(A Historical) Introduction to the Pion Beta Experiment

The experiment

The goal of the Pion-Beta Experiment, which is carried out by an international collaboration, is a measurement of the rate of the pion beta decay, pi+ -> pi0 e+ nu, with a precision of better than 0.5%.

The pion beta decay is one of the most fundamental weak interaction processes. It is analogous to superallowed Fermi transitions in nuclear beta decays. The analysis of nuclear beta decay involves nuclear corrections which are uncertain at the level of a few tenth of a percent. These corrections do not appear in the pion beta process, which, therefore, presents a more stringent test of the weak interaction theory.

The difficulty in measuring the pion beta decay rate is due to the small branching ration of ~ 10-8. Following table gives an overview of the different decay modes of positive pions with the Pion Beta decay in bold letters:

ModeFraction (G(i)/G) G(1)
G(1)mu+ nu(mu) (99.98782+-0.00014)%
G(2)mu+ nu(mu) gamma ( 1.24 +-0.25 )E-4
G(3)e+ nu(e) ( 1.218 +-0.014 )E-4
G(4)e+ nu(e) gamma ( 1.61 +-0.23 )E-7
G(5)e+ nu(e) pi0 ( 1.025 +-0.034 )E-8
G(6)e+ nu(e) e+ e- ( 3.2 +-0.5 )E-9

The most recent and precise determination of the pion beta decay rate was carried out by McFarlane et al., and is in good agreement with theory. However, the measurment uncertainty of ~ 4% is an order of magnitude higher than the theoretical uncertainties.

The detector

Various pictures of parts of the detector are available.

For this experiment we have designed a stopped-pion detector system to detect the two gamma's from the pi0 decay, as well as the e+. Due to the large Michel positron background, the detecotr must be very efficient in the pi0 detection and have excellent background suppression. This is achieved by using a pure CsI calorimeter with a large solid angle coverage and good energy resolution together with an active target and two cylindircal wire chambers for charged particle tracking:

Click on the picture to obtain an enlarged version. This picture is also available as a black & white postscript picture and as a color postscript picture.

The following picture gives an overview of the mechanics holding the CsI calorimeter. The target, chambers and plastic vetos are not shown. The diameter of the detector is about 2 m.

A cross section of this mechanics is shown here:

There are also two computer animations available, which illistrate how the detector will fit in the PiE1 area at PSI:

Movie 1: Opening of the cosmic veto house

MPG format [227k]

Also available in FLI format [2127k]. (Better quality, but larger. You need the AAPLAY.EXE or AAWIN.EXE program on a PC to play this movie)

Movie 2: Flight around the area

MPG format [2127k]

Also available in FLI format [16.5M !!!].

The beam line

The experiment will be performed at the PiE1 beamline at the Paul Scherrer Institute, Switzerland. This beamline, tuned to 116 MeV/c pi+, will give the necessary pion rate of up to 2 * 106 pi+/sec with a low contamination of muons, positrons and protons. Due to a lead collimator in the beamline, the stopping distribution of the pions can be constraint to a 10mm x 10mm (FWHM) spot inside the active target.

Time table

Between 1989 and 1996, several tests were made with the various detector components in a pion beam. In 1997, the detector will be assembled. The first operation is expected in 1998.

S. Ritt, 3. Mar 97