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Register a new userMeasurement of $mathcal{R}(D)$ and $mathcal{R}(D^{ast})$ with a semileptonic tagging method
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We report a measurement of the ratios of branching fractions $mathcal{R}(D) = {cal B}(bar{B} to D tau^- bar{ u}_{tau})/{cal B}(bar{B} to D ell^- bar{ u}_{ell})$ and $mathcal{R}(D^{ast}) = {cal B}(bar{B} to D^* tau^- bar{ u}_{tau})/{cal B}(bar{B} to D^* ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ events recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. The analysis utilizes a method where the tag-side $B$ meson is reconstructed in a semileptonic decay mode, and the signal-side $tau$ is reconstructed in a purely leptonic decay. The measured values are $mathcal{R}(D)= 0.307 pm 0.037 pm 0.016$ and $mathcal{R}(D^{ast})= 0.283 pm 0.018 pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within $0.2$ and $1.1$ standard deviations, respectively, while their combination agrees with the Standard Model predictions within $1.2$ standard deviations.
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The experimental results on the ratios of branching fractions $mathcal{R}(D) = {cal B}(bar{B} to D tau^- bar{ u}_{tau})/{cal B}(bar{B} to D ell^- bar{ u}_{ell})$ and $mathcal{R}(D^*) = {cal B}(bar{B} to D^* tau^- bar{ u}_{tau})/{cal B}(bar{B} to D^* ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon, show a long-standing discrepancy with the Standard Model predictions, and might hint to a violation of lepton flavor universality. We report a new simultaneous measurement of $mathcal{R}(D)$ and $mathcal{R}(D^*)$, based on a data sample containing $772 times 10^6$ $Bbar{B}$ events recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. In this analysis the tag-side $B$ meson is reconstructed in a semileptonic decay mode and the signal-side $tau$ is reconstructed in a purely leptonic decay. The measured values are $mathcal{R}(D)= 0.307 pm 0.037 pm 0.016$ and $mathcal{R}(D^*) = 0.283 pm 0.018 pm 0.014$, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the Standard Model predictions within $0.2$, $1.1$ and $0.8$ standard deviations for $mathcal{R}(D)$, $mathcal{R}(D^*)$ and their combination, respectively. This work constitutes the most precise measurements of $mathcal{R}(D)$ and $mathcal{R}(D^*)$ performed to date as well as the first result for $mathcal{R}(D)$ based on a semileptonic tagging method.
We report the first direct measurement of the inclusive branching fraction ${mathcal B}(B_s rightarrow D_s X)$ via $B_s$ tagging in $e^+e^-toUpsilon$(5S) events. Tagging is accomplished through a partial reconstruction of semileptonic decays $B_s rightarrow D_s X ell u$, where $X$ denotes unreconstructed additional hadrons or photons and $ell$ is an electron or muon. With 121.4 fb$^{-1}$ of data collected at the $Upsilon$(5S) resonance by the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider, we obtain ${mathcal B}(B_s rightarrow D_s X)$ = $(61.6 pm 5.3 pm 2.1)$%, where the first uncertainty is statistical and the second is systematic.
We estimate contributions from Kaluza-Klein excitations of gauge bosons and physical charge scalar for the explanation of the lepton flavor universality violating excess in the ratios $mathcal{R}(D)$ and $mathcal{R}(D^*)$ in 5 dimensional Universal Extra Dimensional scenario with non-vanishing boundary localized terms. This model is conventionally known as non-minimal Universal Extra Dimensional model. We obtain the allowed parameter space in accordance with constraints coming from $B_c to tau u$ decay, as well as those from the electroweak precision tests.
We present nearly final results from our analysis of the form factors for $Bto D^astell u$ decay at nonzero recoil. Our analysis includes 15 MILC asqtad ensembles with $N_f=2+1$ flavors of sea quarks and lattice spacings ranging from $aapprox0.15$ fm down to $0.045$ fm. The valence light quarks employ the asqtad action, whereas the $b$ and $c$ quarks are treated using the Fermilab action. We discuss the impact that our results will have on $ |V_{cb} |$ and $R(D^ast)$.
Recently, the deviation of the ratios $R(D)$, $R(D^{*})$ and $R(J/psi)$ have been found between experimental data and the Standard Model predictions, which may be the hint of New Physics. In this work, we calculate these ratios within the Standard Model by using the improved instantaneous Bethe-Salpeter method. The emphasis is pad to the relativistic correction of the form factors. The results are $R(D)=0.312 ^{+0.006}_{-0.007}$, $R(D^*)= 0.249^{+0.001}_{-0.002}$, $R(D_s)=0.320 ^{+0.009}_{-0.009}$, $R(D^*_s)=0.251 ^{+0.002}_{-0.003}$, $R(eta_c)=0.384 ^{+0.032}_{-0.042}$, and $R(J/psi)=0.267 ^{+0.009}_{-0.011}$, which are consistent with predictions of other models and the experimental data. The semileptonic decay rates and corresponding form factors at zero recoil are also given.
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