Smeared R-ratio from lattice QCD

by Alessandro De Santis (University of Rome "Tor Vergata"​)

Tuesday 30 Apr 2024, 16:00 → 18:00 Europe/Zurich

4/2-037 - TH meeting room (CERN)

The ratio 𝑅(𝐸) of the cross-sections for 𝑒+𝑒−→ hadrons and 𝑒+𝑒−→𝜇+𝜇− is an extremely interesting hadronic observable due to its phenomenological applications. For example, the experimental measurements of 𝑅(𝐸) are used in the dispersive calculations of the leading hadronic vacuum polarization contribution to the muon 𝑔−2 and these are in significant tension with direct lattice calculations and with the muon 𝑔−2 experiment. It is thus very important to probe 𝑅(𝐸) from first-principles and compare it with experiment. In this talk we discuss the first-ever lattice QCD determination of 𝑅(𝐸) smeared with Gaussians, of different widths 𝜎 and central energies, by using the HLT method for extracting smeared spectral densities from Euclidean lattice correlators.  Our theoretical results, compared with the KNT19 compilation of 𝑅(𝐸) smeared with the same Gaussian kernels, shows a tension (about three standard deviations) for 𝜎∼600 MeV and central energies around the 𝜌 resonance peak. Future perspectives, both methodological and phenomenological, are also discussed.