Friday, October 26, 2001, 16:00
WHGA Auditorium
Prof. F.J. Yndurain, Madrid
Abstract:
A new calculation of the hadronic part a_Hadronic
of the anomalous magnetic moment of the muon, a_\mu is described.
For the low energy contributions of order \alpha^2 we carry over an
analysis of the pion form factor F_\pi(t) using recent data both on
e^+e^- -> \pi^+\pi^- and \tau^+ -> \bar\nu_\tau \pi^+\pi^0, including
fully from analyticity properties so we can use also experimental information
on F_\pi(t) at spacelike t. At higher energy we use QCD to supplement
experimental data, which include the recent measurements of e^+e^- ->
hadrons both around 1 GeV and near the \bar{c}c threshold.
This yields a precise determination of the O(\alpha^2) and
O(\alpha^2)+O(\alpha^3) hadronic part of the photon vacuum polarization pieces.
Adding the remaining order \alpha^3 hadronic contributions we find
10^{11}\times a_\mu= 116591849 ± 69 &\nbsp;(e^+e^- + \tau + spacel. Q.c.m.)
10^{11}\times a_\mu= 116591671 ± 71 &\nbsp;(e^+e^- + \tau + spacel. Ch.m.)
depending on whether the light-by-light scattering hadronic contributions are evaluated with the quark constituent model or a chiral model. This deviates from the recent experimental value by 1.1\sigma or 2.1\sigma, respectively.
With the same analysis we also get a precise determinaton of the running electromagnetic coupling, \bar{\alpha}_{Q.E.D.}(M_Z^2)= 1/(128.962 ± 0.017).