle II data. Additionally, we report the lepton-flavor-universality in $ \Omega_{c}^{0} $ → Ω−ℓ+νℓ decay using 922 fb−1 of data collected by the Belle detector.

(Semi-) leptonic D(s) decays at BESIII

BESIII has collected 2.93fb−1 and 6.32fb−1 of e+e− collision data samples at the center-of-mass energies 3.773 and 4.178–4.226 GeV, respectively. We report recent measurements of the (semi-) leptonic decays Ds→ℓ+νℓ (ℓ=μ,τ) and D(s)→Xℓ+νℓ [X=η,K1(1270) or denotes generic decays, and ℓ=e,μ]. The decay constant fDs+, the semileptonic form factor f+η(0) and the CKM matrix elements |Vcd(s)| are determined precisely. These results are important to verify the theoretical calculations of fDs+ and f+η(0), and the unitarity of the CKM matrix. Precision tests of lepton-flavor universality with (semi-) leptonic D(s) decays are also performed.

Measurement of the Drell-Yan forward-backward asymmetry at high dilepton masses in proton-proton collisions at sqrt{s} = 13 TeV

A measurement of the forward-backward asymmetry of pairs of oppositely charged leptons (dimuons and dielectrons) produced by the Drell-Yan process in proton-proton collisions is presented. The data sample corresponds to an integrated luminosity of 138 fb−1 collected with the CMS detector at the LHC at a center-of-mass energy of 13 TeV. The asymmetry is measured as a function of lepton pair mass for masses larger than 170 GeV and compared with standard model predictions. An inclusive measurement across both channels and the full mass range yields an asymmetry of 0.612 ± 0.005 (stat) ± 0.007 (syst). As a test of lepton flavor universality, the difference between the dimuon and dielectron asymmetries is measured as well. No statistically significant deviations from standard model predictions are observed. The measurements are used to set limits on the presence of additional gauge bosons. For a Z′ boson in the sequential standard model the observed (expected) 95% confidence level lower limit on the Z′ mass is 4.4 TeV (3.7 TeV).

Consequences of b→sμ+μ− anomalies on B→K(*)νν¯ , Bs→(η,η′)νν¯ and Bs→ϕνν¯ decay observables

The long persistent discrepancies in $b\,\to\,s\, \ell\, \ell$ quark level transitions continue to be the ideal platform for an indirect search of new physics that lies beyond the SM. The measurements of $R_K$, $R_{K^*}$, $P_5^{\prime}$, $\mathcal{B}(B_s\, \to\, \phi\, \mu^+\,\mu^-)$ and $\mathcal{B}(B_s\, \to\, \mu^+\,\mu^-)$ persistently deviate from the standard model expectations. Similarly, the new tests of lepton flavor universality performed using the isospin partners such as $B^0 \, \to \, K_S^{0} \, \ell\,\ell$ and $B^+ \, \to \, K^{+*} \, \ell\,\ell$ exhibit the same pattern of deviation in $R_{K_S^0}$ and $R_{K^{*+}}$ with the existing results. Motivated by these anomalies we search for the patterns of new physics in the family of flavor changing neutral decays with neutral leptons in the final state undergoing $b \, \to \,s\, \nu \, \bar{\nu}$ quark level transitions. There are close relations between $b\,\to\,s\, \ell\, \ell$ and $b \, \to \,s\, \nu \, \bar{\nu}$ transitions not only in standard model but also in various beyond the standard model scenarios. For beyond the standard model physics under the $\rm SU_L(2)$ gauge symmetry one can relate the left handed charged leptons to the neutrinos. Moreover, there are several advantages of studying $b \, \to \,s\, \nu \, \bar{\nu}$ transitions over $b\,\to\,s\, \ell\, \ell$ as they are free from various hadronic uncertainties beyond the form factors such as the non-factorizable corrections and photonic penguin contributions. Hence, we explore the consequences of $b\, \to\, s\, \mu^+\, \mu^-$ anomalies on $B \, \to \, K^{(*)} \,\nu \, \bar{\nu}$, $B_s \to\, (\eta,\eta') \, \nu\,\bar{\nu}$ and $B_s \, \to \, \phi \, \nu \, \bar{\nu}$ decay observables in SMEFT platform within various 1D and 2D new physics scenarios.

Feasibility study of D_s^+ rightarrow tau ^+ nu _{tau } decay and test of lepton flavor universality with leptonic D_s^+ decays at STCF

We report a sensitive study of D_s^+ rightarrow tau ^+ nu _{tau } decay via tau ^+ rightarrow e^+ nu _e bar{nu }_{tau } with an integrated luminosity of 1 ab^{-1} at the center-of-mass energy of 4.009 GeV at a future Super Tau Charm Facility (STCF). Under the help of the fast simulation software package, the statistical sensitivity for the absolute branching fraction of D_s^+ rightarrow tau ^+ nu _{tau } is determined to be 2times 10^{-4}. Combining with our previous prospect of D_s^+ rightarrow mu ^+ nu _{mu }, the ratio of the branching fractions for D_s^+ rightarrow tau ^+ nu _{tau } over D_s^+ rightarrow mu ^+ nu _{mu } can achieve a relative statistical precision of 0.5%, which will provide the most stringent test of the tau -mu lepton flavor universality in heavy quark decays. Taking the decay constant f_{D_s^+} from lattice QCD calculations or the CKM matrix element |V_{cs}| from the CKMfitter group as an input, the relative statistical uncertainties for |V_{cs}| and f_{D_s^+} are estimated to be 0.3% and 0.2%, respectively.

Extraction of Bs→Ds(*) form factors from Nf=2 lattice QCD

We report on a two-flavor lattice QCD study of the ${B}_{s}\ensuremath{\rightarrow}{D}_{s}$ and ${B}_{s}\ensuremath{\rightarrow}{D}_{s}^{*}$ transitions parametrized, in the heavy quark limit, by the form factors $\mathcal{G}$, and ${h}_{{A}_{1}}$, ${h}_{{A}_{2}}$, and ${h}_{{A}_{3}}$, respectively. In the search for New Physics through tests of lepton flavor universality, ${B}_{s}$ decay channels are complementary to the $B$ decays widely studied at $B$ factories and LHCb, while on the theory side they can be better controlled than the ${B}_{c}$ and ${\mathrm{\ensuremath{\Lambda}}}_{b}$ decays. The purpose of this exploratory two-flavor study is, in preparation for future analyses of lattice QCD simulations with ${N}_{f}&gt;2$ and physical quark masses, to gain experience on a suitable method for a lattice extraction of form factors associated with $b\ensuremath{\rightarrow}c$ currents that may yield tighter control over systematic effects like contamination from excited states and cutoff effects. We obtain the zero-recoil values ${\mathcal{G}}^{{B}_{s}\ensuremath{\rightarrow}{D}_{s}}(1)=1.03(14)$ and ${h}^{{B}_{s}\ensuremath{\rightarrow}{D}_{s}^{*}}(1)=0.85(16)$.

Heavy baryon wave functions, Bakamjian-Thomas approach to form factors, and observables in Λb→Λc(12±)ℓν¯ transitions

Motivated by the calculation of observables in the decays $\Lambda_b \to \Lambda_c\left({1 \over 2}^\pm \right) \ell \overline{\nu}$, as tests of Lepton Flavor Universality, we present a calculation of form factors in the quark model. Our scheme combines a spectroscopic model, providing the internal wave functions, and the Bakamjian-Thomas relativistic formalism to deduce the wave functions in motion and current matrix elements, that amount in the heavy quark limit to the Isgur-Wise (IW) function. For baryons we meet difficulties using standard spectroscopic models, leading us to propose a simple phenomenological model : a Q-pointlike-diquark model, non-relativistic with harmonic oscillator forces, with a reasonable low-lying spectrum and good slope of the IW function. We extract this slope from Lattice data and find $\rho_\Lambda^2 \sim 2$. We are not able to reproduce the right $\rho_\Lambda^2$ when using standard linear + Coulomb potential models, both with three quarks $Qqq$ or in a Q-pointlike-diquark picture. These difficulties seem to derive from the high sensitivity of $\rho_\Lambda^2$ to the structure of the light quark subsystem. After fixing the parameters of our interim model to yield correct spectrum and $\rho_\Lambda^2$, we compute observables. Bjorken sum rule shows that the inelastic IW function is large, and therefore $\Lambda_b \to \Lambda_c \left({1 \over 2}^-, {3 \over 2}^- \right) \ell \overline{\nu}$ could be studied at LHCb. Some observables in the $\tau$ case present zeroes for specific values of $q^2$ that could be tests of the Standard Model.

Charm Physics at BESIII

Recently, BESIII has performed many analyses with the largest e+ e- annihilation data set taken on the mass-thresholds. Those works focus on the measurements of the CKM matrix elements |Vcd(s)|, the transition form factors , tests of lepton flavor universality, precise measurements of branching fraction, studies of interesting intermediate states in the multiple-body decays, and searchs for the new physics. Among those, we present the results with three categorities, the (semi-) leptonic decays, hadronic decays, and the rare decays.

β Decays as Sensitive Probes of Lepton Flavor Universality

Nuclear β decays as well as the decay of the neutron are well-established low-energy probes of physics beyond the standard model (SM). In particular, with the axial-vector coupling of the nucleon g_{A} determined from lattice QCD, the comparison between experiment and SM prediction is commonly used to derive constraints on right-handed currents. Further, in addition to the CKM element V_{us} from kaon decays, V_{ud} from β decays is a critical input for the test of CKM unitarity. Here, we point out that the available information on β decays can be reinterpreted as a stringent test of lepton flavor universality (LFU). In fact, we find that the ratio of V_{us} from kaon decays over V_{us} from β decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in W-μ-ν couplings thanks to a CKM enhancement by (V_{ud}/V_{us})^{2}∼20. From this perspective, recent hints for the violation of CKM unitarity can be viewed as further evidence for LFUV, fitting into the existing picture exhibited by semileptonic B decays and the anomalous magnetic moments of muon and electron. Finally, we comment on the future sensitivity that can be reached with this LFU violating observable and discuss complementary probes of LFU that may reach a similar level of precision, such as Γ(π→μν)/Γ(π→eν) at the PEN and PiENu experiments or even direct measurements of W→μν at an FCC-ee.

A global likelihood for precision constraints and flavour anomalies

We present a global likelihood function in the space of dimension-six Wilson coefficients in the Standard Model Effective Field Theory. The likelihood includes contributions from flavour-changing neutral current B decays, lepton flavour universality tests in charged- and neutral-current B and K decays, meson-antimeson mixing observables in the K, B, and D systems, direct CP violation in Krightarrow pi pi , charged lepton flavour violating B, tau, and muon decays, electroweak precision tests on the Z and W poles, the anomalous magnetic moments of the electron, muon, and tau, and several other precision observables, 265 in total. The Wilson coefficients can be specified at any scale, with the one-loop running above and below the electroweak scale automatically taken care of. The implementation of the likelihood function is based on the open source tools flavio and wilson as well as the open Wilson coefficient exchange format (WCxf) and can be installed as a Python package. It can serve as a basis either for model-independent fits or for testing dynamical models, in particular models built to address the anomalies in B physics. We discuss a number of example applications, reproducing results from the EFT and model building literature.

Leptoquark solution for both the flavor and ANITA anomalies

The ANITA experiment has seen anomalous Earth emergent showers of EeV energies which cannot be explained with Standard Model interactions. In addition, tests of lepton flavor universality in $ R(D^{(\ast)})$ and $R(K^{(\ast)})$ have shown significant deviations from theoretical predictions. It is known that, among single leptoquark solutions, only the chiral vector leptoquark $U_1 \sim (\mathbf{3}, \mathbf{1} , 2/3)$ can simultaneously address the discrepancies. In this paper, we show that the leptoquark motivated by flavor anomalies coupled to a sterile neutrino can also explain the ANITA Anomalous Events. We consider two scenarios, (a) the sterile neutrino, produced via resonant leptoquark mediated neutrino-nucleon interactions, propagates through the Earth without significant attenuation and decays near the surface to a $\tau$ lepton; and (b) a cosmogenic sterile neutrino interacts with the matter near the surface of Earth and generates a $\tau$ lepton. These two scenarios give significantly large survival probabilities even when regeneration effects are not taken into account. In the second scenario, the distribution of emergent tau energy peaks in the same energy range as seen by ANITA.

Analyzing lepton flavor universality in the decays Λb→Λc(*)(12±,32−)+ℓν¯ℓ

Lepton flavor universality can be tested in the semileptonic decays $\Lambda_b\to \Lambda_c^{(\ast)}$ where $\Lambda_c^{(\ast)}$ denotes either the ground state $\Lambda_c(2286)$ (with $J^P=1/2^+$) or its orbital excitations $\Lambda_c(2595)$ (with $J^P=1/2^-$) and $\Lambda_c(2625)$ (with $J^P=3/2^-$). We calculate the differential decay rates as well as the branching fractions of these decays for both tauonic and muonic modes with form factors obtained from a covariant confined quark model previously developed by us. We present results for the rate ratios of the tauonic and muonic modes which provide important tests of lepton flavor universality in forthcoming experiments.