Search

We calculate the complete tree and one-loop matching of the dimension 6 Standard Model Effective Field Theory (SMEFT) with unbroken $U(3)^5$ flavour symmetry to the operators of the Weak Effective Theory (WET) which are responsible for flavour changing neutral current effects among down-type quarks. We also explicitly calculate the effects of SMEFT corrections to input observables on the WET Wilson coefficients, a necessary step on the way to a well-defined, complete prediction. These results will enable high-precision flavour data to be incorporated into global fits of the SMEFT at high energies, where the flavour symmetry assumption is widespread., Comment: 25 pages, 9 figures, including appendix. v2: added references, matches published version

Rare inclusive B decays such as $$ \overline{B} $$ B ¯ → X s(d) ℓ + ℓ − are interesting probes for physics beyond the Standard Model. Due to the complementarity to their exclusive counter- parts, they might shed light on the anomalies currently seen in exclusive b → s transitions. Distinguishing new-physics effects from the Standard Model requires precise predictions and necessitates the control of long distance effects. In the present work we revisit and improve the description of various long distance effects in inclusive decays such as charmo-nium and light-quark resonances, nonfactorizable power corrections, and cascade decays. We then apply these results to a state-of-the-art phenomenological study of $$ \overline{B} $$ B ¯ → Xdℓ + ℓ − , including also logarithmically enhanced QED corrections and the recently calculated five- body contributions. To fully exploit the new-physics potential of inclusive flavour-changing neutral current decays, the $$ \overline{B} $$ B ¯ → Xdℓ + ℓ − observables should be measured in a dedicated Belle II analysis.

The latest LHCb angular analysis of the rare decay B -> K* mu+ mu- shows some discrepancies from the SM up to the 3.7 sigma level. There is a consistent new physics explanation of these anomalies, while it is also reasonable that these anomalies are just statistical fluctuations and/or a direct consequence of underestimated hadronic uncertainties. We briefly discuss possible cross-checks of the various hypotheses with an analysis of the inclusive B -> Xs l+ l- based on the data collected by the B factories Babar and Belle and also based on future opportunities at SuperBelle. We also present a global analysis of the latest LHCb data under the hypothesis of Minimal Flavour Violation. The latter is an important benchmark scenario for new physics models. Any measurements beyond the MFV bounds and relations unambiguously indicate the existence of new flavour structures next to the Yukawa couplings of the Standard Model. However, if new physics is responsible for these discrepancies, we show it is compatible with the MFV hypothesis, so no new flavour structures are needed. Moreover, we analyse the impact of the correlations between the observables based on a Monte Carlo study.

We present a complete method to construct QCD-protected observables based on the exclusive 4-body B-meson decay $ {\bar{B}_d} \to {\bar{K}^{*0}}{\ell^{+} }{\ell^{-} } $ in the low dilepton mass region. The core of the method is the requirement that the constructed quantities should fulfil the symmetries of the angular distribution. We have identified all symmetries of the angular distribution in the limit of massless leptons and explore: a new non-trivial relation between the coefficients of the angular distribution, the possibility to fully solve the system for the K * amplitudes, and the construction of non-trivial observables. We also present a phenomenological analysis of the new physics sensitivity of angular observables in the decay based on QCD factorisation. We further analyse the CP-conserving observables, A T (2) , A T (3) and A T (4) . They are practically free of theoretical uncertainties due to the soft form factors for the full range of dilepton masses rather than just at a single point as for A FB. They also have a higher sensitivity to specific new physics scenarios compared to observables such as A FB. Moreover, we critically examine the new physics reach of CP-violating observables via a complete error analysis due to scale dependences, form factors and Λ QCD/m b corrections. We have developed an ensemble method to evaluate the error on observables from Λ QCD /m b corrections. Finally, we explore the experimental prospects of CP-violating observables and find that they are rather limited. Indeed, the CP-conserving (averaged) observables A T (i) (with i = 2, 3, 4) will offer a better sensitivity to large CP phases and may be more suitable for experimental analysis.

We discuss the large set of observables available from the angular distributions of the decay d → *0l+l−. We present a NLO analysis of all observables based on the QCD factorization approach in the low-dilepton mass region and an estimate of Λ/mb corrections. Moreover, we discuss their sensitivity to new physics. We explore the experimental sensitivities at LHCb (10 fb−1) and SuperLHCb (100 fb−1) based on a full-angular fit method and explore the sensitivity to right handed currents. We also show that the previously discussed transversity amplitude AT(1) cannot be measured at the LHCb experiment or at future B factory experiments as it requires a measurement of the spin of the final state particles.

We investigate sum rules for heavy-to-light transition form factors at large recoil derived from correlation functions with interpolating currents for light pseudoscalar or vector fields in soft-collinear effective theory (SCET). We consider both, factorizable and non-factorizable contributions at leading power in the Lambda/m_b expansion and to first order in the strong coupling constant alpha_s, neglecting contributions from 3-particle distribution amplitudes in the B-meson. We pay particular attention to various sources of parametric and systematic uncertainties. We also discuss certain form factor ratios where part of the hadronic uncertainties related to the B-meson distribution amplitude and to logarithmically enhanced alpha_s corrections cancel.

We investigate to what extent the experimental information on $B \to \pi\pi$ branching fractions and CP asymmetries can be used to better understand the QCD dynamics in these decays. For this purpose we decompose the independent isospin amplitudes into factorizable and non-factorizable contributions. The former can be estimated within the framework of QCD factorization for exclusive $B$ decays. The latter vanish in the heavy-quark limit, $m_b \to \infty$, and are treated as unknown hadronic parameters. We discuss at some length in which way the non-factorizable contributions are treated in different theoretical and phenomenological frameworks. We point out the potential differences between the phenomenological treatment of power-corrections in the ``BBNS approach'', and the appearance of power -suppressed operators in soft-collinear effective theory (SCET). On that basis we define a handful of different (but generic) scenarios where the non-factorizable part of isospin amplitudes is parametrized in terms of three or four unknowns, which can be constrained by data. We also give some short discussion on the implications of our analysis for $B \to \pi K$ decays. In particular, since non-factorizable QCD effects in $B \to \pi \pi$ may be large, we cannot exclude sizeable non-factorizable effects, which violate $SU(3)_F$ flavour symmetry, or even isospin symmetry (via long-distance QED effects). This may help to explain certain puzzles in connection with isospin-violating observables in $B \to \pi K$ decays.