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Register a new userIsospin extrapolation as a method to study inclusive $bar{B} to X_{s} ell^{+}ell^{-}$ decays
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A novel approach to reconstruct inclusive $bar{B} to X_{s} ell^{+}ell^{-}$ decays is presented. The method relies on isopsin symmetry to extrapolate the semi-inclusive signature $X_{b}to K^{+} ell^{+}ell^{-} X$ to the fully inclusive rate in $B^{+}$ and $B^{0}$ decays. We investigate the possibility to measure branching fractions and other observables such as lepton universality ratios and $CP$ asymmetries. As a proof of concept, fast simulation is used to compare the $X_{b}to K^{+} ell^{+}ell^{-} X$ signature with a fully inclusive approach. Several experimental advantages are seen which have the potential to make measurements of inclusive $bar{B} to X_{s} ell^{+}ell^{-}$ decays tractable at a hadron collider.
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We calculate tree-level contributions to the inclusive rare $bar B to X_{s(d)} , ell^+ell^-$ decays. At the partonic level they stem from the five-particle process $b to s(d) , q bar q , ell^+ell^-$, with $q in {u,d,s}$. While for $b to d$ transitions such five-body final states contribute at the same order in the Wolfenstein expansion compared to the three-body partonic decay, they are CKM suppressed in $b to s$ decays. In the perturbative expansion, we include all leading-order contributions, as well as partial next-to-leading order QCD and QED effects. In the case of the differential branching ratio, we present all results completely analytically in terms of polylogarithmic functions of at most weight three. We also consider the differential forward-backward asymmetry, where all except one interference could be obtained analytically. From a phenomenological point of view the newly calculated contributions are at the percent level or below.
Rare inclusive $B$ decays such as $bar{B}to X_{s(d)} ell^+ell^-$ are interesting probes for physics beyond the Standard Model. Due to the complementarity to their exclusive counterparts, they might shed light on the anomalies currently seen in exclusive $b to s$ 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 charmonium and light-quark resonances, nonfactorisable power corrections, and cascade decays. We then apply these results to a state-of-the-art phenomenological study of $bar{B}to X_d ell^+ell^-$, 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 $bar{B}to X_d ell^+ell^-$ observables should be measured in a dedicated Belle II analysis.
Rare semileptonic $b to s ell^+ ell^-$ transitions provide some of the most promising frameworks to search for new physics effects. Recent analyses of these decays have indicated an anomalous behaviour in measurements of angular distributions of the decay $B^0to K^*mu^+mu^-$ and lepton-flavour-universality observables. Unambiguously establishing if these deviations have a common nature is of paramount importance in order to understand the observed pattern. We propose a novel approach to independently and complementary probe this hypothesis by performing a simultaneous amplitude analysis of $bar{B}^0 to bar{K}^{*0} mu^+mu^-$ and $bar{B}^0 to bar{K}^{*0} e^+e^-$ decays. This method enables the direct determination of observables that encode potential non-equal couplings of muons and electrons, and are found to be insensitive to nonperturbative QCD effects. If current hints of new physics are confirmed, our approach could allow an early discovery of physics beyond the standard model with LHCb run II data sets.
With the first data being recorded at Belle II, we are at the brink of a new era in quark flavour physics. The many exciting new opportunities for Belle~II include a full angular analysis of inclusive ${bar B to X_{s} , ell^+ell^-}$ which has the potential to reveal new physics, in particular by its interplay with the exclusive $b to s ell^+ell^-$ counterparts studied extensively at LHCb. In this paper, we present fully updated Standard Model predictions for all angular observables necessary for this endeavour. These predictions are tailored to Belle II and include an elaborate study of the treatment of collinear photons which become crucial when aiming for the highest precision. In addition, we present a phenomenological study of the potential for Belle II to reveal possible new physics in the inclusive decay channel, both in an independent manner and in combination with exclusive modes.
Ratios of branching fractions of semileptonic B decays, $(B to H mu mu)$ over $(B to H ee)$ with $H=K, K^*,X_s, K_0(1430), phi, ldots$ are sensitive probes of lepton universality. In the Standard Model, the underlying flavor changing neutral current process $brightarrow s ell ell$ is lepton flavor universal. However models with new flavor violating physics above the weak scale can give substantial non-universal contributions. The leading contributions from such new physics can be parametrized by effective dimension six operators involving left- or right-handed quarks. We show that in the double ratios $R_{X_s}/R_K$, $R_{K^*}/R_K$ and $R_phi/R_K$ the dependence on new physics coupling to left-handed quarks cancels out. Thus a measurement of any of these double ratios is a clean probe of flavor nonuniversal physics coupling to right-handed quarks. We also point out that the observables $R_{X_s}$, $R_{K^*}$, $R_{K_0(1430)}$ and $R_phi$ depend on the same combination of Wilson coefficients and therefore satisfy simple consistency relations.
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