MAM Seminar : Muon anomalous magnetic moment (MAMM): a 2.3 to 4.8 sigma discrepancy (or isospin breaking in tau->pi pi nu_tau decays)

Wednesday February 20th 2013, 2:00pm - Coffee room 

Speaker: Pablo Roig

Abstract: The MAMM is one of the most precise low-energy measurements, which should in principle allow for a stringent test of beyond the SM contributions.

Although the QED and EW parts have been computed with striking precision, there is an unavoidable uncertainty due to the hadronic contributions (mainly the pi pi one) in the non-perturbative QCD regime which are therefore data-driven.
 If the hadronic e^+e? cross-section data are used the discrepancy is ~3.6 sigma (assuming initial state radiation is well understood), whereas the tau hadronic width yields a ~2.3 sigma difference (assuming isospin corrections are controlled). Theoretical approaches to both sets of data obtain eventually larger discrepancies, up to 4.8 sigma.
 Motivated by this situation we started a reanalysis of both kinds of data using a dispersive representation of the vector two-pion form factor including isospin corrections which will be sketched.


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