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Andrew Lytle

Publications and source records attributed to Andrew Lytle.

31 records · Page 2Linked to original sources
arXiv2020

$B_c \rightarrow J/ψ$ Form Factors for the full $q^2$ range from Lattice QCD

We present the first lattice QCD determination of the $B_c \rightarrow J/ψ$ vector and axial-vector form factors. These will enable experimental information on the rate for $B_c$ semileptonic decays to $J/ψ$ to be converted into a value for $V_{cb}$. Our calculation covers the full physical $q^2$ range of the decay and uses non-perturbatively renormalised lattice currents. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks on the second generation MILC ensembles of gluon field configurations including $u$, $d$, $s$ and $c$ HISQ sea quarks. Our HISQ heavy quarks have masses ranging upwards from that of $c$; we are able to reach that of the $b$ on our finest lattices. This enables us to map out the dependence on heavy quark mass and determine results in the continuum limit at the $b$. We use our form factors to construct the differential rates for $B_c^- \rightarrow J/ψμ^- \barν_μ$ and obtain a total rate with $7\%$ uncertainty: $Γ(B_c^-\rightarrow J/ψμ^-\barν_μ)/|η_{\mathrm{EW}}V_{cb}|^2 = 1.73(12)\times 10^{13} ~\mathrm{s}^{-1}$. Including values for $V_{cb}$, $η_{\mathrm{EW}}$ and $τ_{B_c}$ yields a branching fraction for this decay mode of 0.0150(11)(10)(3) ~with uncertainties from lattice QCD, $η_\mathrm{EW}V_{cb}$ and $τ_{B_c}$ respectively.

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arXiv2020

Lattice $B \to D^{(*)}$ form factors, $R(D^{(*)})$, and $|V_{cb}|$

I discuss recent progress in lattice calculations of $B \to D^{(*)} \ell ν$ form factors, important for the precision determination of $|V_{cb}|$ in the Standard Model (SM), and for testing SM expectations of lepton flavor universality in observables $R(D^{(*)})$. I also discuss progress in calculations of the related $b \to c$ semileptonic decays $B_s \to D_s^{(*)} \ell ν$ and $B_c \to J/ψ\, \ell ν$ now experimentally accessible at the LHC.

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arXiv2020

Nucleon mass and isovector couplings in 2+1-flavor dynamical domain-wall lattice QCD near physical mass

We report nucleon mass, isovector vector and axial-vector charges, and tensor and scalar couplings, calculated using two recent 2+1-flavor dynamical domain-wall fermions lattice-QCD ensembles generated jointly by the RIKEN-BNL-Columbia and UKQCD collaborations. These ensembles were generated with Iwasaki $\times$ dislocation-suppressing-determinant-ratio gauge action at inverse lattice spacing of 1.378(7) GeV and pion mass values of 249.4(3) and 172.3(3) MeV. The nucleon mass extrapolates to a value $m_N = 0.950(5)$ GeV at physical point. The isovector vector charge renormalizes to unity in the chiral limit, narrowly constraining excited-state contamination in the calculation. The ratio of the isovector axial-vector to vector charges shows a deficit of about ten percent. The tensor coupling no longer depends on mass and extrapolates to 1.04(5) in $\overline {\rm MS}$ 2-GeV renormalization at physical point, in a good agreement with the value obtained at the lightest mass in our previous calculations and other calculations that followed. The scalar charge, though noisier, does not show mass dependence and is in agreement with other calculations.

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arXiv2019

$χ$SF near the electroweak scale

We employ the chirally rotated Schrödinger functional ($χ$SF) to study two-point fermion bilinear correlation functions used in the determination of $Z_{A,V,S,P,T}$ on a series of well-tuned ensembles. The gauge configurations, which span renormalisation scales from 4 to 70~GeV, are generated with $N_{\rm f}=3$ massless flavors and Schrödinger Functional (SF) boundary conditions. Valence quarks are computed with $χ$SF boundary conditions. We show preliminary results on the tuning of the $χ$SF Symanzik coefficient $z_f$ and the scaling of the axial current normalization $Z_{\rm A}$. Moreover we carry out a detailed comparison with the expectations from one-loop perturbation theory. Finally we outline how automatically $\mathrm{O}(a)$-improved $B_{\rm K}$ matrix elements, including BSM contributions, can be computed in a $χ$SF renormalization scheme.

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arXiv2018

$B_c$ spectroscopy using highly improved staggered quarks

We report on a calculation of $B_c$ ground state and radial excitation energies, obtained from heavy-charm highly improved staggered quark (HISQ) correlators computed on MILC gauge ensembles, with lattice spacings down to $a=0.044$ fm. Using HISQ valence quarks on progressively finer lattices allows us to simulate up to the $b$-quark mass. In particular we focus on the $B_c(2S)$ energy, which we compare with O(α_s)-improved non-relativistic QCD results computed on the same ensembles and recent experimental results from ATLAS.

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arXiv2017

Light meson form factors at high $Q^2$ from lattice QCD

Measurements and theoretical calculations of meson form factors are essential for our understanding of internal hadron structure and QCD, the dynamics that bind the quarks in hadrons. The pion electromagnetic form factor has been measured at small space-like momentum transfer $|q^2| < 0.3$~GeV$^2$ by pion scattering from atomic electrons and at values up to $2.5$~GeV$^2$ by scattering electrons from the pion cloud around a proton. On the other hand, in the limit of very large (or infinite) $Q^2=-q^2$, perturbation theory is applicable. This leaves a gap in the intermediate $Q^2$ where the form factors are not known. As a part of their 12 GeV upgrade Jefferson Lab will measure pion and kaon form factors in this intermediate region, up to $Q^2$ of $6$~GeV$^2$. This is then an ideal opportunity for lattice QCD to make an accurate prediction ahead of the experimental results. Lattice QCD provides a from-first-principles approach to calculate form factors, and the challenge here is to control the statistical and systematic uncertainties as errors grow when going to higher $Q^2$ values. Here we report on a calculation that tests the method using an $η_s$ meson, a 'heavy pion' made of strange quarks, and also present preliminary results for kaon and pion form factors. We use the $n_f=2+1+1$ ensembles made by the MILC collaboration and Highly Improved Staggered Quarks, which allows us to obtain high statistics. The HISQ action is also designed to have small discretisation errors. Using several light quark masses and lattice spacings allows us to control the chiral and continuum extrapolation and keep systematic errors in check.

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arXiv2016

Looking at the deconfinement transition using Wilson flow

Wilson flow is an effective tool for constructing renormalized composite operators. We explore use of the Wilson flow to construct renormalized order parameters for the deconfinement transition in SU(3) gauge theory. We discuss renormalization of the Polyakov loop, and of gluon condensates.

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arXiv2016

Using Wilson flow to study the SU(3) deconfinement transition

We explore the use of Wilson flow to study the deconfinement transition in SU(3) gauge theory. We use the flowed Polyakov loop as a renormalized order parameter for the transition, and use it to renormalize the Polyakov loop. We also study the flow properties of the electric and magnetic gluon condensates, and demonstrate that the difference of the flowed operators shows rapid change across the transition point.

hep-lat↗
arXiv2016

$B_c$ decays from highly improved staggered quarks and NRQCD

We calculate semileptonic form factors for the decays $B_c \to η_c \, l ν$ and $B_c \to J/ψ\, l ν$ over the entire $q^2$ range, using a highly improved lattice quark action for charm at several lattice spacings down to $a=0.045$ fm. We have two ways of treating the $b$ quark: either with an $O(α_s)$ improved NRQCD formalism or by extrapolating a heavy mass $m_h$ to $m_b$ in the relativistic formalism. Comparison of the two approaches provides an important cross-check of methodologies in lattice QCD. Nonperturbative renormalisation of the currents in the relativistic theory also allows us then to fix NRQCD-charm normalisation for $b$ to $c$ decays such as $B \to D$ and $B \to D^*$.

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arXiv2016

Nucleon structure from 2+1-flavor dynamical DWF ensembles

Nucleon isovector vector- and axialvector-current form factors, the renormalized isovector transversity and scalar charge, and the bare quark momentum and helicity moments of isovector structure functions are reported with improved statistics from two recent RBC+UKQCD 2+1-flavor dynamical domain-wall fermions ensembles: Iwasaki\(\times\)DSDR gauge \(32^3\times64\) at inverse lattice spacing of 1.38 GeV and pion mass of 249 and 172 MeV.

hep-lat↗
arXiv2016

Semileptonic $B_c$ decays from full lattice QCD

We present first lattice QCD results for semileptonic form factors for the decays $B_c \to η_c l ν$ and $B_c \to J/ψl ν$ over the full $q^2$ range, using both improved non-relativistic QCD (NRQCD) and fully relativistic (HISQ) formalisms. These can be viewed as prototype calculations for pseudoscalar to pseudoscalar and pseudoscalar to vector decays involving a $b \to c$ transition. In particular we can use information from the relativistic computations to fix the NRQCD current normalisations, which can then be used in improved computations of decays such as $B \to D l ν$ and $B \to D^* l ν$.

hep-lat↗
arXiv2015

Wilson flow with naive staggered quarks

Scale setting for QCD with two flavours of staggered quarks is examined using Wilson flow over a factor of four change in both the lattice spacing and the pion mass. The statistics needed to keep the errors in the flow scale fixed is found to increase approximately as the inverse square of the lattice spacing. Tree level improvement of the scales t_0 and w_0 is found to be useful in most of the range of lattice spacings we explore. The scale uncertainty due to remaining lattice spacing effects is found to be about 3%. The ratio w_0/\sqrt{t_0} is N_f dependent and we find its continuum limit to be 1.106 \pm 0.007 (stat) \pm 0.005 (syst) for m_πw_0 \simeq 0.3.

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