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Register a new userUpdate on $Bto D^ast ell u$ form factor at zero-recoil using the Oktay-Kronfeld action
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We present an update on the calculation of $bar{B}to D^ast ell bar{ u}$ semileptonic form factor at zero recoil using the Oktay-Kronfeld bottom and charm quarks on $N_f=2+1+1$ flavor HISQ ensembles generated by the MILC collaboration. Preliminary results are given for two ensembles with $aapprox 0.12$ and $0.09$ fm and $M_piapprox 310$ MeV. Calculations have been done with a number of valence quark masses, and the dependence of the form factor on them is investigated on the $aapprox 0.12$ fm ensemble. The excited state is controlled by using multistate fits to the three-point correlators measured at 4--6 source-sink separations.
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We report recent progress in calculating semileptonic form factors for the $bar{B} to D^ast ell bar{ u}$ and $bar{B} to D ell bar{ u}$ decays using the Oktay-Kronfeld (OK) action for bottom and charm quarks. We use the second order in heavy quark effective power counting $mathcal{O}(lambda^2)$ improved currents in this work. The HISQ action is used for the light spectator quarks. We analyzed four $2+1+1$-flavor MILC HISQ ensembles with $aapprox 0.09,mathrm{fm}$, $0.12,mathrm{fm}$ and $M_pi approx 220,mathrm{MeV}$, $310,mathrm{MeV}$: $a09m220$, $a09m310$, $a12m220$, $a12m310$. Preliminary results for $Bto D^astell u$ decays form factor $h_{A_1}(w)$ at zero recoil ($w=1$) are reported. Preliminary results for $B to D,ell u$ decays form factors $h_pm(w)$ over a kinematic range $1<w<1.3$ are reported as well.
We present preliminary blinded results from our analysis of the form factors for $Brightarrow D^astell u$ decay at non-zero recoil. Our analysis includes 15 MILC asqtad ensembles with $N_f=2+1$ flavors of sea quarks and lattice spacings ranging from $aapprox 0.15$ 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 $left|V_{cb}right|$ and $R(D^ast)$.
The current status of the lattice-QCD calculations of the form factors of the $Bto D^astell u$ semileptonic decay is reviewed. Particular emphasis is given to the most mature calculation at non-zero recoil coming from the Fermilab Lattice and MILC collaborations. Blinded, preliminary results for the form factors are shown, including a preliminary, but detailed error budget. The lattice results seem to favor a large slope at small recoil, in contrast to the latest untagged results coming from the Belle collaboration. A comprehensive comparison between the latest BGL $z$ expansions of Belle, Babar, the lattice and a joint BGL fit including lattice and Belle data is presented, and a roadmap to improve the current calculation is discussed. The current implications for $V_{cb}$ and $R(D^ast)$ are discussed.
The exclusive semileptonic decay $B rightarrow pi ell u$ is a key process for the determination of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$ from the comparison of experimental rates as a function of $q^2$ with theoretically determined form factors. The sensitivity of the form factors to the $u/d$ quark mass has meant significant systematic uncertainties in lattice QCD calculations at unphysically heavy pion masses. Here we give the first lattice QCD calculations of this process for u/d quark masses going down to their physical values, calculating the $f_0$ form factor at zero recoil to 3%. We are able to resolve a long-standing controversy by showing that the soft-pion theorem result $f_0(q^2_{max}) = f_B/f_{pi}$ does hold as $m_{pi} rightarrow 0$. We use the Highly Improved Staggered Quark formalism for the light quarks and show that staggered chiral perturbation theory for the $m_{pi}$ dependence is almost identical to continuum chiral perturbation theory for $f_0$, $f_B$ and $f_{pi}$. We also give results for other processes such as $B_s rightarrow K ell u$.
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 $R(D^ast)$.
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