$D$-Meson Semileptonic Form Factors at Zero Momentum Transfer in (2+1+1)-Flavor Lattice QCD

We present a calculation of the $$D\to K \ell \nu$$ and $$D\to\pi \ell \nu$$ semileptonic form factors at $q^2=0$, which enable determinations of the CKM matrix elements $$\lvert{V_{cs}}\rvert$$ and $$\lvert{V_{cd}}\rvert$$, respectively. We use gauge-field configurations generated by the MILC collaboration with four flavors of highly-improved staggered (HISQ) quarks, analyzing several ensembles including those with physical pion masses and approximate lattice spacings ranging from 0.12~fm to 0.042~fm. We also use the HISQ action for the valence quarks. We employ twisted boundary conditions to calculate the form factors at zero momentum transfer directly. We use heavy-light-meson chiral perturbation theory modified for energetic pions and kaons, and supplemented by terms to describe the lattice-spacing dependence, to obtain preliminary results at the physical point and in the continuum limit.