Friday, July 5, 2002, 16:00
WHGA Auditorium
Prof. P. Debevec, Univ. of Illinois
Abstract:
The g-factors of the electron and the muon are not exactly 2. They are
slightly larger.
The deviation from the 2, the g-factor anomaly or g-2, arises from the
coupling to virtual
fields and particles. For the electron only virtual photons, and
electron-
positron pairs
are important. Theory (quantum electrodynamics) and experiment agree to
an
astounding
precision. For the muon with 207 times the electron mass, virtual
fields
and particles
from both the strong and the weak interaction are also important. These
contributions
can also be calculated to high precision within the framework of the
Standard Model.
In March, 2001 the Brookhaven National Laboratory E821 collaboration
announced a new
measurement of the muon g-2, based on data it obtained in 1999, at a
precision of 1.3
parts per million. This value differed from the accepted Standard Model
prediction
by 2.6 standard deviations. Following the announcement, the implication
of
such a deviation
to Standard Model extensions was widely discussed, and, concurrently,
the
Standard Model
prediction itself was carefully examined. The explanation of new
physics
now appears less
likely as an error was subsequently uncovered in one part of the
Standard
Model calculation,
and the deviation was reduced from 2.6 to 1.6 standard deviations. The
story is, however,
far from over. The E821 collaboration obtained a significantly larger
data
sample in 2000
(again with positive muons) and in 2001 (with negative muons). The
analysis of the 2000 data
is nearing completion, and the precision in g-2 is expected to improve
by a
factor of two.
The talk will present an overview of the theory of the muon g-2, the
experiment by which
it is measured, and the current status of the analysis.