The 39th International Symposium on Lattice Field Theory (Lattice 2022)

Probing the R-ratio on the lattice

11 Aug 2022, 10:00

20m

CP1-HSZ/0.001 (CP1-HSZ) - HS3 (CP1-HSZ)

Oral Presentation QCD in searches for physics beyond the Standard Model QCD in searches for physics beyond the Standard Model

Speaker

Alessandro De Santis (University and INFN of Rome Tor Vergata)

Description

The ratio of the cross sections for $e^{+}{e}^{-}\to hadrons$ and $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ at c.o.m energy $E$, i.e. $R(E)$, is an extremely interesting observable. Its measurements are used in dispersive analyses of the leading hadronic vacuum polarization (HVP) contributing to the muon $g-2$, and the results of these analyses for a certain window observable are in significant tension with those coming from recent accurate lattice computations. It is thus very important to determine $R(E)$ from first-principles and compare it with experiment. In this talk we study $R(E)$ through a smearing in energy with different kernels $f(E)$. Indeed, by changing the shape of the smearing kernel one obtains an infinite number of observables, $R[f]$, that probe $R(E)$ in different ways. In particular, choosing $f(E)=\text{exp}(-Et)$ yields the Euclidean lattice correlator of two electromagnetic hadronic currents. This is a primary quantity from the lattice viewpoint, which we compare with its experimental counterpart directly obtained from the measured $R(E)$. We also use a recently proposed method for extracting smeared spectral densities from Euclidean lattice correlators in order to compute $R[f]$ for smearing kernels $f$ chosen as Gaussians of different width and central energy. Our still preliminary numerical results are obtained using state-of-the-art ETMC ensembles with $N=2+1+1$ dynamical quark flavours at three values of the lattice spacing ($\ge$ 0.06 fm), large volumes and physical pion mass.

Presentation materials

talk.pdf