Test Of Lepton Flavor Universality Research Articles

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.

Testing lepton flavour universality in semileptonic \u039bb\u2009\u2192\u2009\u039bc* decays

Lepton Flavour Universality tests with semileptonic Λb → Λc* decays are important to corroborate the present anomalies in the similar ratios {R_D}_{{}^{left(ast right)}} , and can provide complementary constraints on possible origins of these anomalies beyond the Standard Model. In this paper we provide — for the first time — all the necessary theoretical ingredients to perform and interpret measurements of {R}_{Lambda_c^{*}} at the LHCb experiment. For this, we revisit the heavy-quark expansion of the relevant hadronic matrix elements, and provide their expressions to order αs and 1/m accuracy. Moreover, we study the sensitivity to the form factor parameters given the projected size and purity of upcoming and future LHCb datasets of {Lambda}_bto {Lambda}_c^{*}mu overline{nu} decays. We demonstrate explicitly the need to perform a simultaneous fit to both Λc* final states. Finally, we provide projections for the uncertainty of {R}_{Lambda_c^{*}} based on the form factors analysis from semimuonic decays and theoretical relations based on the heavy-quark expansion.

B decay anomalies at LHCb

The LHCb collaboration has provided a plethora of precise measurements of flavour observables in the last years. In the B meson sector some of these results show consistent deviations from Standard Model predictions with a clear tendency to specific New Physics scenarios. B decay anomalies are found in particular related to lepton flavour universality tests and angular observables in Flavour-Changing-Neutral-Current transitions. I review here the LHCb measurements and their experimental caveats. Results from Lattice are crucial in the coming years to verify if these anomalies are explained by QCD effects or if they are unambiguous hints of physics beyond the Standard Model.

Tests of Lepton Flavour Universality at LHCb

In the Standard Model the electroweak coupling of the gauge bosons to leptons is independent of the lepton flavour. Semileptonic and rare decays of b quarks provide an ideal laboratory to test this property. Any violation of Lepton Flavour Universality would be a clear sign of physics beyond the Standard Model. In this work a review of the Lepton Flavour Universality tests performed using data collected by the LHCb experiment in 2011 and 2012 at a centre of mass energy of 7 and 8 TeV is presented.

Bs0\u2009\u2192\u2009\u2113+\u2113\u2212\u03b3 as a test of lepton flavor universality

We discuss a number of strategies to reduce the ℬ(Bs0 → ℓ+ℓ−γ) theoretical error, and make such a measurement a new probe of the interactions that are interesting in the light of present-day flavor discrepancies. In particular, for low di-lepton invariant mass we propose to exploit the close parenthood between ℬ(Bs0 → ℓ+ℓ−γ) and the measured ℬ(Bs0 → ϕ(→K+K−)γ). For high q2, conversely, we exploit the fact that the decay is dominated by two form-factor combinations, plus contributions from broad charmonium that we model accordingly. We construct the ratio Rγ , akin to RK and likewise sensitive to lepton-universality violation. Provided the two rates in this ratio are integrated in a suitable region that minimises bremsstrahlung contributions while maximising statistics, the ratio is very close to unity and the form-factor dependence cancels to an extent that makes it a new valuable probe of lepton-universality violating contributions in the effective Hamiltonian. We finally speculate on additional ideas to extract short-distance information from resonance regions, which are theoretically interesting but statistically limited at present.