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DESCRIPTION:Peter Winter\, University of Illinois at Urbana-Champaign \n\nT
he muon group at Illinois is performing three experiments at the Paul Sche
rrer Institute all measuring the muon lifetime with high precision. The Mu
Lan experiment uses a simple soccer-ball like scintillator array to detect
the decay positrons. We collected twice 10 12 muon decays in
two different target materials to obtain the final precision of 1 ppm whi
ch will give a 20 times better determination of the Fermi constant G F . A first result was recently published [1] which already improv
ed the precision of G F to 5 ppm. The muon capture experiment
MuCap uses a negative muon beam stopped in a time projection chamber as a
n active target filled with ultra-pure hydrogen gas. The elementary captur
e process μ\; - +p →\; n+ ν\; offers a rare (0.1
5%) but additional disappearance channel. The measured difference of the p
ositive and negative muon's lifetime determines the rate of the capture pr
ocess to a final precision of 1%. This can be used to derive an improved v
alue of the proton's pseudoscalar form factor g P to 7% preci
sion. A first result gP = 7.3 \; 1.1 has been published [2]. This i
s a first precise\, unambigous determintation of gP and an important test
of QCD symmetries. Recently\, we started a new experiment\, MuSun [3]\, th
at will start a first commissioning run at the end of 2008. Here\, a measu
rement of the μ\; - +d →\; n+n+ ν\; provides a b
enchmark of the understanding of weak processes in the two nucleon-system.
It was shown\, that other weak reactions involving the two nucleon system
(pp →\; de + ν\; or ν\; +d reactions) are relat
ed to the same low-energy constant\, characterizing the two nucleon system
at short distances. This constant is not well constrained and therefore t
he MuSun experiment comes closest to calibrating these basic astrophysical
reactions under terrestrial conditions.\n\n[1] Phys. Rev. Lett. 99\, 0320
01 (2007)\n[2] Phys. Rev. Lett. 99\, 032002 (2007)\n[3] http://www.npl.uiu
c.edu/exp/musun/documents/prop07.pdf\n
DTSTART:20090323T193000Z
LOCATION:Physics Building\, Room 204
SUMMARY:In a muon's lifetime: From Fermi's constant to "calibrating" the su
n
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