arXiv subjects

Jongjeong Kim

Publications and source records attributed to Jongjeong Kim.

At least 19 recordsLinked to original sources

arXiv2017

Masses and decay constants of pions and kaons in mixed-action staggered chiral perturbation theory

Lattice QCD calculations with different staggered valence and sea quarks can be used to improve determinations of quark masses, Gasser-Leutwyler couplings, and other parameters relevant to phenomenology. We calculate the masses and decay constants of flavored pions and kaons through next-to-leading order in staggered-valence, staggered-sea mixed-action chiral perturbation theory. We present the results in the valence-valence and valence-sea sectors, for all tastes. As in unmixed theories, the taste-pseudoscalar, valence-valence mesons are exact Goldstone bosons in the chiral limit, at non-zero lattice spacing. The results reduce correctly when the valence and sea quark actions are identical, connect smoothly to the continuum limit, and provide a way to control light quark and gluon discretization errors in lattice calculations performed with different staggered actions for the valence and sea quarks.

hep-lat↗

arXiv2015

Kaon BSM B-parameters using improved staggered fermions from $N_f=2+1$ unquenched QCD

We present results for the matrix elements of the additional $ΔS=2$ operators that appear in models of physics beyond the Standard Model (BSM), expressed in terms of four BSM $B$-parameters. Combined with experimental results for $ΔM_K$ and $ε_K$, these constrain the parameters of BSM models. We use improved staggered fermions, with valence HYP-smeared quarks and $N_f=2+1$ flavors of "asqtad" sea quarks. The configurations have been generated by the MILC collaboration. The matching between lattice and continuum four-fermion operators and bilinears is done perturbatively at one-loop order. We use three lattice spacings for the continuum extrapolation: $a\approx 0.09$, $0.06$ and $0.045\;$fm. Valence light-quark masses range down to $\approx m_s^{\rm phys}/13$ while the light sea-quark masses range down to $\approx m_s^{\rm phys}/20$. Compared to our previous published work, we have added four additional lattice ensembles, leading to better controlled extrapolations in the lattice spacing and sea-quark masses. We report final results for two renormalization scales, $μ=2\;\text{GeV}$ and $3\;\text{GeV}$, and compare them to those obtained by other collaborations. Agreement is found for two of the four BSM $B$-parameters ($B_2$ and $B_3^\text{SUSY}$). The other two ($B_4$ and $B_5$) differ significantly from those obtained using RI-MOM renormalization as an intermediate scheme, but are in agreement with recent preliminary results obtained by the RBC-UKQCD collaboration using RI-SMOM intermediate schemes.

hep-lat↗

arXiv2014

Non-perturbative Renormalization of Four-Fermion Operators Relevant to $B_K$ with Staggered Quarks

We present preliminary results of matching factors of the four-fermion operators relevant to $B_K$, which are obtained using the non-perturbative renormalization (NPR) method in the RI-MOM scheme with HYP-smeared improved staggered fermions. We use the MILC asqtad coarse ($a \cong 0.12\,$fm) ensembles with $20^3 \times 64$ geometry and $am_{\ell}/am_s = 0.01/0.05$. We compare NPR results with those of one-loop perturbative matching.

hep-lat↗

arXiv2014

Toolkit for staggered $ΔS=2$ matrix elements

A recent numerical lattice calculation of the kaon mixing matrix elements of general $ΔS=2$ four-fermion operators using staggered fermions relied on two auxiliary theoretical calculations. Here we describe the methodology and present the results of these two calculations. The first concerns one-loop matching coefficients between staggered lattice operators and the corresponding continuum operators. Previous calculations with staggered fermions have used a non-standard regularization scheme for the continuum operators, and here we provide the additional matching factors needed to connect to the standard regularization scheme. This is the scheme in which two-loop anomalous dimensions are known. We also observe that all previous calculations of this operator matching using staggered fermions have overlooked one matching step in the continuum. This extra step turns out to have no impact on three of the five operators (including that relevant for $B_K$), but does affect the other two operators. The second auxiliary calculation concerns the two-loop renormalization group (RG) evolution equations for the $B$-parameters of the $ΔS=2$ operators. For one pair of operators, the standard analytic solution to the two-loop RG equations fails due to a spurious singularity introduced by the approximations made in the calculation. We give a non-singular expression derived using analytic continuation, and check the result using a numerical solution to the RG equations. We also describe the RG evolution for "golden" combinations of $B$-parameters, and give numerical results for RG evolution matrices needed in the companion lattice calculation.

hep-lat↗

arXiv2014

Improved determination of $B_K$ with staggered quarks

We present results for the kaon mixing parameter $B_K$ obtained using improved staggered fermions on a much enlarged set of MILC asqtad lattices. Compared to our previous publication, which was based largely on a single ensemble at each of the three lattice spacings $a\approx 0.09\;$fm, $0.06\;$fm and $0.045\;$fm, we have added seven new fine and four new superfine ensembles, with a range of values of the light and strange sea-quark masses. We have also increased the number of measurements on one of the original ensembles. This allows us to do controlled extrapolations in the light and strange sea-quark masses, which we do simultaneously with the continuum extrapolation. This reduces the extrapolation error and improves the reliability of our error estimates. Our final result is $\hat{B}_K = 0.7379 \pm 0.0047 (\text{stat}) \pm 0.0365 (\text{sys})$.

hep-lat↗

arXiv2013

Heavy-meson semileptonic decays for the Standard Model and beyond

We calculate the form factors for the semileptonic decays $B_s\to K\ellν$ and $B\to K\ell\ell$ with lattice QCD. We work at several lattice spacings and a range of light quark masses, using the MILC 2+1-flavor asqtad ensembles. We use the Fermilab method for the $b$ quark. We obtain chiral-continuum extrapolations for $E_K$ up to $\sim1.2$ GeV and then extend to the entire kinematic range with the model-independent $z$ expansion.

hep-lat↗

arXiv2013

Flavored pions and kaons at next-to-leading order in mixed-action staggered chiral perturbation theory

Different versions of improved staggered fermions can be used as valence quarks to reduce discretization effects in lattice QCD calculations while increasing statistics on existing staggered gauge ensembles. Such mixed-action simulations can be used to improve determinations of light quark masses, Gasser-Leutwyler couplings, decay constants, and other parameters relevant to particle phenomenology. We recall the generalization of ordinary, unmixed staggered chiral perturbation theory required to describe data from lattice calculations with a mixed action such as with HYP staggered valence quarks and asqtad sea quarks. We calculate the next-to-leading order loop diagrams contributing to the masses and decay constants of the flavored pseudo-Goldstone bosons of all tastes and here report results for the decay constants and valence-valence masses.

hep-lat↗

arXiv2013

Update on $B_K$ and $\varepsilon_K$ with staggered quarks

We update our results for $B_K$ obtained using HYP-smeared staggered valence quarks on the MILC asqtad lattices. In the last year, we have added 5 new measurments on the fine ($a\approx 0.09\;$fm) ensembles, and 2 new measurements on the superfine ($a\approx 0.06\;$fm) ensembles. These allow a simultaneous extrapolation in $a^2$ and sea quark masses, reducing the corresponding systematic error significantly. Our updated result is $\hat{B}_K = 0.738 \pm 0.005 (\text{stat}) \pm 0.034 (\text{sys})$.

hep-lat↗

arXiv2013

Beyond the Standard Model B-parameters with improved staggered fermions in $N_f=2+1$ QCD

We calculate the kaon mixing B-parameters for operators arising generically in theories of physics beyond the standard model. We use HYP-smeared improved staggered fermions on the $N_f = 2+1$ MILC asqtad lattices. Operator matching is done perturbatively at one-loop order. Chiral extrapolations are done using "golden combinations" in which one-loop chiral logarithms are absent. For the combined sea-quark mass and continuum extrapolation, we use three lattice spacings: $a \approx 0.045, 0.06$ and $0.09 \text{fm}$. Our results have a total error of 5-6%, which is dominated by the systematic error from matching and continuum extrapolation. For two of the BSM $B$-parameters, we agree with results obtained using domain-wall and twisted-mass dynamical fermions, but we disagree by $(4-5)σ$ for the other two.

hep-lat↗

arXiv2013

Non-perturbative Renormalization of Bilinear Operators with Improved Staggered Quarks

We present renormalization factors for the bilinear operators obtained using the non-perturbative renormalization method (NPR) in the RI-MOM scheme with improved staggered fermions on the MILC asqtad lattices ($N_f = 2+1$). We use the MILC coarse ensembles with $20^3 \times 64$ geometry and $am_{\ell}/am_s = 0.01/0.05$. We obtain the wave function renormalization factor $Z_q$ from the conserved vector current and the mass renormalization factor $Z_m$ from the scalar bilinear operator. We also present preliminary results of renormalization factors for other bilinear operators.

hep-lat↗

arXiv2013

Neutral kaon mixing from new physics: matrix elements in $N_f=2+1$ QCD

We present results for matrix elements of $ΔS=2$ four-fermion operators arising generically in models of new physics. These are needed to constrain such models using the measured values of $\varepsilon_K$ and $ΔM_K$. We use lattice QCD with 2+1 flavors of improved staggered fermions on lattices generated by the MILC collaboration. We extrapolate to the continuum from three lattice spacings ranging down to $a\approx 0.045\;$fm. Total errors are $\sim 5-6%$, arising primarily from our use of one-loop matching between lattice and continuum operators. For two of the matrix elements, our results disagree significantly from those obtained using different fermion discretizations.

hep-lat↗

arXiv2013

Kaon semileptonic vector form factor and determination of |V_{us}| using staggered fermions

Using staggered fermions and twisted boundary conditions, we calculate the K meson semileptonic decay vector form factor at zero momentum transfer. The HISQ formulation is used for the valence quarks, while the sea quarks are simulated with the asqtad action (MILC N_f=2+1 configurations). For the chiral and continuum extrapolation we use two-loop continuum CHPT, supplemented by partially quenched staggered CHPT at one loop. Our result is f_+^{Kπ}(0) = 0.9667+-0.0023+-0.0033, where the first error is statistical and the second is the sum in quadrature of the systematic uncertainties. This result is the first N_f=2+1 calculation with two lattice spacings and a controlled continuum extrapolation. It is also the most precise result to date for the vector form factor and, although the central value is larger than previous unquenched lattice calculations, it is compatible with them within errors. Combining our value for f_+^{Kπ}(0) with the latest experimental measurements of K semileptonic decays, we obtain |V_{us}| = 0.2238+-0.0009+-0.0005, where the first error is from f_+^{Kπ}(0) and the second one is experimental. As a byproduct of our calculation, we obtain the combination of low-energy constants [C_{12}^r+C_{34}^r-(L_5^r)^2](M_ρ) = (3.62+-1.00)x10^{-6}.

hep-lat↗

arXiv2012

Recent progress in calculation of $B_K$ using staggered fermions

We report on recent progress in the calculation of $B_K$ using HYP-smeared improved staggered fermions on the MILC asqtad lattices. We have added measurements on fine ($a\sim 0.09 $fm) and superfine ($a\sim 0.06 $fm) ensembles at different values of the light sea quark mass ($a m_\ell$), as well as increased the statistics on some other ensembles. We find that the results on the fine lattices show a significantly stronger $am_\ell$ dependence than those on the superfine and coarse ($a\sim 0.12 $fm) lattices. We discuss different methods for accounting for these new results when doing the $a m_\ell$ and continuum extrapolations.

hep-lat↗

arXiv2012

Beyond the Standard Model corrections to $K^0-\bar{K}^0$ mixing

We calculate the B-parameters for operators arising in theories of new physics beyond the standard model (BSM) using HYP-smeared improved staggered fermions on the MILC asqtad lattices with N_f = 2+1 flavors. We use three different lattice spacings (a ~ 0.045, 0.06 and 0.09 fm) at a fixed ratio of light to strange quarks, m_l/m_s = 1/5, to obtain the continuum results. Operator matching is done using perturbative matching at one-loop order, and results are run to 2 or 3 GeV using two-loop running in the MS-bar scheme. For the chiral and continuum extrapolations, we use SU(2) staggered chiral perturbation theory. We present preliminary results with only statistical errors.

hep-lat↗

arXiv2012

Refining new-physics searches in B -> D tau nu decay with lattice QCD

The semileptonic decay channel B -> D tau nu is sensitive to the presence of a scalar current, such as that mediated by a charged-Higgs boson. Recently the BaBar experiment reported the first observation of the exclusive semileptonic decay B -> D tau nu, finding an approximately 2-sigma disagreement with the Standard-Model prediction for the ratio R(D)=BR(B->D tau nu)/BR(B->D l nu), where l=e,mu. We compute this ratio of branching fractions using hadronic form factors computed in unquenched lattice QCD and obtain R(D) = 0.316(12)(7), where the errors are statistical and total systematic, respectively. This result is the first Standard-Model calculation of R(D) from ab initio full QCD. Its error is smaller than that of previous estimates, primarily due to the reduced uncertainty in the scalar form factor f_0(q^2). Our determination of R(D) is approximately 1-sigma higher than previous estimates and, thus, reduces the tension with experiment. We also compute R(D) in models with electrically charged scalar exchange, such as the type II two-Higgs doublet model. Once again, our result is consistent with, but approximately 1-sigma higher than, previous estimates for phenomenologically relevant values of the scalar coupling in the type II model. As a byproduct of our calculation, we also present the Standard-Model prediction for the longitudinal polarization ratio P_L (D)= 0.325(4)(3).

hep-ph↗

arXiv2012

B_s->D_s/B->D Semileptonic Form-Factor Ratios and Their Application to BR(B^0_s->μ^+μ^-)

We calculate form-factor ratios between the semileptonic decays \bar{B}->D^+\ell^-\barν and \bar{B}_s->D_s^+\ell^-\barν with lattice QCD. These ratios are a key theoretical input in a new strategy to determine the fragmentation fractions of the neutral B decays, which are needed for measurements of BR(B^0_s-> μ^+μ^-). We use the MILC ensembles of gauge configurations with 2+1 flavors of sea quarks at two lattice spacings of approximately 0.12 fm and 0.09 fm. We use the model-independent z-parametrization to extrapolate our simulation results at small recoil toward maximum recoil. Our results for the form-factor ratios are $f_0^{(s)}(M^2_π)/f_0^{(d)}(M^2_K) =1.046(44)_{stat.}(15)_{syst.}$ and $f_0^{(s)}(M^2_π)/f_0^{(d)}(M^2_π)=1.054(47)_{stat.}(17)_{syst.}$. In contrast to a QCD sum-rule calculation, no significant departure from U-spin (d<->s) symmetry is observed.

hep-lat↗

arXiv2012

Kaon $B$-parameter from improved staggered fermions in $N_f=2+1$ QCD

We present a calculation of the kaon $B$-parameter, $B_K$, using lattice QCD. We use improved staggered valence and sea fermions, the latter generated by the MILC collaboration with $N_f=2+1$ light flavors. To control discretization errors, we use four different lattice spacings ranging down to $a\approx 0.045\;$fm. The chiral and continuum extrapolations are done using SU(2) staggered chiral perturbation theory. Our final result is $\hat{B}_K = 0.727 \pm 0.004 (\text{stat}) \pm 0.038 (\text{sys})$, where the dominant systematic error is from our use of truncated (one-loop) matching factors.

hep-lat↗

arXiv2011

Continuum extrapolation of $B_K$ with staggered fermions

We report on recent progress in the calculation of $B_K$ using HYP-smeared staggered fermions on the MILC asqtad lattices. Our main focus is on the continuum extrapolation, which is done using (up to) four different lattice spacings---$a\approx$ 0.12, 0.09, 0.06 and 0.045 fm. Since Lattice 2010, we have reduced the statistical errors on the $a\approx 0.09\;$fm lattices by a factor of $\sim 3$, and roughly doubled the size of the $a\approx0.045\;$fm ensemble. We find that these improvements have a very significant impact on the continuum extrapolation, with the $a\approx 0.12\;$fm data lying outside the range of applicability of simple functional forms. Hence we use only the three smallest lattice spacings to perform the extrapolation, finding $\hat{B}_K = B_K(\text{RGI}) = 0.725 \pm 0.004(\text{stat}) \pm 0.038(\text{sys}) $. This value is consistent with our published value from 2010 (based the three coarsest lattice spacings), but has smaller errors.

hep-lat↗