Experimental Constraints on Left-Right Symmetric Models from Muon Decay

The TWIST Collaboration has completed a new measurement of the energy-angle spectrum of positrons from the decay of highly polarized muons. A simultaneous measurement of the muon decay parameters $\rho$, $\delta$, and ${\mathcal{P}}_{\mu }^{\pi }\xi$ tests the standard model in a purely leptonic process and provides improved limits for relevant extensions to the standard model. Specifically, for the generalized left-right symmetric model $|(g_R/g_L)\zeta |<0.020$ and $(g_L/g_R)m_2>578\mathrm{GeV}/c^2$, both 90% C.L.

Figure 1

Residuals for the fit of one nominal data set to simulation in units of standard deviations. The fiducial region is outlined.

Figure 2

Summary of published central values and total uncertainties for $\rho$, $\delta$, and ${\mathcal{P}}_{\mu }^{\pi }\xi$ [5, 6, 7, 14, 15], along with the results of this analysis. Vertical lines represent the SM values.

Figure 3

Allowed region (90% C.L.) of mixing angle ($\zeta$) and heavy $W$ mass ($m_2$) for the general left-right symmetric model.