# General Spin Analysis from Angular Correlations in Two-Body Decays

## TL;DR

This paper develops a comprehensive helicity formalism to analyze particle spins and couplings in two-body decays, connecting decay distributions across reference frames, with applications to Standard Model and new physics processes.

## Contribution

It introduces a general analytic framework for relating decay helicity amplitudes and distributions in different frames, applicable to various spins and couplings, including new physics scenarios.

## Key findings

- Formulas for decay helicity amplitudes and distributions are derived.
- The framework is demonstrated with Standard Model processes and a new vectorlike top quark decay.
- The approach enables spin and coupling determination in complex decay chains.

## Abstract

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 $\tau^-\to \rho^-\nu_\tau\to (\pi^-\pi^0)\nu_\tau$ and the sequential process $e^-e^+\to t\bar{t}$ followed by $t\to W^+ b \to (\ell^+\nu_\ell)b$, and one non-standard decay process of a new vectorlike heavy top quark, $T\to Z t$, followed by $Z\to \ell^-\ell^+$. All the useful formulas directly applicable to any combinations of spins and any types of couplings in the two-body decay $X_2\to Y X_1$ followed by suitable $Y$ two-body decays processes are collected and described in detail.