Fitting strategies for the SNS nEDM experiment in the presence of time-varying magnetic fields

The Spallation Neutron Source (SNS) neutron electric dipole moment experiment (nEDM) performs a Larmor spin precession technique on ultracold neutrons and $^3$He. The spin-dependent interaction of the ultracold neutrons and polarized $^3$He co-magnetometer atoms will produce sinusoidal oscillations in the scintillation light that is sensitive to the difference in the neutrons' and 3He atoms' precession frequencies, $f_{3n}$. If the neutron EDM is non-zero, the value of $f_{3n}$ will shift upon reversal of the relative directions of the magnetic and electric fields. The precession frequency of the $^3$He will be measured via detection of the precessing 3He magnetization in SQUID pickup loops. We carried out simulations of the SQUID and scintillation rate signals in the presence of time-varying magnetic fields and then performed a global fit by minimizing the $\chi^2$ made up of the two signals. We present different fitting strategies for a global fit to the scintillation light and the 3He co-magnetometer signal in the presence of time-varying magnetic fields.