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What does g-2 really test?



Hi all-
Both the unfortunate Brookhaven news release and the submitted
paper claim a 2.6 sigma disagreement between the results to date and
theory. The relevant question is: what theory is at issue?
There are 3 contributions to g-2; these are quantum
electrodynamic (QED), weak and hadronic. The first two together comprise
the standard model of electroweak interactions. The theoretical
uncertainty, 6.7 in some units, comes from the hadronic correction
which is 692.4+-6.2 in those units (1% uncertainty).
The hadronic correction comes from a single paper by Davier and
Hoecker, Phys. Lett. B 435 (1998) 427. It attempts, as I understand it,
to calculate the contributions of certain quark loops to the muon g-2.
These contributions are deduced from cross sections for hadron production
in e+e- collisions and from data on tau-decay into final states with two
pions. These data are then used to evaluate certain dispersion integrals.
I think it fair to say, therefore, that what is really being
tested in the new g-2 experiment is the ability of theorists to calculate
the hadronic corrections to g-2 and to understand the uncertainties in
such calculations. If, for example, the true uncertainty in the hadronic
corrections is 5%, rather than 1%, then there is no disagreement to talk
about.
It is very premature, in any event, to argue that disagreement
between theory and experiment signals the need for new physics. Such
disagreement may only signal the need for more reliable ways to measure
hadronic contributions or, in the alternative, provide a unique measure
of the hadronic contributions.
Regards,
Jack

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