- #1
lriuui0x0
- 101
- 25
- Homework Statement
- A particle ##P## of mass ##M## decays into a particle ##R## with mass ##0 < m < M## and a massless particle. Calculate the speed of ##R## in ##P##'s rest frame
- Relevant Equations
- ##m^2c^2 = E^2/c^2 - \mathbf{p}^2##
I have attempted a solution using conservation of momentum. Could people help check if this solution is correct (the result looks weird), as the problem doesn't have solution with it.
$$
\begin{aligned}
\begin{pmatrix}Mc \\ 0\end{pmatrix} &= \begin{pmatrix}E_R/c \\ \mathbf{p}_R\end{pmatrix} + \begin{pmatrix}E_\gamma/c \\ \mathbf{p}_\gamma \end{pmatrix} \\
\begin{pmatrix}Mc \\ 0\end{pmatrix} &= \begin{pmatrix}\sqrt{m^2c^2 + \mathbf{p}_R^2} \\ \mathbf{p}_R\end{pmatrix} + \begin{pmatrix}|\mathbf{p}_R| \\ -\mathbf{p}_R \end{pmatrix} \\
M^2c^2 &= m^2c^2 + 2(\sqrt{m^2c^2 + \mathbf{p}_R^2}|\mathbf{p}_R| + \mathbf{p}_R^2) \\
M^2c^2 &= m^2c^2 + 2(m\gamma v\sqrt{m^2c^2 + m^2\gamma^2v^2} + m^2\gamma^2v^2) \\
M^2c^2 &= m^2c^2 + 2(m^2\gamma cv\sqrt{1 + \gamma^2(1-\gamma^{-2})} + m^2\gamma^2v^2) \\
M^2c^2 &= m^2c^2 + 2(m^2\gamma^2cv + m^2\gamma^2v^2) \\
M^2c^2 &= m^2(c^2 + 2\gamma^2cv + \gamma^2v^2) \\
M^2c^2 &= m^2(c^2 + \frac{2cv}{1-\frac{v^2}{c^2}} + \frac{v^2}{1-\frac{v^2}{c^2}}) \\
M^2c^2 &= m^2\frac{c^4 - c^2v^2 + 2c^3v + c^2v^2}{c^2 - v^2} \\
M^2 &= m^2\frac{c^2 + 2cv}{c^2 - v^2} \\
M^2c^2 - M^2v^2 &= m^2c^2 + 2m^2cv \\
M^2v^2 + 2m^2cv + (m^2-M^2)c^2 &= 0 \\
v^2 + 2r^2cv +(r^2-1)c^2 &= 0 \\
v &= (\sqrt{r^4-r^2+1} - r^2)c \\
\end{aligned}
$$
where we have replaced ##r = \frac{m}{M}##.
$$
\begin{aligned}
\begin{pmatrix}Mc \\ 0\end{pmatrix} &= \begin{pmatrix}E_R/c \\ \mathbf{p}_R\end{pmatrix} + \begin{pmatrix}E_\gamma/c \\ \mathbf{p}_\gamma \end{pmatrix} \\
\begin{pmatrix}Mc \\ 0\end{pmatrix} &= \begin{pmatrix}\sqrt{m^2c^2 + \mathbf{p}_R^2} \\ \mathbf{p}_R\end{pmatrix} + \begin{pmatrix}|\mathbf{p}_R| \\ -\mathbf{p}_R \end{pmatrix} \\
M^2c^2 &= m^2c^2 + 2(\sqrt{m^2c^2 + \mathbf{p}_R^2}|\mathbf{p}_R| + \mathbf{p}_R^2) \\
M^2c^2 &= m^2c^2 + 2(m\gamma v\sqrt{m^2c^2 + m^2\gamma^2v^2} + m^2\gamma^2v^2) \\
M^2c^2 &= m^2c^2 + 2(m^2\gamma cv\sqrt{1 + \gamma^2(1-\gamma^{-2})} + m^2\gamma^2v^2) \\
M^2c^2 &= m^2c^2 + 2(m^2\gamma^2cv + m^2\gamma^2v^2) \\
M^2c^2 &= m^2(c^2 + 2\gamma^2cv + \gamma^2v^2) \\
M^2c^2 &= m^2(c^2 + \frac{2cv}{1-\frac{v^2}{c^2}} + \frac{v^2}{1-\frac{v^2}{c^2}}) \\
M^2c^2 &= m^2\frac{c^4 - c^2v^2 + 2c^3v + c^2v^2}{c^2 - v^2} \\
M^2 &= m^2\frac{c^2 + 2cv}{c^2 - v^2} \\
M^2c^2 - M^2v^2 &= m^2c^2 + 2m^2cv \\
M^2v^2 + 2m^2cv + (m^2-M^2)c^2 &= 0 \\
v^2 + 2r^2cv +(r^2-1)c^2 &= 0 \\
v &= (\sqrt{r^4-r^2+1} - r^2)c \\
\end{aligned}
$$
where we have replaced ##r = \frac{m}{M}##.