February-02-2001

Technical Proposal, PSI Proposal R-97-05:

Precision Measurement of Singlet Capture in a Hydrogen TPC

The CAP Experiment

V.A. Andreev, D.V. Balin, A.A. Fetisov, V.A. Ganzha, V.I. Jatsoura,
A.G. Krivshich, E.M. Maev, O.E. Maev, G.E. Petrov, S. Sadetsky, G.N. Schapkin,
G.G. Semenchuk, M. Soroka, A.A. Vorobyov, N.I. Voropaev
Petersburg Nuclear Physics Institute (PNPI), Gatchina 188350, Russia


P.U. Dick, A. Dijksman, J. Egger, D. Fahrni, W.D. Herold, V. Markushin,
C. Petitjean, R. Schmidt
Paul Scherrer Institute, PSI, CH-5232 Villigen, Switzerland


T.A. Case, K.M. Crowe, S.J. Freedman, B. Gartner, B. Lauss
University of California Berkeley, UCB and LBNL, Berkeley, CA 94720, USA


D.B. Chitwood, P.T. Debevec, F. E. Gray, D.W. Hertzog,
P. Kammel, C.J. G. Onderwater, C.C. Polly,

University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA

J. Deutsch, J. Govaerts, R. Prieels
Université Catholique de Louvain, B-1348 Louvain-La-Neuve, Belgium


F.J. Hartmann
Technische Universität München, D-85747 Garching, Germany

R. M. Carey

Boston University, Boston, MA 02215, USA

Abstract The goal of this project is to improve considerably the knowledge of the rate $\Lambda_s$ for the basic electroweak capture reaction of a negative muon on a free proton. The proposed measurement to 1% precision determines the induced pseudoscalar coupling constant g$_{P}$ to better than 7%. It will provide stringent tests of theoretical predictions based on Standard Model symmetries and the chiral perturbation theory. These basic QCD predictions for g$_{P}$ are currently in 4$\sigma$ disagreement with a recent experiment on radiative $\mu p$ capture at TRIUMF. Our proposed method is essentially free from molecular uncertainties which have clouded the interpretation of earlier experiments with high density targets.

This experiment is a muon lifetime measurement in ultra-pure deuterium-depleted hydrogen gas at 10 bar. A time projection chamber (TPC) made from bakeable low-outgasing material will be built acting as active muon stop detector which also can view the decay electrons. The gas purity of 10$^{-8}$ is produced and maintained using chemical cleaning and recirculation methods. The main electron detector consists of 2 Sindrum chambers and a plastic scintillation hodoscope surrounding the TPC pressure vessel in cylindrical geometry with 75% solid angle. A 70 Gauss magnetic field will be applied to neutralize spin rotation effects in the $\mu^+$ control measurements. A new data acquisition system was developed using continuous dead-time free TDC electronics, designed for beam rates up to 50 kHz.

We present the results of test runs in 1998-2000, where our method was tested with a full size prototype TPC, and discuss the systematics of this experiment. We request 4 + 2 weeks of beamtime for testing the final apparatus and 2 months for a first production run.