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Register a new userImplications of new evidence for lepton-universality violation in $bto sell^+ell^-$ decays
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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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Motivated by deviations with respect to Standard Model predictions in $bto sell^+ell^-$ decays, we evaluate the global significance of the new physics hypothesis in this system, including the look-elsewhere effect for the first time. We estimate the trial-factor with psuedo-experiments and find that it can be as large as seven. We calculate the global significance for the new physics hypothesis by considering the most general description of a non-standard $bto sell^+ell^-$ amplitude of short-distance origin. Theoretical uncertainties are treated in a highly conservative way by absorbing the corresponding effects into a redefinition of the Standard Model amplitude. Using the most recent measurements of LHCb, ATLAS and CMS, we obtain the global significance to be $3.9$ standard deviations.
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.
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.
In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available $bto sell^+ell^-$ data ($ell=e,mu$) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by $5,sigma$ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4$,sigma$ level. We discuss the impact of LFU-violating New Physics on the observable $P_5^prime$ from $B to K^*mu^+mu^-$ and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.
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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