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Register a new userSearch for new physics in $b to q ell ell$ decays
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Authors
Francesco Dettori
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Recent results obtained in experiments at the LHC in the field of rare $b$-hadron decays are reviewed in this contribution, with a focus on $bto qellell$ processes. A general status is presented as well as recently completed measurements.
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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.
We study the impact of contact interactions involving two leptons (electrons or muons) and two $b$-quarks ($b bar{b} ell^+ ell^-$) on the high-mass di-lepton region at the LHC. We consider different selections of $b$-tagged jet multiplicities in the di-lepton final states: inclusive (no selection), 0, 1 and 2 $b$-tagged jets, and show that the single $b$-jet selection significantly improves the sensitivity to New Physics (NP) in the form of the $b bar{b} ell^+ ell^-$ contact term. We obtain a better sensitivity compared to the currently existing searches of NP in the di-lepton inclusive channel. In particular, the expected limits go beyond competitive bounds set by LEP (for electrons) on the scale of NP, $Lambda$, by a factor of $1.2-3.1$, depending on the chirality structure of the operator. In addition, the expected limits on $Lambda$, set by using a non-resonant LHC di-lepton inclusive search, are expected to be improved by a factor of $1.3-1.4$ for both electrons and muons.
Rare decays of heavy-flavoured particles provide an ideal laboratory to look for deviations from the Standard Model, and explore energy regimes beyond the LHC reach. Decays proceeding via electroweak penguin diagrams are excellent probes to search for New Physics, and $b to s ell^{+}ell^{-}$ processes are particularly interesting since they give access to many observables such as branching fractions, asymmetries and angular observables. Recent results from the LHCb experiment are reviewed.
We present the prospects of an angular analysis of the $Lambda_b to Lambda(1520)ell^+ell^-$ decay. Using the expected yield in the current dataset collected at the LHCb experiment, as well as the foreseen ones after the LHCb upgrades, sensitivity studies are presented to determine the experimental precision on angular observables related to the lepton distribution and their potential to identify New Physics. The forward-backward lepton asymmetry at low dilepton invariant mass is particularly promising. NP scenarios favoured by the current anomalies in $bto sell^+ell^-$ decays can be distinguished from the SM case with the data collected between the Run 3 and the Upgrade 2 of the LHCb experiment.
The branching fractions of the decays $B^{+} to eta ell^{+} u_{ell}$ and $B^{+} to eta^{prime} ell^{+} u_{ell}$ are measured, where $ell$ is either an electron or a muon, using a data sample of $711,{rm fb}^{-1}$ containing $772 times 10^6 Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. To reduce the dependence of the result on the form factor model, the measurement is performed over the entire $q^2$ range. The resulting branching fractions are ${cal B}(B^{+} rightarrow eta ell^{+} u_{ell}) = (2.83 pm 0.55_{rm (stat.)} pm 0.34_{rm (syst.)}) times 10^{-5}$ and ${cal B}(B^{+} rightarrow eta ell^{+} u_{ell}) = (2.79 pm 1.29_{rm (stat.)} pm 0.30_{rm (syst.)}) times 10^{-5}$.
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