Renormalization of heavy-light currents in moving NRQCD

TL;DR

This paper presents the one-loop renormalization of heavy-light currents in moving NRQCD, improving theoretical precision for lattice QCD calculations of B-meson decays relevant to Standard Model tests.

Contribution

The paper computes one-loop corrections to heavy-light vector and tensor operators in moving NRQCD, automating Feynman rule generation and addressing divergences for enhanced accuracy.

Findings

Calculated one-loop renormalization factors for heavy-light currents.

Implemented automated Feynman rule generation for complex actions.

Analyzed divergences in lattice and continuum loop integrals.

Abstract

Heavy-light decays such as $B \to \pi \ell \nu$, $B \to K^{*} \gamma$ and $B \to K^{(*)} \ell \ell$ can be used to constrain the parameters of the Standard Model and in indirect searches for new physics. While the precision of experimental results has improved over the last years this has still to be matched by equally precise theoretical predictions. The calculation of heavy-light form factors is currently carried out in lattice QCD. Due to its small Compton wavelength we discretize the heavy quark in an effective non-relativistic theory. By formulating the theory in a moving frame of reference discretization errors in the final state are reduced at large recoil. Over the last years the formalism has been improved and tested extensively. Systematic uncertainties are reduced by renormalizing the m(oving)NRQCD action and heavy-light decay operators. The theory differs from QCD only for large loop momenta at the order of the lattice cutoff and the calculation can be carried out in perturbation theory as an expansion in the strong coupling constant. In this paper we calculate the one loop corrections to the heavy-light vector and tensor operator. Due to the complexity of the action the generation of lattice Feynman rules is automated and loop integrals are solved by the adaptive Monte Carlo integrator VEGAS. We discuss the infrared and ultraviolet divergences in the loop integrals both in the continuum and on the lattice. The light quarks are discretized in the ASQTad and highly improved staggered quark (HISQ) action; the formalism is easily extended to other quark actions.