Nuclear De-Excitation Following Muon Capture and the Bound Muon Decay Anomaly

Abstract

Experiments have been performed 1 " 3 to measure the rate of negative muon decay, l±~ -*e~ + v + v, when the muon is bound in the Coulomb field of a nucleus. Figure 1 presents the data of Yovanovitch 1 for the ratio of the bound decay rate to the free decay rate (rate for JU + decay or rate for muons bound to a low-Z nucleus), d dThe curve is a theoretical curve. 4 While the theoretical predictions are always less than R = 1 the experimental results rise well above R = 1 in the iron region, yet return quickly close to theory for 30 Zn, and fall well below the theo-retical curve for very large Z. The presence of this anomaly has been verified in detail by combining measurements by Lederman and Weinrich 2 of A^~/A^ and measurements of A^, 3 the total disappearance rate. These points are not included in Fig. 1 simply to avoid clutter. The other mode of disappearance of negative muons in matter is muon capture for which the basic reaction isTo determine whether the bound muon decay anomaly is really due to detection of y rays associated with muon capture rather than electrons from muon decay, we can take the difference between the experimental and theoretical values for R and divide by A c /A^+. Table I presents the values of (R' eX g -R th) A d*/ A c calculated using experimentally determined capture rates. 5 The near constancy of the values in Table I makes it appear that the anomaly is due to the detection of 1% of the muon capture events and that this sensitivity to muon capture mysteriously stops at Zn. The initially sharp Z dependence of the anomaly is just the Z dependence of muon capture rates ~Z 4 .To understand how background due to muon capture could appear in such careful experiments, let us consider some of the nuclear processes Table I. Proportionality of the bound muon decay anomaly to the capture rate.