Physics at Virginia

"In a muon's lifetime: From Fermi's constant to "calibrating" the sun"

Peter Winter , University of Illinois at Urbana-Champaign
[Host: Don Crabb]
The muon group at Illinois is performing three experiments at the Paul Scherrer Institute all measuring the muon lifetime with high precision. The MuLan 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 which will give a 20 times better determination of the Fermi constant G F . A first result was recently published [1] which already improved 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 an active target filled with ultra-pure hydrogen gas. The elementary capture process μ - +p → n+ ν offers a rare (0.15%) but additional disappearance channel. The measured difference of the positive and negative muon's lifetime determines the rate of the capture process to a final precision of 1%. This can be used to derive an improved value of the proton's pseudoscalar form factor g P to 7% precision. A first result gP = 7.3  1.1 has been published [2]. This is a first precise, unambigous determintation of gP and an important test of QCD symmetries. Recently, we started a new experiment, MuSun [3], that will start a first commissioning run at the end of 2008. Here, a measurement of the μ - +d → n+n+ ν provides a benchmark 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 related to the same low-energy constant, characterizing the two nucleon system at short distances. This constant is not well constrained and therefore the MuSun experiment comes closest to calibrating these basic astrophysical reactions under terrestrial conditions. [1] Phys. Rev. Lett. 99, 032001 (2007) [2] Phys. Rev. Lett. 99, 032002 (2007) [3] http://www.npl.uiuc.edu/exp/musun/documents/prop07.pdf
Nuclear Physics Seminar
Monday, March 23, 2009
3:30 PM
Physics Building, Room 204
Note special date.
Note special room.

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