Semileptonic B-meson Decays Research Articles

Self-consistent optimization of the z expansion for B -meson decays

We discuss the self-consistency imposed by the analyticity of regular parts of form factors, appearing in the z expansion for semileptonic B-meson decays, when fitted in different kinematic regions. Relying on the uniqueness of functions defined by analytic continuation, we propose four metrics which measure the departure from the ideal analytic self-consistency. We illustrate the process using Belle data for B→Dℓνℓ with the two kinematic regions chosen as the five low-z and the five high-z bins. For this specific example, the metrics provide consistent indications that some choices (order of truncation, Boyd-Grinstein-Lebed or Bourrely-Caprini-Lellouch) made in the form of the z expansion can be optimized. However, other choices (z origin, location of isolated poles and threshold constraints) appear to have very little effect on these metrics. On the other hand, changing the kinetic regions affects the results and should also be considered in the optimization process. We briefly discuss the implication for optimization of the z expansion for nucleon form factors relevant for neutrino oscillation experiments. Published by the American Physical Society 2024

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.

Scotogenic Dirac neutrino mass models embedded with leptoquarks: one pathway to address the flavor anomalies and the neutrino masses together

If the leptoquarks proposed to account for the intriguing anomalies observed in the semileptonic B-meson decays, R_{D^{(*)}} and R_{K^{(*)}}, as well as in the anomalous magnetic moment of the muon, (g-2)_mu , can be embedded into the scotogenic Dirac neutrino mass models, all these flavor anomalies, together with the origin of neutrino masses and the nature of dark matter, could be potentially addressed in a unified picture. Among the minimal seesaw, one-loop, and two-loop realizations of the dimension-4 effective operator {mathcal {L}}_{4} for the Dirac neutrino masses, we show that plenty of diagrams associated with the two-loop realizations of {mathcal {L}}_{4} can support the coexistence of leptoquarks and dark matter candidates. After a simple match of these leptoquarks with the ones introduced to accommodate all the flavor anomalies, we establish the scotogenic Dirac neutrino mass models embedded with leptoquarks, which could address all the problems mentioned above.

Frac{left|{V}_{ub}right|}{left|{V}_{cb}right|} and quest for new physics

Charged current semi-leptonic decays of B-meson are important for a precise determination of the CKM elements. The CKM elements |Vub| and |Vcb| show a discrepancy between the exclusive and inclusive determinations. These determinations are however masked with hadronic and other uncertainties, and thus can’t be unambiguously taken as implying new physics. In the present study, we propose a new observable: the ratio of these two CKM elements, {R}_Vequiv frac{left|{V}_{ub}right|}{left|{V}_{cb}right|} , which is found to receive negligible corrections due to hadronic as well as QED effects. Interestingly, RV as constructed from exclusive determinations of |Vub| and |Vcb| agrees quite well with that constructed from the inclusive determinations of these CKM elements. Hence, we propose that RV is a better and cleaner observable, and can serve as an excellent tool for the test of the Standard Model. We also provide an example of its probing power.

Revisiting the B-physics anomalies in R-parity violating MSSM

In recent years, several deviations from the Standard Model predictions in semileptonic decays of B-meson might suggest the existence of new physics which would break the lepton-flavour universality. In this work, we have explored the possibility of using muon sneutrinos and right-handed sbottoms to solve these B-physics anomalies simultaneously in R-parity violating minimal supersymmetric standard model. We find that the photonic penguin induced by exchanging sneutrino can provide sizable lepton flavour universal contribution due to the existence of logarithmic enhancement for the first time. This prompts us to use the two-parameter scenario (C^mathrm{V}_9, , C^mathrm{U}_9) to explain b rightarrow s ell ^+ ell ^- anomaly. Finally, the numerical analyses show that the muon sneutrinos and right-handed sbottoms can explain b rightarrow s ell ^+ ell ^- and R(D^{(*)}) anomalies simultaneously, and satisfy the constraints of other related processes, such as B rightarrow K^{(*)} nu bar{nu } decays, B_s-bar{B}_s mixing, Z decays, as well as D^0 rightarrow mu ^+ mu ^-, tau rightarrow mu rho ^0, B rightarrow tau nu , D_s rightarrow tau nu , tau rightarrow K nu , tau rightarrow mu gamma , and tau rightarrow mu mu mu decays.

Recent varvec{B} physics anomalies: a first hint for compositeness?

We scrutinize the recently further strengthened hints for new physics in semileptonic B-meson decays, focusing on the ‘clean’ ratios of branching fractions R_K and R_{K^*} and examining to which pattern of new effects they point to. We explore in particular the hardly considered, yet fully viable, option of new physics in the right-handed electron sector and demonstrate how a recently proposed framework of leptons in composite Higgs setups naturally solves both the R_K and R_{K^*} anomalies via a peculiar structure of new physics effects, predicted by minimality of the model and the scale of neutrino masses. Finally, we also take into account further observables, such as mathcal{B}(B_s rightarrow mu ^+mu ^-), varDelta M_{B_s}, and angular observables in B rightarrow K^{*} mu ^+ mu ^- decays, to arrive at a comprehensive picture of the model concerning (semileptonic) B decays. We conclude that – since it is in good agreement with the experimental situation in flavor physics and also allows to avoid ultra-light top partners – the model furnishes a very promising scenarios of Higgs compositeness in the light of LHC data.

Third family quark–lepton unification at the TeV scale

We construct a model of quark–lepton unification at the TeV scale based on an SU(4) gauge symmetry, while still having acceptable neutrino masses and enough suppression in flavor changing neutral currents. An approximate U(2) flavor symmetry is an artifact of family-dependent gauge charges leading to a natural realization of the CKM mixing matrix. The model predicts sizeable violation of PMNS unitarity as well as a gauge vector leptoquark U1μ=(3,1,2/3) which can be produced at the LHC – both effects within the reach of future measurements. In addition, recently reported experimental anomalies in semi-leptonic B-meson decays, both in charged b→cτν and neutral b→sμμ currents, can be accommodated.

Flavor non-universal gauge interactions and anomalies in B-meson decays**Supported by the Grant-in-Aid for Innovative Areas (16H06490)

Motivated by flavor non-universality and anomalies in semi-leptonic B-meson decays, we present a general and systematic discussion about how to construct anomaly-free U(1)′ gauge theories based on an extended standard model with only three right-handed neutrinos. If all standard model fermions are vector-like under this new gauge symmetry, the most general family non-universal charge assignments, (a,b,c) for three-generation quarks and (d,e,f) for leptons, need satisfy just one condition to be anomaly-free, 3(a+b+c) = − (d+e+f). Any assignment can be linear combinations of five independent anomaly-free solutions. We also illustrate how such models can generally lead to flavor-changing interactions and easily resolve the anomalies in B-meson decays. Probes with mixing, decay into τ±, dilepton and dijet searches at colliders are also discussed.

Annihilation semileptonic decays of B mesons and the model dependence on the choice of the distribution amplitude

The rare semileptonic B-meson decay B → ϕl+l− is considered. The partial decay rate integrated over the 1–8 GeV2 interval of the dilepton invariant mass squared is computed with a 10% systematic uncertainty arising from the model dependence of the distribution amplitude. The purely perturbative contribution to the decay probability amounts to ℬ ∼ 10−12, which suggests that the LHC experiments could observe the B → ϕl+l− decay in a few years of data taking.

Probing lepton flavour (universality) violation at NA62 and future kaon experiments

Recent results from the LHC’s first run have revealed intriguing departures from lepton flavour universality in the semi-leptonic decays of B-mesons. We discuss the complementary role that rare kaon decays can provide in testing new physics explanations of these flavour anomalies. In the framework of minimal flavour violation, we relate the chiral low-energy constants involved in and with the new physics Wilson coefficients of the b → s effective Hamiltonian. We comment on the determination of these low-energy constants at NA62 and future kaon experiments, as well as the required improvements in sensitivity necessary to test the B-physics anomalies in the kaon sector.

Semileptonic B decays at Belle

Recent results from the Belle experiment on semileptonic decays of B-mesons are reviewed, including their effect on the determination of the CKM matrix elements |Vcb| and |Vub|.

Measurement of the top quark charge at the atlas detector

We have investigated the possibility of reconstructing the top quark charge through measurement of its decay product charges at the ATLAS detector. Verification of the hypothesis about alternative interpretation of top-quark experimental data has been considered as well. The method of “semileptonic B-meson decay” was applied for reconstructing the b-jet charge. A statistical significance of more than 5σ can be achieved using this method after analyzing 1 fb−1 of the tt-pairs data. The analysis was carried out with HERWIG and PYTHIA generators and using the GEANT4 detector simulation software package.

Determination of the Branching Fraction forB→XcℓνDecays and of|Vcb|from Hadronic-Mass and Lepton-Energy Moments

We determine the inclusive B --> Xc l nu branching fraction, the CKM matrix element |Vcb|, and other heavy-quark parameters from a simultaneous fit to moments of the hadronic-mass and lepton-energy distributions in semileptonic B-meson decays, measured as a function of the lower limit on the lepton energy, using data recorded with the BABAR detector. Using Heavy Quark Expansions (HQEs) to order 1/mb^3, we extract BR_cenu=(10.61 +- 0.16(exp) +- 0.06(HQE))% and |Vcb| = (41.4 +- 0.4(exp) +- 0.4(HQE) +- 0.6(th)) 10^-3. The stated errors refer to the experimental, HQE, and additional theoretical uncertainties.

Radiative and semi-leptonic B-meson decay spectra: Sudakov resummation beyond logarithmic accuracy and the pole mass

The inclusive spectra of radiative and semi-leptonic B-meson decays near the endpoint is computed taking into account renormalons in the Sudakov exponent (Dressed Gluon Exponentiation). In this framework we demonstrate the factorization of decay spectra into hard, jet and soft functions and discuss the universality of the latter two. Going beyond perturbation theory the soft function, which we identify as the longitudinal momentum distribution in an on-shell b quark, is replaced by the b-quark distribution in the B meson. The two differ by power corrections. We show how the resummation of running-coupling effects can be used to perform consistent separation to power accuracy between perturbative and non-perturbative contributions. In particular, we prove that the leading infrared renormalon ambiguity in the Sudakov exponent cancels against the one associated with the definition of the pole mass. This cancellation allows us to identify the non-perturbative parameter that controls the shift of the perturbative spectrum in the heavy-quark limit as the mass difference between the meson and the quark.

The forward backward asymmetries ofB→Xsτ+τ−in the MSSM

The relatively clean theoretical probes of the Standard Model (SM), and the various theories beyond the SM, provided by radiative, semi-leptonic and (purely) leptonic decays of B-mesons have become increasingly important. Due to the large number of possible distributions in the semi-leptonic decays based on the quark level transition $b \to s \ell^+ \ell^-$ (not just the branching ratio), these transitions have become very useful. A study of the Forward-Backward asymmetries for the inclusive decay ($B \to X_s \ell^+ \ell^-$) is carried out in this paper. This study shall be performed in the SM and a minimal supersymmetric extensions of the SM, namely the mSUGRA model.

Direct resummation of the leading renormalons in inclusive semileptonic B decay

We present a Borel resummation method for the QCD corrections in inclusive, charmless, semileptonic B-meson decay. The renormalon contributions are resummed to all orders by employing a bilocal expansion of the Borel transform that accurately accounts for the first infrared renormalons in the Borel plane. The renormalons in the pole mass and the QCD expansion are resummed separately, and a precise relation is obtained between a properly defined pole mass and the MS mass. The inclusive decay rate is calculated to three-loop order using an estimate of the yet unknown NNLO coefficient.

Scale setting forαsbeyond leading order

We present a general procedure for incorporating higher-order information into the scale-setting prescription of Brodsky, Lepage and Mackenzie. In particular, we show how to apply this prescription when the leading coefficient or coefficients in a series in the strong coupling alpha_s are anomalously small and the original prescription can give an unphysical scale. We give a general method for computing an optimum scale numerically, within dimensional regularization, and in cases when the coefficients of a series are known. We apply it to the heavy quark mass and energy renormalization in lattice NRQCD, and to a variety of known series. Among the latter, we find significant corrections to the scales for the ratio of e+e- to hadrons over muons, the ratio of the quark pole to MSbar mass, the semi-leptonic B-meson decay width, and the top decay width. Scales for the latter two decay widths, expressed in terms of MSbar masses, increase by factors of five and thirteen, respectively, substantially reducing the size of radiative corrections.

Semi-leptonic decays heavy-light to heavy-light

We present results for the QCD matrix elements involved in semi-leptonic decays of B-mesons into pseudo scalar heavy light states. The application of NRQCD heavy quarks allows for quark masses around the physical b-quark. We investigate the dependence of the form factors on the external momenta and looked at the mass dependence at zero recoil. For the first time, results for radially excited decay products are presented.

Determination of the HQET parameterλ1from an inclusive semileptonicBmeson decay spectrum

We estimate the heavy quark effective theory parameter ${\ensuremath{\lambda}}_{1}$ from the inclusive semileptonic B-meson decay spectrum. By using recent CLEO double lepton tagging data of $\stackrel{\ensuremath{\rightarrow}}{B}\mathrm{Xe}\ensuremath{\nu}$, which show the lepton momentum as low as 0.6 GeV, we extracted ${\ensuremath{\lambda}}_{1}\ensuremath{\sim}\ensuremath{-}0.58 {\mathrm{GeV}}^{2}$. We also derived $\overline{\ensuremath{\Lambda}}\ensuremath{\sim}0.46 \mathrm{GeV}$ and $|{V}_{\mathrm{cb}}|=0.041\ifmmode\pm\else\textpm\fi{}0.002$.

R-parity violation and semileptonic decays of B-meson

We investigate the effects of R-parity violation on the semileptonic decays of B-meson in the minimal supersymmetric standard model with explicit R-parity violation and discuss its physical implications. We find that the semileptonic decays of B-meson can be largely affected by the R-parity violation.