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Let's begin with a general amplitude for the two-body decay of a state with angular momentum quantum numbers ''J'',''m''. Specifically, we want to know the amplitude of this state for having daughter 1 with momentum direction <math>\Omega=(\phi,\theta)</math> in the center of mass reference frame, and helicity <math>\lambda_1</math>, while daughter 2 has direction <math>-\Omega=(\phi+\pi,\pi-\theta)</math> and helicity <math>\lambda_2</math>.   
 
Let's begin with a general amplitude for the two-body decay of a state with angular momentum quantum numbers ''J'',''m''. Specifically, we want to know the amplitude of this state for having daughter 1 with momentum direction <math>\Omega=(\phi,\theta)</math> in the center of mass reference frame, and helicity <math>\lambda_1</math>, while daughter 2 has direction <math>-\Omega=(\phi+\pi,\pi-\theta)</math> and helicity <math>\lambda_2</math>.   
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<table>
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Let U be the decay operator from the initial state into the given 2-body final state.  Insertion of the complete set of helicity basis vectors gives
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<math>
<td><math>
   
\langle \Omega \lambda_1 \lambda_2 | U | J m \rangle
 
\langle \Omega \lambda_1 \lambda_2 | U | J m \rangle
 
=
 
=
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| J m \lambda_1 \lambda_2 \rangle \langle J m \lambda_1 \lambda_2 |
 
| J m \lambda_1 \lambda_2 \rangle \langle J m \lambda_1 \lambda_2 |
 
U | J m \rangle
 
U | J m \rangle
</math></td>
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</math>
<td>
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insertion of the complete set of helicity basis vectors
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</td>
 
</td>
 
</tr>
 
</tr>

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