Bottomonium spectrum at order v^6 from domain-wall lattice QCD: precise results for hyperfine splittings

TL;DR

This paper presents precise lattice QCD calculations of the bottomonium spectrum, including hyperfine splittings, using domain-wall fermions and NRQCD, achieving results in excellent agreement with experimental data.

Contribution

The study introduces high-precision bottomonium spectrum calculations at order v^6 using domain-wall lattice QCD and improved methods for hyperfine splitting predictions.

Findings

Lattice results agree with experimental bottomonium energy splittings.

Calculated 1S hyperfine splitting: 60.3 MeV with uncertainties.

Predicted 2S hyperfine splitting: 23.5 MeV with uncertainties.

Abstract

The bottomonium spectrum is computed in dynamical 2+1 flavor lattice QCD, using NRQCD for the b quarks. The main calculations in this work are based on gauge field ensembles generated by the RBC and UKQCD collaborations with the Iwasaki action for the gluons and a domain-wall action for the sea quarks. Lattice spacing values of approximately 0.08 fm and 0.11 fm are used, and simultaneous chiral extrapolations to the physical pion mass are performed. As a test for gluon discretization errors, the calculations are repeated on two ensembles generated by the MILC collaboration with the Luscher-Weisz gauge action. Gluon discretization errors are also studied in a lattice potential model using perturbation theory for four different gauge actions. The nonperturbative lattice QCD results for the radial and orbital bottomonium energy splittings obtained from the RBC/UKQCD ensembles are found to be in excellent agreement with experiment. To get accurate results for spin splittings, the spin-dependent order-v^6 terms are included in the NRQCD action, and suitable ratios are calculated such that most of the unknown radiative corrections cancel. The cancellation of radiative corrections is verified explicitly by repeating the calculations with different values of the couplings in the NRQCD action. Using the lattice ratios of the S-wave hyperfine and the 1P tensor splitting, and the experimental result for the 1P tensor splitting, the 1S hyperfine splitting is found to be 60.3+-5.5(stat)+-5.0(syst)+-2.1(exp) MeV, and the 2S hyperfine splitting is predicted to be 23.5+-4.1(stat)+-2.1(syst)+-0.8(exp) MeV.