Probing the Energy-Smeared R Ratio Using Lattice QCD

We present a first-principles lattice QCD investigation of the R ratio between the e+e- cross section into hadrons and into muons. By using the method of Ref. [1], that allows one to extract smeared spectral densities from Euclidean correlators, we compute the R ratio convoluted with Gaussian smearing kernels of widths of about 600 MeV and central energies from 220 MeV up to 2.5 GeV. Our theoretical results are compared with the corresponding quantities obtained by smearing the KNT19 compilation [2] of R-ratio experimental measurements with the same kernels and, by centering the Gaussians in the region around the p-resonance peak, a tension of about 3 standard deviations is observed. From the phenomenological perspective, we have not included yet in our calculation QED and strong isospin-breaking corrections, and this might affect the observed tension. From the methodological perspective, our calculation demonstrates that it is possible to study the R ratio in Gaussian energy bins on the lattice at the level of accuracy required in order to perform precision tests of the standard model.

Automated summary

We perform a lattice QCD calculation of the R ratio convoluted with Gaussian smearing kernels to investigate the tension between our theoretical results and experimental measurements. Our calculation demonstrates the possibility of studying the R ratio on the lattice with the required level of accuracy for precision tests of the standard model. The observed tension may be attributed to the omission of QED and strong isospin-breaking corrections.