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3. Rare Pion-Decay Experiments

The intention of the proposed pion beta decay experiment is a precise determination of the decay rate p +-> p 0e+ n e relative to the rate for the decay p +->e+ n e as will be discussed in section 4.1. In order to deal with the occurring experimental difficulties, a precise knowledge of the experimental technique and the calibration methods of previous experiments on both of the two pion decay modes is required. The presentation of previous experiments includes thus a discussion of their calibration methods (see 3.1.1) and low energy tail corrections (see 3.2), since some of these calibration methods are a central subject of the present work.

An overview of the decay channels of pions, muons and p 0's is given in tables 3.1 and 3.2.


Process
R = /[Gamma]

p +->µ+ n µ
(99.98770±0.00004)%

p +->µ+ n µ g
(1.24±0.25)·10-4

p +->e+ n e
(1.230±0.004)·10-4

p +->e+ n e g
(1.61±0.23)·10-7

p +-> p 0e+ n e
(1.025±0.034)·10-8

Table 3.1: Pion decay modes [PDG 94]


Process
R = /[Gamma]

Process
R = /[Gamma]

µ+->e+ n eµ
(98.6±0.4)%

p 0-> g g
(98.798±0.032)%

µ+->e+ n eµ g
(1.4±0.4)%

p 0->e+e- g
(1.198±0.032)%

µ+->e+ n eµ e+e-
(3.4±0.4)·10-5



Table 3.2: Muon and p 0 decay modes [PDG 94] Figure 3.1: Michel spectrum of positrons from muon decays at rest. The main decay of the pion ( p +->µ+ n µ) produces a monoenergetic µ+ which decays with a lifetime of [tau]µ [congruent] 2.2µs mainly into µ+->e+ n e µ ( p µe chain). For stopped pion decay experiments where the pion is assumed to be at rest before decaying, the muon has a kinetic energy of Tµ [congruent] 4.12 MeV. Since the range of the µ+ is very small ( about 1mm in scintillator material) the µ+ is also stopped in the target and the muon decay occurs at rest too. The energy spectrum of the e+ from the decay is the Michel spectrum, shown in Fig. 3.1, with a maximum energy of 52.83 MeV. Positrons from the p µe chain are the main background for all stopped pion decay experiments.

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