Precise Measurement of Muon Capture on the Proton

The aim of the μCap experiment is a 1% measurement of the singlet capture rate ΛS for the basic electro-weak reaction μ + p → n + νμ. This observable is sensitive to the weak form-factors of the nucleon, in particular to the induced pseudoscalar coupling constant gP. 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 μ− in hydrogen gas and the comparison with the free μ+ lifetime. The μ− experiment will be performed in ultra-clean, deuterium-depleted H2 gas at 10 bar. Low density compared to liquid H2 is chosen to avoid uncertainties due to ppμ 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 4π.