On lepton flavour universality in semileptonic Bc → ηc, J/ψ decays

157

234

We perform a general model-independent analysis of b → cτν τ transitions, including measurements of R D , R D * , their q 2 differential distributions, the recently measured longitudinal D * polarization F D * L , and constraints from the B c → τν τ lifetime, each of which has significant impact on the fit. A global fit to a general set of Wilson coefficients of an effective low-energy Hamiltonian is presented, the solutions of which are interpreted in terms of hypothetical new-physics mediators. From the obtained results we predict selected b → cτν τ observables, such as the baryonic transition Λ b → Λ c τν τ , the ratio R J/ψ , the forward-backward asymmetries A D ( * ) FB , the τ polarization asymmetries P D ( * ) τ , and the longitudinal D * polarization fraction F D * L . The latter shows presently a slight tension with any new-physics model, such that an improved measurement could have an important impact. We also discuss the potential change due the very recently announced preliminary R D ( * ) measurement by the Belle collaboration.

Section: Introductionmentioning

confidence: 85%

Global fit to b → cτν transitions

Murgui

Peñuelas

Jung

et al. 2019

157

234

“…where the form factors entering dΓ(B c → J/ψµν µ )/dq 2 are known to some extent from lattice calculation [57], and are reproduced to a satisfactory precision by the QCDSR method explained in detail in Sec.2. The form factors for B c → J/ψ transition were already briefly presented in [18]. Here the used parameters differ a little from ones used there, due to a necessary update.…”

Section: )mentioning

confidence: 99%

“…, (1.6) which for B → K * , B → D * consistently show 2 − 3 σ lower values than predicted in the Standard Model (SM). Recently, also the potential sign of the lepton-flavour nonuniversality was observed for semileptonic B c → J/ψ decay [17][18][19]. With upgraded detectors in the next run of LHC, it would be possible to sample enough data in LHCb experiment for the analysis of other semileptonic B c decays.…”

Section: Introductionmentioning

confidence: 99%

“…Otherwise, for the b quark we use the so called "potential subtracted" mass [36], which is coincidentally very close to both the Υ(1S) scheme mass [37] and the kinetic scheme mass [38,39], whereas the c-quark the mass is then given by varying the ratio Z of the two masses given by the QCD lattice calculation [40,41], keeping in mind that we do not use the MS masses, and this ratio for pole masses tends to be lower as higher order corrections are included -which is why we choose to use a somewhat lower value of Z. The same method described above was already used for calculating B c → η c , J/ψ transition form factors in [18]. All parameters used are listed in Table 2.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

|Vub| determination and testing of lepton flavour universality in semileptonic Bc → D(∗) decays

Leljak

Melić

2020

Self Cite

In light of prospects for measurements of B c → D ( * ) lν decays in the upcoming Upgrade II of the LHC, we show that by using calculated B c → D ( * ) form factors a competitive extraction of the |V ub | CKM matrix element from the B c → Dµν µ decay might be possible. To minimize experimental and theoretical uncertainties we provide the ratio |V ub |/|V cb | by normalizing the B c → D ( * ) µν µ to B c → J/ψµν µ decay. We also briefly examine the suggestion to extract |V ub |/|V cs | from the theoretically interesting ratio of B c → D 0 eν e and B c → B s eν e decay rates in the zero-recoil limit. With the present average value of |V ub |, the predicted branching ratios are estimated to be BR(B c → D 0 µν µ ) = (2.4 ± 0.4) · 10 −5 and BR(B c → D * µν µ ) = (7 ± 3) · 10 −5 , and the semileptonic ratios for testing the lepton flavour universality in these B c decays are R c (D 0 ) = 0.64 ± 0.05 and R c (D * ) = 0.55 ± 0.05. We also provide q 2 distributions and various angular observables of B c → D ( * ) lν decays. arXiv:1909.01213v2 [hep-ph] 4 Sep 2019 Contents 1 Introduction 1 2 Calculation of B c → D ( * ) form factors 4 2.1 High q 2 extrapolation of the form factors 9 3 Semileptonic B c → D ( * ) decays and the |V ub | determination 13 3.1 Predictions for the decay rates and angular observables 13 3.2 |V ub | determination and the |V ub |/|V cb | ratio 19 3.3 The |V ub |/|V cs | ratio 21 4 Conclusion 23 A Three-point sum rule contributions to form factors 24 A.1 Perturbative contributions 24 A.2 Non-local quark-condensate contributions 26 A.2.1 B c → P transition 27 A.2.2 B c → V transition 29 A.3 Gluon condensate contributions 30 B QCDSR parameters inB c → D ( * ) and B c → J/ψ, B c → B s form factor calculations and discussion about their q 2 dependence 32 B.1 Parameters in the B c → J/ψ, B s 3ptSR calculations 32 B.2 Covariance matrices 33 B.3 z-series fits 34 C Differential q 2 distributions and |V ub | extraction 35 C.1 Differential (q 2 , cos θ l ) distributions of semileptonic B c → D ( * ) decays 35 C.2 Binned q 2 distributions of ∆ζ D 0 and R D 0 J/ψ 36 R c (D * ) ≡ B(B c → D * τν τ ) B(B c → D * µν µ ) = 0.55 ± 0.05, (3.9)which measure the ratios of branching fractions of the semileptonic decays to a τ lepton to the branching fractions to a muon. Once measured the ratios in (3.9) will serve as a an

“…measured recently by LHCb to be R J/ψ = 0.71 ± 0.17 ± 0.18 [41]. Although the ratio is also measured to be enhanced with respect to the SM prediction R SM J/ψ ≈ 0.25-0.29 [42][43][44][45][46][47][48][49][50][51][52][53][54], the central value of the measurement shows a very large effect that cannot be wellaccommodated with BSM contributions [37]. The observable f D * L , the longitudinal polarisation of the D * in B → D * τ ν, also differs from the SM expectation by ∼ 1.6σ:…”

Section: Charged Current Processesmentioning

confidence: 99%

A near-minimal leptoquark model for reconciling flavour anomalies and generating radiative neutrino masses

Bigaran

Gargalionis

Volkas

2019

We introduce two scalar leptoquarks, the SU(2) L isosinglet denoted φ ∼ (3, 1, −1/3) and the isotriplet ϕ ∼ (3, 3, −1/3), to explain observed deviations from the standard model in semi-leptonic B-meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables R D ( * ) , R K ( * ) , and angular observables in b → sµµ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from ∆L = 2 effective operators. Introducing the vector-like quark χ ∼ (3, 2, −5/6) necessary for lepton-number violation, we consider the contribution of both leptoquarks to the generation of radiative neutrino mass. We find that constraints permit simultaneously accommodating the flavour anomalies while also explaining the relative smallness of neutrino mass without the need for cancellation between leptoquark contributions. A characteristic prediction of our model is a rate of muon-electron conversion in nuclei fixed by the anomalies in b → sµµ and neutrino mass; the COMET experiment will thus test and potentially falsify our scenario. The model also predicts signatures that will be tested at the LHC and Belle II.