$A_β[E_β]$ in $^{37}$K decay: new physics with opposite $β$ helicity

TL;DR

This work extends the analysis of $^{37}$K beta decay to allow the beta asymmetry to depend on energy via the helicity factor $m_eta/E_eta$, enabling sensitivity to a Fierz interference term $b$ and to Lorentz scalar and tensor lepton-quark couplings. By combining a detailed optical-pumping-based polarization scheme, precise detector geometry, and GEANT4-based simulations, the authors extract $A_eta[E_eta]$ and $b$ with a two-parameter fit, finding $b = 0.033 \pm 0.084\ \text{(stat)} \pm 0.039\ \text{(syst)}$ and $A_eta = -0.5738 \pm 0.0082\ \text{(stat)} \pm 0.0041\ \text{(syst)}$, consistent with the Standard Model ($b=0$, $A_eta$ SM value). The analysis demonstrates complementarity to neutron decay and high-energy constraints, maps the allowed region for scalar and tensor couplings, and identifies dominating systematics (beta scattering) with a path toward significant improvements. The results thus constrain non-universal new physics scenarios and set the stage for an order-of-magnitude sensitivity enhancement with planned instrumental upgrades, potentially probing TeV-scale new physics via precision nuclear beta decay observables.

Abstract

By extending our analysis and simulations of our $^{37}$K $β$-decay data set to allow the $β$ asymmetry with respect to nuclear spin to vary with $β$ energy $E_β$, we have gained sensitivity to new physics that depends on a helicity factor for the $β$, $m_β/E_β$. In particular, we constrain Lorentz scalar and tensor quark-lepton interaction strengths at a sensitivity complementary to the similar Fierz interference term in neutron $β$ decay. Our result for that new physics is a Fierz interference term $b$ = 0.033 $\pm$ 0.084 (stat) $\pm$ 0.039 (syst), consistent with the standard model electroweak interaction value $b=0$. We consider presently achieved complementarity to $β$-decay and particle physics experiments, along with projectable technical improvements to our method.

Figures (5)

• Figure 1: TRINAT during the optical pumping time. Shown are $\beta$ telescopes (double-sided silicon strip detector DSSD $\Delta E$ backed by plastic scintillator), mirrors for optical pumping light and its beams, magnetic field coils, electric field electrodes, and microchannel plates for electron and ion detection. Distance between trap cloud and ion MCP is 9.7 cm.
• Figure 2: Improved shakeoff electron (SOE) timing wrt the $\beta$ compared to Fig. 4 of Ref. Fenker2018, and the resulting smaller background. The data timing cut is shown. The summed simulation reproduces the data well in the region of interest. 10% of real coincidences are from electrons not firing eMCP, scattering, and returning in the E field 14 ns later-- note this late 10% of the prompt $\beta$-$\gamma$ coincidences do not fall in the ROI.
• Figure 3: Superratio asymmetry from "Dataset B" (one-half of the useful data) with 1$\sigma$ statistical uncertainties, compared to the GEANT4 simulation with best fit $b$ and $A_\beta$ to Set B and to all data (almost same), and the residuals.
• Figure 4: A $\chi^2$ map of all experimental data compared to a simulated parameter space of $A_\beta$ and $b$, showing in solid lines the 1 $\sigma$ 68% confidence limits from a change in $\chi^2$ of 2.3 from the minimum. Dashed lines are 90% CL. The larger, red ellipses determine the more general measurement of $b$ from Eq. 1. The tighter, black ellipses add the $chi^2$ contribution from the predicted center value of $A_{\beta}$ from $Ft$ information (see text), which determines $C_S+C'_S$ vs. $C_T+C'_T$ in Fig. 5 below.
• Figure 5: Exclusion plot at 1 $\sigma$ of Lorentz scalar and tensor lepton-quark couplings to left-handed $\nu$'s and wrong-handed $\beta$'s. Consistency between neutron $\beta$ decay, nuclear $\beta$ decay, and particle physics requires a non-universal interaction with scale less than TeV. Our present $^{37}$K constraints from $b$ (solid black lines), along with our tighter constraints combining $b$, $A_{\beta}$ and $fT$ info (black ellipse), are also detailed in text.