Two-body decay threshold
= Two-body decay threshold
{title2=$M\ge m_1+m_2$}
In the rest frame of the parent <particle>, <four-momentum conservation> requires $M=E_1+E_2\ge m_1+m_2$. For two equal masses the daughter <momentum> magnitude is $\tfrac12\sqrt{M^2-4m^2}$; the <two-body decay phase space> closes at $M=2m$. For a finite constant <decay amplitude>, the <decay width> tends to zero at this threshold.