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Register a new userMeasuring the Breaking of Lepton Flavour Universality in $Bto K^*ell^+ell^-$
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We propose measurements of weighted differences of the angular observables in the rare decays $B to K^*ell^+ell^-$. The proposed observables are very sensitive to the difference between the Wilson coefficients $mathcal{C}_9^{(e)}$ and $mathcal{C}_9^{(mu)}$ for decays into electrons and muons, respectively. At the same time, the charm-induced hadronic contributions are kinematically suppressed to $lesssim 7% (4%)$ in the region $1,$GeV$^2 leq q^2 leq 6,$GeV$^2$, as long as LFU breaking occurs only in $mathcal{C}^{(ell)}_{9}$. This suppression becomes stronger for the region of low hadronic recoil, $q^2 geq 15,$GeV$^2$.
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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.
We present a measurement of angular observables and a test of lepton flavor universality in the $Bto K^ast ell^+ell^-$ decay, where $ell$ is either $e$ or $mu$. The analysis is performed on a data sample corresponding to an integrated luminosity of $711~mathrm{fb}^{-1}$ containing $772times 10^{6}$ $Bbar B$ pairs, collected at the $Upsilon(4S)$ resonance with the Belle detector at the asymmetric-energy $e^+e^-$ collider KEKB. The result is consistent with Standard Model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of $2.6sigma$.
We present a measurement of $R_{K^{ast}}$, the branching fraction ratio ${{cal B}(Bto K^ast mu^+ mu^-)}$/ ${{cal B}(Bto K^ast e^+ e^-)}$, for both charged and neutral $B$ mesons. The ratio for the charged case, $R_{K{^{ast +}}}$, is the first measurement ever performed. In addition, we report absolute branching fractions for the individual modes in bins of the squared dilepton invariant mass, $q^2$. The analysis is based on a data sample of $711~mathrm{fb}^{-1}$, containing $772times 10^{6}$ $Bbar B$ events, recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The obtained results are consistent with Standard Model expectations.
A measurement of the ratio of branching fractions of the decays $B^+to K^+mu^+mu^-$ and $B^+to K^+e^+e^-$ is presented. The proton-proton collision data used correspond to an integrated luminosity of $5.0,$fb$^{-1}$ recorded with the LHCb experiment at centre-of-mass energies of $7$, $8$ and $13,$TeV. For the dilepton mass-squared range $1.1 < q^2 < 6.0,$GeV$^2!/c^4$ the ratio of branching fractions is measured to be $R_K = {0.846,^{+,0.060}_{-,0.054},^{+,0.016}_{-,0.014}}$, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $R_K$ to date and is compatible with the Standard Model at the level of 2.5 standard deviations.
Motivated by renewed evidence for new physics in $b to sellell$ transitions in the form of LHCbs new measurements of theoretically clean lepton-universality ratios and the purely leptonic $B_stomu^+mu^-$ decay, we quantify the combined level of discrepancy with the Standard Model and fit values of short-distance Wilson coefficients. A combination of the clean observables $R_K$, $R_{K^*}$, and $B_sto mumu$ alone results in a discrepancy with the Standard Model at $4.0sigma$, up from $3.5sigma$ in 2017. One-parameter scenarios with purely left-handed or with purely axial coupling to muons fit the data well and result in a $sim 5 sigma$ pull from the Standard Model. In a two-parameter fit of %$C_9$ and $C_{10}$, new-physics contributions with both vector and axial-vector couplings to muons the allowed region is much more restricted than in 2017, principally due to the much more precise result on $B_s to mu^+ mu^-$, which probes the axial coupling to muons.Including angular observables data restricts the allowed region further.A by-product of our analysis is an updated average of $text{BR}(B_s to mu^+ mu^-) = (2.8pm 0.3) times 10^{-9}$.
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