Hadronic Contributions to New Physics Searches

Precise determination of the low-energy hadronic contribution to the muon g−2 from analyticity and unitarity - an improved analysis

29 Sept 2016, 17:15

35m

Muon g-2

Speaker

Irinel Caprini (IFIN-HH Bucharest (RO))

Summary

The two-pion low-energy contribution to the anomalous magnetic moment of
the muon, $a_{\mu }\equiv (g-2{)}_{\mu }/2$, expressed as an integral over the
modulus squared of the pion electromagnetic form factor, brings a
relatively large contribution to the theoretical error, since the low
accuracy of experimental measurements in this region is amplified by the
drastic increase of the integration kernel. We derive stringent
constraints on the two-pion contribution by exploiting analyticity and
unitarity of the pion electromagnetic form factor. To avoid the poor
knowledge of the modulus of this function, we use instead its phase, known
with high precision in the elastic region from Roy equations for pion-pion
scattering via the Fermi-Watson theorem. Above the inelastic threshold we
adopt a conservative integral condition on the modulus, determined from
data and perturbative QCD. Additional high precision data on the modulus
in the range $0.65-0.71$ GeV, obtained from $e^{+}{e}^{-}$ annihilation and
$\tau$-decay experiments, are used to improve the predictions on the
modulus
at lower energies by means of a parametrization-free analytic
extrapolation. The results are optimal for a given input and do not depend
on the unknown phase of the form factor above the inelastic threshold.
The present work improves a previous analysis based on the same technique,
including more experimental data and employing better statistical tools
for their treatment. We obtain for the contribution to $a_{\mu }$ from below
0.63 GeV the value $(133.258\pm 0.723)\times {10}^{-10}$, which amounts
to a reduction of the theoretical error by about $6\times {10}^{-11}$.

Presentation materials

Caprini.pdf