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Register a new userTest of lepton universality in beauty-quark decays
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The Standard Model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton-proton collision data collected with the LHCb detector at CERNs Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the Standard Model, such as a new fundamental interaction between quarks and leptons.
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A precision test of lepton flavour universality has been performed by measuring the ratio RK of kaon leptonic decay rates K+ --> e+nu and K+ --> mu+nu in a sample of 59813 reconstructed K+ --> e+nu candidates with (8.71 +- 0.24)% background contamination. The result RK = (2.487 +- 0.013) * 10^{-5} is in agreement with the Standard Model expectation.
We propose a novel strategy to test lepton flavor universality (LFU) in top decays, applicable to top pair production at colliders. Our proposal exploits information in kinematic distributions and mostly hinges on data-driven techniques, thus having very little dependence on our theoretical understanding of top pair production. Based on simplified models accommodating recent hints of LFU violation in charged current B meson decays, we show that existing LHC measurements already provide non-trivial information on the flavor structure and the mass scale of such new physics (NP). We also project that the measurements of LFU in top decays at the high-luminosity LHC could reach a precision at the percent level or below, improving the sensitivity to LFU violating NP in the top sector by more than an order of magnitude compared to existing approaches.
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.
We present measurements of the branching fractions for the decays $Bto K mu^{+}mu^{-}$ and $Bto K e^{+}e^{-}$, and their ratio ($R_{K}$), using a data sample of 711 $fb^{-1}$ that contains $772 times 10^{6}$ $Bbar{B}$ events. The data were collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The ratio $R_{K}$ is measured in five bins of dilepton invariant-mass-squared ($q^{2}$): $q^{2} in (0.1, 4.0), (4.0, 8.12), (1.0, 6.0)$, $(10.2, 12.8)$ and ($>14.18) GeV^{2}/c^{4}$, along with the whole $q^2$ region. The $R_{K}$ value for $q^{2} in (1.0, 6.0) GeV^{2}/c^{4}$ is $1.03^{+0.28}_{-0.24} pm 0.01$. The first and second uncertainties listed are statistical and systematic, respectively. All results for $R_{K}$ are consistent with Standard Model predictions. We also measure $C!P$-averaged isospin asymmetries in the same $q^{2}$ bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the $q^{2}in(1.0,6.0) GeV^{2}/c^{4}$ bin in the mode with muon final states. The measured differential branching fractions, ${dcal B}/{dq^{2}}$, are consistent with theoretical predictions for charged $B$ decays, while the corresponding values are below the expectations for neutral $B$ decays. We have also searched for lepton-flavor-violating $B rightarrow Kmu^{pm}e^{mp}$ decays and set $90%$ confidence-level upper limits on the branching fraction in the range of $10^{-8}$ for $B^{+} rightarrow K^{+}mu^{pm}e^{mp}$, and $B^{0} rightarrow K^{0}mu^{pm}e^{mp}$ modes.
A test of lepton universality, performed by measuring the ratio of the branching fractions of the $B^{0} rightarrow K^{*0}mu^{+}mu^{-}$ and $B^{0} rightarrow K^{*0}e^{+}e^{-}$ decays, $R_{K^{*0}}$, is presented. The $K^{*0}$ meson is reconstructed in the final state $K^{+}pi^{-}$, which is required to have an invariant mass within 100$mathrm{,MeV}c^2$ of the known $K^{*}(892)^{0}$ mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3$mathrm{,fb}^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8$mathrm{,TeV}$. The ratio is measured in two regions of the dilepton invariant mass squared, $q^{2}$, to be begin{eqnarray*} R_{K^{*0}} = begin{cases} 0.66~^{+~0.11}_{-~0.07}mathrm{,(stat)} pm 0.03mathrm{,(syst)} & textrm{for } 0.045 < q^{2} < 1.1~mathrm{,GeV^2}c^4 , , 0.69~^{+~0.11}_{-~0.07}mathrm{,(stat)} pm 0.05mathrm{,(syst)} & textrm{for } 1.1phantom{00} < q^{2} < 6.0~mathrm{,GeV^2}c^4 , . end{cases} end{eqnarray*} The corresponding 95.4% confidence level intervals are $[0.52, 0.89]$ and $[0.53, 0.94]$. The results, which represent the most precise measurements of $R_{K^{*0}}$ to date, are compatible with the Standard Model expectations at the level of 2.1--2.3 and 2.4--2.5 standard deviations in the two $q^{2}$ regions, respectively.
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