General Spin Analysis from Angular Correlations in Two-Body Decays

Determining the spin of any new particle and measuring its couplings to other particles and/or itself are crucial in reconstructing the structure of any quantum field theory containing the particle. A general helicity formalism is employed to describe the polarization of the particle $Y$ in a two-body decay $X_2\to Y{X}_1$ with polarized $X_2$ for the purpose of diagnosing the dynamical properties of three involved particles and for determining their spins altogether. We perform a general and comprehensive analytic analysis with our special focus on grasping fully how to connect the decay helicity amplitudes and decay distributions in the $X_2$ rest frame and those in a laboratory frame with $X_2$ moving with a non-zero velocity through Wick helicity rotation on helicity states and amplitudes. This theoretical framework is demonstrated in a detailed illustrative manner with the Standard Model (SM) processes, the sequential process $e^{-}{e}^{+}\to Z\to {\tau }^{-}{\tau }^{+}$ followed by $\tau^-\to \rho^-\nu_\tau\to (\pi^-\pi^0)\nu_\tau$andthesequentialprocess$e^-e^+\to t\bar{t}$followedby$t\to W^+ b \to (\ell^+\nu_\ell)b$, and one non-standard decay process of a new vectorlike heavy top quark, $T\to Zt$, followed by $Z\to {\mathrm{\ell }}^{-}{\mathrm{\ell }}^{+}$. All the useful formulas directly applicable to any combinations of spins and any types of couplings in the two-body decay $X_2\to Y X_1$followedbysuitable$Y$ two-body decays processes are collected and described in detail.