Probing the Energy-Smeared R Ratio Using Lattice QCD

Area of Science:

• High Energy Physics
• Quantum Chromodynamics (QCD)
• Particle Physics

Background:

• The R ratio, defined as the ratio of the electron-positron cross section into hadrons versus muons, is a crucial observable in particle physics.
• Precise theoretical calculations of the R ratio are essential for testing the Standard Model of particle physics.
• Lattice QCD provides a non-perturbative approach to calculating fundamental quantities in particle physics.

Purpose of the Study:

• To perform a first-principles lattice QCD investigation of the R ratio.
• To compare theoretical predictions with experimental measurements of the R ratio.
• To assess the feasibility of lattice QCD for precision Standard Model tests.

Main Methods:

• Employed a method to extract smeared spectral densities from Euclidean correlators.
• Computed the R ratio convoluted with Gaussian smearing kernels (width ~600 MeV) at central energies from 220 MeV to 2.5 GeV.
• Compared lattice QCD results with smeared experimental data from the KNT19 compilation.

Main Results:

• Observed a tension of approximately 3 standard deviations between theoretical and experimental R ratios when centered around the rho-resonance peak.
• The calculation demonstrates the potential for lattice QCD to achieve the accuracy needed for precision Standard Model tests.
• Acknowledged that QED and strong isospin-breaking corrections were not included and may influence the observed tension.

Conclusions:

• Lattice QCD calculations can probe the R ratio in specific energy bins with sufficient accuracy for precision tests.
• The observed tension highlights areas for further theoretical refinement and experimental investigation.
• This work validates the methodological approach for future precision studies of the R ratio on the lattice.