13 research outputs found
On the contribution of the electromagnetic dipole operator to the decay amplitude
We construct a factorization theorem that allows to systematically include
QCD corrections to the contribution of the electromagnetic dipole operator in
the effective weak Hamiltonian to the decay
amplitude. We first rederive the known result for the leading-order QED box
diagram, which features a double-logarithmic enhancement associated to the
different rapidities of the light quark in the meson and the
energetic muons in the final state. We provide a detailed analysis of the
cancellation of the related endpoint divergences appearing in individual
momentum regions, and show how the rapidity logarithms can be isolated by
suitable subtractions applied to the corresponding bare factorization theorem.
This allows us to include in a straightforward manner the QCD corrections
arising from the renormalization-group running of the hard matching coefficient
of the electromagnetic dipole operator in soft-collinear effective theory, the
hard-collinear scattering kernel, and the -meson distribution amplitude.
Focusing on the contribution from the double endpoint logarithms, we derive a
compact formula that resums the leading-logarithmic QCD corrections.Comment: 33 pages, 3 figure
Dispersive analysis of B → K (*) and B s → ϕ form factors
We propose a stronger formulation of the dispersive (or unitarity) bounds à la Boyd-Grinstein-Lebed (BGL), which are commonly applied in analyses of the hadronic form factors for B decays. In our approach, the existing bounds are split into several new bounds, thereby disentangling form factors that are jointly bounded in the common approach. This leads to stronger constraints for these objects, to a significant simplification of our numerical analysis, and to the removal of spurious correlations among the form factors. We apply these novel bounds to B¯→K¯∗ and B¯s→ϕ form factors by fitting them to purely theoretical constraints. Using a suitable parametrization, we take into account the form factors’ below-threshold branch cuts arising from on-shell B¯sπ0 and B¯sπ0π0 states, which so-far have been ignored in the literature. In this way, we eliminate a source of hard-to-quantify systematic uncertainties. We provide machine readable files to obtain the full set of the B¯→K¯∗ and B¯s→ϕ form factors in and beyond the entire semileptonic phase space
Heavy-Quark Expansion for Form Factors and Unitarity Bounds beyond the Limit
We carry out a comprehensive analysis of the full set of form factors for spectator quarks within the framework of
the Heavy-Quark Expansion (HQE) to order . In addition to the available lattice QCD calculations we make use of
two new sets of theoretical constraints: we produce for the first time
numerical predictions for the full set of form
factors using Light-Cone Sum Rules with -meson distribution amplitudes.
Furthermore, we reassess the QCD three-point sum rule results for the
Isgur-Wise functions entering all our form factors for both and
spectator quarks. These additional constraints allow us to go beyond the
commonly used assumption of symmetry for the
form factors, especially in the unitarity constraints which we impose
throughout our analysis. We find the coefficients of the IW functions emerging
at to be consistent with the naive
expectation, indicating a good convergence of the HQE. While we do not find
significant breaking, the explicit treatment of as compared to a
simple symmetry assumption renders the unitarity constraints more effective. We
find that the (pseudo)scalar bounds are saturated to a large degree, which
affects our theory predictions. We analyze the phenomenological consequences of
our improved form factors by extracting from decays and producing theoretical predictions for the
lepton-flavour universality ratios , , and ,
as well as the - and polarization fractions for the modes.Comment: 16 pages, 3 figures, 7 tables, includes ancillary files; v2: minor
changes to the text, conclusions unchanged, 2 missing files added, as
accepted for publication in EPJ
A puzzle in decays and extraction of the fragmentation fraction
We provide updated predictions for the hadronic decays and . They are based on
results for the QCD factorization amplitudes at
leading power and on recent results for the
form factors up to order in the
heavy-quark expansion. We give quantitative estimates of the matrix elements
entering the hadronic decay amplitudes at order for the first time. Our results are very precise, and uncover a
substantial discrepancy between the theory predictions and the experimental
measurements. We explore two possibilities for this discrepancy:
non-factorizable contributions larger than predicted by the QCD factorization
power counting, and contributions beyond the Standard Model. We determine the
fragmentation fraction for the CDF, D0 and LHCb experiments for both
scenarios.Comment: 13 pages, 4 tables: v2: minor modifications, accepted for publication
in EPJ
Lepton-flavour non-universality of angular distributions in and beyond the Standard Model
We analyze in detail the angular distributions in decays, with a focus on lepton-flavour non-universality. We
investigate the minimal number of angular observables that fully describes
current and upcoming datasets, and explore their sensitivity to physics beyond
the Standard Model (BSM) in the most general weak effective theory. We apply
our findings to the current datasets, extract the non-redundant set of angular
observables from the data, and compare to precise SM predictions that include
lepton-flavour universality violating mass effects. Our analysis shows that the
current presentation of the experimental data is not ideal and prohibits the
extraction of the full set of relevant BSM parameters, since the number of
independent angular observables that can be inferred from data is limited to
only four. We uncover a tension between data and predictions that
is hidden in the redundant presentation of the Belle 2018 data on decays. This tension specifically involves observables that
probe lepton-flavour universality. However, we find inconsistencies in
these data, which renders results based on it suspicious. Nevertheless, we
discuss which generic BSM scenarios could explain the tension, in the case that
the inconsistencies do not affect the data materially. Our findings highlight
that non-universality in the SM, introduced by the finite muon mass, is
already significant in a subset of angular observables with respect to the
experimental precision.Comment: 18 pages, 2 figures, 3 table
Recommended from our members
Non-factorisable contributions of strong-penguin operators in Λ b → Λ ℓ + ℓ − decays
Acknowledgements: We thank Marzia Bordone for her contributions in the early stage of this project. This research is supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under grant 396021762 – TRR 257. The work of N.G. has been partially supported by STFC consolidated grants ST/T000694/1 and ST/X000664/1.We investigate for the first time a certain class of non-factorisable contributions of the four-quark operators O3−6 in the weak effective Hamiltonian to the Λb → Λℓ+ℓ− decay amplitude. We focus on the case where a virtual photon is radiated from one of the light constituents of the Λb baryon, in the kinematic situation of large hadronic recoil with an energetic Λ baryon in the final state. The effect on the suitably defined “non-local form factors” is calculated using the light-cone sum rule approach for a correlator with an interpolating current for the light Λ baryon. We find that this approach requires the introduction of new soft functions that generalise the standard light-cone distribution amplitudes (LCDAs) for the heavy Λb baryon. We give a heuristic discussion of their properties and a model that relates them to the standard LCDAs. Within this framework, we provide numerical results for the size of the non-local form factors considered
and form factors from QCD light-cone sum rules
We perform the first calculation of form factors in the semileptonic decays
and
using QCD light-cone sum rules (LCSRs) with -meson distribution amplitudes.
In this calculation the -quark mass is finite. Analytical expressions for
two-particle contributions up to twist four are obtained. To disentangle the
and contributions in the LCSRs, we suggest a novel approach
that introduces a combination of two interpolating currents for these charmed
mesons. To fix all the parameters in the LCSRs, we use the two-point QCD sum
rules for the decay constants of and mesons augmented by a
single experimental input, that is the decay
width. We provide numerical results for all and
form factors. As a byproduct, we also obtain the - and -meson decay
constants and predict the lepton-flavour universality ratios and
.Comment: 30 pages, 2 figures, published versio