Background

Constant

\[A(t) = A_{\mathrm{bg}}\]

A time-independent background asymmetry, used additively in essentially every composite model. Physically it accounts for muons that stop outside the sample — silver sample holders, cryostat walls, and tails — and for any residual detector imbalance that survives the α calibration. Because all relaxation functions satisfy \(G(0) = 1\), the asymmetry amplitudes and the background obey the sum rule \(\sum_i A_i + A_{\mathrm{bg}} = A_0\) with \(A_0\) the calibrated total asymmetry; a fitted background that drifts across a run series usually signals a changing beam spot rather than sample physics, and is worth fixing from a calibration measurement.

Name	Symbol	Unit	Description
A_bg	\(A_{\mathrm{bg}}\)	%	Time-independent background level.

Note the distinction from a relaxing background: muons stopping in silver relax so slowly (\(\lambda \lesssim 0.01\;\mu s^{-1}\)) that a constant is almost always adequate, but a holder containing nuclear moments may need its own slow Gaussian or StaticGKT_ZF term instead.