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Register a new userMeasurement of the Lepton Forward-Backward Asymmetry in $B rightarrow X_s ell^+ ell^-$ Decays with a Sum of Exclusive Modes
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We report the first measurement of the lepton forward-backward asymmetry ${cal A}_{rm FB}$ as a function of the squared four-momentum of the dilepton system, $q^2$, for the electroweak penguin process $B rightarrow X_s ell^+ ell^-$ with a sum of exclusive final states, where $ell$ is an electron or a muon and $X_s$ is a hadronic recoil system with an $s$ quark. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ pairs recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. ${cal A}_{rm FB}$ for the inclusive $B rightarrow X_s ell^+ ell^-$ is extrapolated from the sum of 10 exclusive $X_s$ states whose invariant mass is less than 2 GeV/$c^2$. For $q^2 > 10.2$ GeV$^2$/$c^2$, ${cal A}_{rm FB} < 0$ is excluded at the 2.3$sigma$ level, where $sigma$ is the standard deviation. For $q^2 < 4.3$ GeV$^2$/$c^2$, the result is within 1.8$sigma$ of the Standard Model theoretical expectation.
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We report a measurement of the ratios of Wilson coefficients $A_9/A_7$ and $A_{10}/A_7$ in $B to K^* ell^+ ell^-$. The result is obtained from a data sample containing 386 million $Bbar{B}$ pairs that was collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider.
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.
We use 772$times 10^6$ $B bar{B}$ meson pairs collected at the $Upsilon(4S)$ resonance with the Belle detector to measure the branching fraction for $bar{B} rightarrow X_s gamma$. Our measurement uses a sum-of-exclusives approach in which 38 of the hadronic final states with strangeness equal to $+1$, denoted by $X_s$, are reconstructed. The inclusive branching fraction for $M_{X_s}<$ 2.8 GeV/$c^2$, which corresponds to a minimum photon energy of 1.9 GeV, is measured to be ${cal B}(bar{B} rightarrow X_s gamma)=(3.51pm0.17pm0.33)times10^{-4}$, where the first uncertainty is statistical and the second is systematic.
Using the most general effective Hamiltonian comprising scalar,vector and tensor type interactions, we have written the branching ratio, the forward-backward (FB) asymmetry and the normalized FB asymmetry as functions of the new Wilson coefficients. It is found that the branching ratio depends on all new coefficients,but the dependence of asymmetries on coefficients could be analyzed only for one Wilson coefficient.
The ratio of branching fractions of the decays $Lambda^{0}_{b}to pK^{-}e^{+}e^{-}$ and $Lambda^{0}_{b}to pK^{-}mu^{+}mu^{-}$, $R^{-1}_{pK}$, is measured for the first time using proton-proton collision data corresponding to an integrated luminosity of 4.7 $fb^{-1}$ recorded with the LHCb experiment at center-of-mass energies of 7, 8 and 13 TeV. In the dilepton mass-squared range $0.1 < q^{2} < 6.0$ $GeV^{2}/c^{4}$ and the $pK^{-}$ mass range $m(pK^{-}) < 2600$ $MeV/c^{2}$, the ratio of branching fractions is measured to be $R^{-1}_{pK} = 1.17 ^{+0.18}_{-0.16} pm 0.07$, where the first uncertainty is statistical and the second systematic. This is the first test of lepton universality with b baryons and the first observation of the decay $Lambda^{0}_{b}to pK^{-}e^{+}e^{-}$.
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