2,759 research outputs found
Complete 2-loop quantum electrodynamic contributions to the muon lifetime in the Fermi model
The complete 2-loop QED contributions to the muon lifetime have been
calculated analytically in the Fermi theory. The exact result for the effects
of virtual and real photons, virtual electrons, muons and hadrons as well as
e+e- pair creation is
Delta Gamma^(2)=Gamma_0(alpha/pi)^2[(156815/5184)-(1036/27)zeta(2)
-(895/36)zeta(3)+(67/8)zeta(4)
+53zeta(2)ln(2)-(0.042+/-0.002)]
where Gamma_0 is the tree-level width. This eliminates the theoretical error
in the extracted value of the Fermi coupling constant, G_F, which was
previously the source of the dominant uncertainty. The new value is
G_F=(1.16637 +/- 0.00001) x 10^-5 GeV^-2
with the error being entirely experimental. Several experiments are planned
for the next generation of muon lifetime measurements and these can proceed
unhindered by theoretical uncertainties.Comment: 9 pages, LaTeX, uses sprocl.sty, amsmath.sty, amssymb.sty and
axodraw.sty. To appear in the Proceedings of the IVth International Symposium
on Radiative Corrections (RADCOR98), Barcelona, Spain, 8-12 September, 1998,
edited by J. Sol
Gauge Invariance and the Unstable Particle
It is shown how to construct exactly gauge-invariant S-matrix elements for
processes involving unstable gauge particles such as the boson. The
results are applied to derive a physically meaningful expression for the
cross-section and thereby provide a solution to the
long-standing problem of the unstable particle.Comment: 8 pages LaTeX. Uses aipproc.sty and epsfig.sty. Talk presented at 1st
Latin American Symposium on High-energy Physics, SILAFAE-I, Merida, November
1--5, 199
Gauge Invariance in the Process
The process is considered as an example of the problems associated with
maintaining gauge invariance in matrix elements involving unstable particles.
It is shown how to construct a matrix element that correctly treats width
effects for the intermediate unstable boson and that is both and
gauge-invariant. gauge-invariance is maintained by
Laurent expansion in kinematic invariants and
gauge-invariance is enforced by means of a projection operator under which the
exact matrix element is invariant.Comment: Case of gauge-invariance addressed. Substantial changes in
text. 9 pages LaTeX, also available from via anonymous ftp from
ftp://feynman.physics.lsa.umich.edu/pub/preprints/UM-TH-96-02.p
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