Precise measurement of muon capture on the proton

The aim of the muCap experiment is a 1% measurement of the singlet capture rate Lambda(S) for the basic electro-weak reaction mu + p --> n + nu(mu). This observable is sensitive to the weak form-factors of the nucleon, in particular to the induced pseudoscalar coupling constant g(p). It will provide a rigorous test of theoretical predictions based on the Standard Model and effective theories of QCD. The present method is based on high precision lifetime measurements of mu(-) in hydrogen gas and the comparison with the free mu(+) lifetime. The mu(-) experiment will be performed in ultra-clean, deuterium-depleted H-2 gas at 10 bar. Low density compared to liquid H-2 is chosen to avoid uncertainties due to ppmu formation. A time projection chamber acts as a pure hydrogen active target. It defines the muon stop position in 3D and detects rare background reactions. Decay electrons are tracked in cylindrical wire-chambers and a scintillator array covering 75% of 4pi