arXiv subjects

Heechang Na

Publications and source records attributed to Heechang Na.

At least 19 recordsLinked to original sources

arXiv2018

Form factor ratios for $B_s \rightarrow K \, \ell \, ν$ and $B_s \rightarrow D_s \, \ell \, ν$ semileptonic decays and $|V_{ub}/V_{cb}|$

We present a lattice quantum chromodynamics determination of the ratio of the scalar and vector form factors for two semileptonic decays of the $B_s$ meson: $B_s \rightarrow K \ell ν$ and $B_s \rightarrow D_s \ell ν$. In conjunction with future experimental data, our results for these correlated form factors will provide a new method to extract $|V_{ub}/V_{cb}|$, which may elucidate the current tension between exclusive and inclusive determinations of these Cabibbo-Kobayashi-Maskawa mixing matrix parameters. In addition to the form factor results, we determine the ratio of the differential decay rates, and forward-backward and polarization asymmetries, for the two decays.

hep-lat↗

arXiv2017

$B_s \to D_s \ell ν$ Form Factors and the Fragmentation Fraction Ratio $f_s/f_d$

We present a lattice quantum chromodynamics determination of the scalar and vector form factors for the $B_s \rightarrow D_s \ell ν$ decay over the full physical range of momentum transfer. In conjunction with future experimental data, our results will provide a new method to extract $|V_{cb}|$, which may elucidate the current tension between exclusive and inclusive determinations of this parameter. Combining the form factor results at non-zero recoil with recent HPQCD results for the $B \rightarrow D \ell ν$ form factors, we determine the ratios $f^{B_s \rightarrow D_s}_0(M_π^2) / f^{B \rightarrow D}_0(M_K^2) = 1.000(62)$ and $f^{B_s \rightarrow D_s}_0(M_π^2) / f^{B \rightarrow D}_0(M_π^2) = 1.006(62)$. These results give the fragmentation fraction ratios $f_s/f_d = 0.310(30)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(6)_{\mathrm{theor.}}(38)_{\mathrm{latt.}} $ and $f_s/f_d = 0.307(16)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(23)_{\mathrm{theor.}}(44)_{\mathrm{latt.}}$, respectively. The fragmentation fraction ratio is an important ingredient in experimental determinations of $B_s$ meson branching fractions at hadron colliders, in particular for the rare decay ${\cal B}(B_s \rightarrow μ^+ μ^-)$. In addition to the form factor results, we make the first prediction of the branching fraction ratio $R(D_s) = {\cal B}(B_s\to D_sτν)/{\cal B}(B_s\to D_s\ellν) = 0.301(6)$, where $\ell$ is an electron or muon. Current experimental measurements of the corresponding ratio for the semileptonic decays of $B$ mesons disagree with Standard Model expectations at the level of nearly four standard deviations. Future experimental measurements of $R(D_s)$ may help understand this discrepancy.

hep-lat↗

arXiv2016

$B_{(s)}\to D_{(s)}$ semileptonic decays with NRQCD-HISQ valence quarks

We present a calculation of the form factors, $f_0$ and $f_+$, for the $B_{(s)} \to D_{(s)}$ semileptonic decays. Our work uses the MILC $n_f=2+1$ AsqTad configurations with NRQCD and HISQ valence quarks at four values of the momentum transfer $q^2$. We provide results for the chiral-continuum extrapolations of the scalar and vector form factors.

hep-lat↗

arXiv2016

Decay constants $f_B$ and $f_{B_s}$ and quark masses $m_b$ and $m_c$ from HISQ simulations

We present a progress report on our calculation of the decay constants $f_B$ and $f_{B_s}$ from lattice-QCD simulations with highly-improved staggered quarks. Simulations are carried out with several heavy valence-quark masses on $(2+1+1)$-flavor ensembles that include charm sea quarks. We include data at six lattice spacings and several light sea-quark masses, including an approximately physical-mass ensemble at all but the smallest lattice spacing, 0.03 fm. This range of parameters provides excellent control of the continuum extrapolation to zero lattice spacing and of heavy-quark discretization errors. Finally, using the heavy-quark effective theory expansion we present a method of extracting from the same correlation functions the charm- and bottom-quark masses as well as some low-energy constants appearing in the heavy-quark expansion.

hep-lat↗

arXiv2016

$B \rightarrow D l ν$ Form Factors at Non-Zero Recoil and Extraction of $|V_{cb}|$

We present a lattice QCD calculation of the $B \rightarrow D l ν$ semileptonic decay form factors $f_+(q^2)$ and $f_0(q^2)$ for the entire physical $q^2$ range. Non-relativistic QCD (NRQCD) bottom quarks and Highly Improved Staggered Quark (HISQ) charm and light quarks are employed together with $N_f = 2+1$ MILC gauge configurations. A joint fit to our lattice and BaBar experimental data allows an extraction of the CKM matrix element $|V_{cb}|$. We also determine the phenomenologically interesting ratio $R(D) = {\cal B}(B \rightarrow D τν_τ) / {\cal B}(B \rightarrow D l ν_l)$ ($l = e, μ$). We find $|V_{cb}|_{excl.}^{B \rightarrow D} = 0.0402(17)(13)$, where the first error consists of the lattice simulation errors and the experimental statistical error and the second error is the experimental systematic error. For the branching fraction ratio we find $R(D) = 0.300(8)$.

hep-lat↗

arXiv2015

Decay constants $f_B$ and $f_{B_s}$ from HISQ simulations

We give a progress report on a project aimed at a high-precision calculation of the decay constants $f_B$ and $f_{B_s}$ from simulations with HISQ heavy and light valence and sea quarks. Calculations are carried out with several heavy valence-quark masses on ensembles with 2+1+1 flavors of HISQ sea quarks at five lattice spacings and several light sea-quark mass ratios $m_{ud}/m_s$, including approximately physical sea-quark masses. This range of parameters provides excellent control of the continuum limit and of heavy-quark discretization errors. We present a preliminary error budget with projected uncertainties of 2.2~MeV and 1.5~MeV for $f_B$ and $f_{B_s}$, respectively.

hep-lat↗

arXiv2014

Searching for the $X(3872)$ and $Z_c^+(3900)$ on HISQ lattices

We present preliminary simulation results for the I = 0 charmonium state $X(3872)(1^{++})$ and the I = 1 charmonium state $Z_c^+(3900)(1^{+-})$. The study is performed on gauge field configurations with 2+1+1 flavors of highly improved staggered sea quarks (HISQ) with clover (Fermilab interpretation) charm quarks and HISQ light valence quarks. Since the $X(3872)$ lies very close to the open charm $D \bar D^*$ threshold, we use a combination of $\bar c c$ and $D \bar D^* + \bar D D^*$ interpolating operators. For the $Z_c^+(3900)$ we use a combination of $J/ψπ$ and $D \bar D^* + \bar D D^*$ channels. This is the first such study with HISQ sea quarks and light valence quarks. To this end, we describe a variational method for treating staggered quarks that incorporates both oscillating and non-oscillating components.

hep-lat↗

arXiv2014

$B_s \to K \ell ν$ form factors from lattice QCD

We report the first lattice QCD calculation of the form factors for the standard model tree-level decay $B_s\to K \ellν$. In combination with future measurement, this calculation will provide an alternative exclusive semileptonic determination of $|V_{ub}|$. We compare our results with previous model calculations, make predictions for differential decay rates and branching fractions, and predict the ratio of differential branching fractions between $B_s\to Kτν$ and $B_s\to Kμν$. We also present standard model predictions for differential decay rate forward-backward asymmetries, polarization fractions, and calculate potentially useful ratios of $B_s\to K$ form factors with those of the fictitious $B_s\toη_s$ decay. Our lattice simulations utilize NRQCD $b$ and HISQ light quarks on a subset of the MILC Collaboration's $2+1$ asqtad gauge configurations, including two lattice spacings and a range of light quark masses.

hep-lat↗

arXiv2014

Standard Model predictions for B -> Kll with form factors from lattice QCD

We calculate, for the first time using unquenched lattice QCD form factors, the Standard Model differential branching fractions $dB/dq^2(B \to Kll)$ for $l=e, μ, τ$ and compare with experimental measurements by Belle, BABAR, CDF, and LHCb. We report on $\mathcal{B}(B \to Kll)$ in $q^2$ bins used by experiment and predict $\mathcal{B}(B \to K ττ) = (1.44 \pm 0.15) 10^{-7}$. We also calculate the ratio of branching fractions $R^μ_e = 1.00023(63)$ and predict $R^τ_l = 1.159(40)$, for $l=e, μ$. Finally, we calculate the "flat term" in the angular distribution of the differential decay rate $F_H^{e, μ, τ}$ in experimentally motivated $q^2$ bins.

hep-ph↗

arXiv2013

B and Bs semileptonic decay form factors with NRQCD/HISQ quarks

We discuss our ongoing effort to calculate form factors for several B and Bs semileptonic decays. We have recently completed the first unquenched calculation of the form factors for the rare decay B -> K ll. Extrapolated over the full kinematic range of q^2 via model-independent z expansion, these form factor results allow us to calculate several Standard Model observables. We compare with experiment (Belle, BABAR, CDF, and LHCb) where possible and make predictions elsewhere. We discuss preliminary results for Bs -> K l nu which, when combined with anticipated experimental results, will provide an alternative exclusive determination of |Vub|. We are exploring the possibility of using ratios of form factors for this decay with those for the unphysical decay Bs -> eta_s as a means of significantly reducing form factor errors. We are also studying B -> pi l nu, form factors for which are combined with experiment in the standard exclusive determination of |Vub|. Our simulations use NRQCD heavy and HISQ light valence quarks on the MILC 2+1 dynamical asqtad configurations.

hep-lat↗

arXiv2013

Rare decay B -> K ll form factors from lattice QCD

We calculate, for the first time using unquenched lattice QCD, form factors for the rare decay B -> Kll in and beyond the Standard Model. Our lattice QCD calculation utilizes a nonrelativistic QCD formulation for the b valence quarks, the highly improved staggered quark formulation for the light valence quarks, and employs the MILC 2+1 asqtad ensembles. The form factor results, based on the z expansion, are valid over the full kinematic range of q^2. We construct the ratios f0/f+ and fT/f+, which are useful in constraining new physics and verifying effective theory form factor symmetry relations. We also discuss the calculation of Standard Model observables.

hep-lat↗

arXiv2013

$|V_{cd}|$ and $|V_{cs}|$ from Semileptonic and Leptonic decays using lattice QCD methods

In this paper, we review recent calculations of the CKM matrix elements $|V_{cd}|$ and $|V_{cs}|$ from semileptonic and leptonic decays of $D$ and $D_s$ mesons using lattice QCD methods.

hep-lat↗

arXiv2012

Precise Determinations of the Decay Constants of B and D mesons

Recently we studied the B, Bs, D and Ds meson decay constants using various treatments for the heavy quark. For B mesons, we determined fB, fBs, and fBs/fB with NRQCD bottom quarks. We then combined the ratio fBs/fB and another very precise determination from HPQCD for fBs using heavy HISQ quarks, and extracted fB with 2% total errors. We also calculated fD, fDs, and fDs/fD using HISQ charm quarks. Here we review our results and briefly discuss their implications for the determination of the CKM matrix elements |Vcd| and |Vcs|.

hep-lat↗

arXiv2012

Matching heavy-light currents with NRQCD and HISQ quarks

We calculate the one loop renormalisation parameters for the heavy-light axial-vector and vector currents using lattice perturbation theory. We use NonRelativistic QCD (NRQCD) heavy quarks and the Highly Improved Staggered Quark (HISQ) action for the light quarks. We present results for heavy-light currents with massless HISQ quarks and briefly discuss the extension to heavy-heavy currents with massive HISQ quarks.

hep-lat↗

arXiv2012

Form factors for B and B_s semileptonic decays with NRQCD/HISQ quarks

We discuss preliminaries of a calculation of the form factors for the semileptonic decays B -> pi lv, B_s -> K lv, and B -> K ll. We simulate with NRQCD heavy and HISQ light valence quarks on the MILC 2+1 dynamical asqtad configurations. The form factors are calculated over a range of momentum transfer to allow determination of their shape and the extraction of |V_ub|. Additionally, we are calculating ratios of these form factors to those for the unphysical decay B_s -> eta_s. We are studying the possibility of combining these precisely determined ratios with future calculations of B_s ->eta_s using HISQ b-quarks to generate form factors with significantly reduced errors.

hep-lat↗

arXiv2012

The B and B_s Meson Decay Constants from Lattice QCD

We present a new determination of the B and B_s meson decay constants using NRQCD b-quarks, HISQ light and strange valence quarks and the MILC collaboration N_f=2+1 lattices. The new calculations improve on HPQCD's earlier work with NRQCD b-quarks by replacing AsqTad with HISQ valence quarks, by including a more chiral MILC fine ensemble in the analysis, and by employing better tuned quark masses and overall scale. We find f_B = 0.191(9)GeV, f_{B_s} = 0.228(10)GeV and f_{B_s}/f_B = 1.188(18). Combining the new value for f_{B_s}/f_B with a recent very precise determination of the B_s meson decay constant based on HISQ b-quarks, f_{B_s} = 0.225(4)GeV, leads to f_B = 0.189(4)GeV. With errors of just 2.1% this represents the most precise f_B available today.

hep-lat↗

arXiv2012

|V_cd| from D Meson Leptonic Decays

We present an update of the D meson decay constant f_D using the Highly Improved Staggered Quark (HISQ) action for valence charm and light quarks on MILC N_f = 2+1 lattices. The new determination incorporates HPQCD's improved scale r_1^{N_f = 2+1} = 0.3133(23) fm, accurately retuned bare charm quark masses and data from an ensemble that is more chiral than in our previous calculations. We find f_D = 208.3(3.4) MeV. Combining the new f_D with D -> μν branching fraction data from CLEO-c, we extract the CKM matrix element |V_cd| = 0.223(10)_{exp.}(4)_{lat.}. This value is in excellent agreement with |V_cd| from D semileptonic decays and from neutrino scattering experiments and has comparable total errors. We determine the ratio between semileptonic form factor and decay constant and find [f^{D -> π}_+(0) / f_D ]_{lat.} = 3.20(15) GeV^{-1} to be compared with the experimental value of [f^{D -> π}_+(0) / f_D ]_{exp.} = 3.19(18) GeV^{-1}. Finally, we mention recent preliminary but already more accurate D -> μν branching fraction measurements from BES III and discuss their impact on precision |V_cd| determinations in the future.

hep-lat↗

arXiv2012

Approaching Conformality with Ten Flavors

We present first results for lattice simulations, on a single volume, of the low-lying spectrum of an SU(3) Yang-Mills gauge theory with ten light fermions in the fundamental representation. Fits to the fermion mass dependence of various observables are found to be globally consistent with the hypothesis that this theory is within or just outside the strongly-coupled edge of the conformal window, with mass anomalous dimension consistent with 1 over the range of scales simulated. We stress that we cannot rule out the possibility of spontaneous chiral-symmetry breaking at scales well below our infrared cutoff. We discuss important systematic effects, including finite-volume corrections, and consider directions for future improvement.

hep-ph↗