Determination of the Muon Lifetime in $^{76}$Se with the MONUMENT experiment

TL;DR

This work provides a precise measurement of the muon lifetime in $^{76}$Se via ordinary muon capture, using MONUMENT with an HPGe array and scintillator tagging at PSI. Two independent analyses (ALPACA and MIDAS) plus multiple gamma and muonic X-ray lines yield a combined lifetime of $\tau = (135.1 \pm 0.5)\ \text{ns}$, a result that agrees with QRPA calculations using an unquenched axial-vector coupling $g_A$ and with semi-empirical Primakoff–type predictions. The study corrects a previous higher value and establishes a robust benchmark for nuclear-model inputs in neutrinoless double beta decay analyses, highlighting little to no quenching in the QRPA framework for this system. The findings have implications for interpreting $^{76}$Ge onbb decay constraints and motivate extending similar measurements to other nuclides relevant to onbb physics.

Abstract

Ordinary muon capture provides a benchmark for the nuclear physics models of neutrinoless double beta decay under comparable momentum transfer conditions. The total capture strength defines the lifetime of the muonic atom. The muon lifetime in $^{76}$Se, the daughter nucleus of $^{76}$Ge, was determined with improved accuracy by the MONUMENT collaboration, using an array of high-purity germanium detectors and a set of scintillator counters at the $π$E1 muon beam line of the Paul Scherrer Institute. The new value of (135.1 $\pm$ 0.5) ns agrees with phenomenological calculations based on the quasiparticle random phase approximation with unquenched axial-vector coupling.

Figures (7)

• Figure 1: Schematic comparison of *omc and *bb decay. *omc on ${^{A}\IfNoValueTF{Z+2}{}{_{Z+2}}\textrm{Y}}$ provides experimental access to the right virtual transition of *bb decay, populating the intermediate nucleus ${^{A}\IfNoValueTF{Z+1}{}{_{Z+1}}\textrm{W}}$ under similar momentum transfer as *onbb decay. The subsequent gamma emission after *omc allows to tag the capture process and determine the muon lifetime in the target nucleus. For the *bb-decay isotope ${^{76}\IfNoValueTF{-NoValue-}{}{_{-NoValue-}}\textrm{Ge}}$, the daughter isotope and *omc target is ${^{76}\IfNoValueTF{-NoValue-}{}{_{-NoValue-}}\textrm{Se}}$, which transforms into ${^{76}\IfNoValueTF{-NoValue-}{}{_{-NoValue-}}\textrm{As}}$ after *omc and other ${^{}\IfNoValueTF{-NoValue-}{}{_{-NoValue-}}\textrm{As}}$ isotopes after neutron emission.
• Figure 2: Measurement principle of monument. No signal in the scintillator counter C$_0$, coincident signals in C$_1$ and C$_2$, and no signal in C$_3$ identify muons that stopped in the target, whereas the hpge detector array provides spectroscopic information about the subsequent emission of gammas and muonic x rays from the target.
• Figure 3: Fragment of a two-dimensional histogram based on the midas data, spanning the hpge detector energy and the time since the muon signal. The muon lifetime becomes apparent in the exponential extent of the gamma lines towards later times, while the muonic x rays appear prompt, evidencing the timing resolution. The data was recorded with detector 5.
• Figure 4: Time profile extraction in alpaca. The intensities were extracted by a combined fit to all 40 time slices, each of which is 32 ns long, using shared fep parameters, but individual backgrounds. The slices are shown in consecutive order, using 8 windows and 5 colors. Their indices indicate the slice number. The muonic x ray at 466 keV dominates the earlier times slices, whereas the gamma line at 469 keV prevails until later times. The data was recorded with detector 6, which achieves proper separation of the two lines.
• Figure 5: Lifetime fit in alpaca. The lifetime was obtained by a combined fit of the muonic x ray, providing the prompt timing response, and the gamma line, whose exponential decay encodes the muon lifetime. The data correspond to the muonic x ray line at 456 keV and gamma line at 419 keV recorded with detector 4.