DV Calculations of 4-momentum components

Calculations of 4-momentum components

Initial Lab Frame

Assume the incoming electron has momentum of 11000 MeV in the positive z direction.

[math]p_{1\ z}\equiv p_{1}=11000 MeV[/math]

The moller electron is initially at rest

[math]p_{2}\equiv 0[/math]

The total energy in this frame

[math]E\equiv \sqrt{p^2+m^2}[/math]

[math]E\equiv \sqrt{(p_{1}+p_{2})^2+(m_{1}+m_{2})^2}[/math]

[math]\Longrightarrow E\equiv \sqrt{(11000 MeV)^2+(.511 MeV+.511 MeV)^2}\approx 11000 MeV[/math]

Center of Mass Frame

Using the definition

[math]E^2\equiv p^2c^2+m^2c^4[/math]

[math]\Longrightarrow p^2=\frac {E^2}{c^2}-m^2 c^2[/math]

We can use 4-momenta vectors, i.e. [math]{\mathbf P}\equiv \left(\begin{matrix} E/c\\ p_x \\ p_y \\ p_z \end{matrix} \right)[/math]

we can use the fact that the scalar product of a 4-momenta with itself,

[math]{\mathbf P_1}\cdot {\mathbf P_1}=E_1E_1-\vec p_1\cdot \vec p_1 c^2=m_1^2c^4=s[/math]

is invariant

Using this, the sum of two 4-momenta forms a 4-vector as well

[math]{\mathbf P_1}+ {\mathbf P_2}= \left( \begin{matrix}E_1+E_2\\\vec p_1 c+\vec p_2 c\end{matrix} \right)= {\mathbf P}[/math]

The length of this four-vector is an invariant as well

[math]{\mathbf P^2}=({\mathbf P_1}+{\mathbf P_2})^2=(E_1+E_2)^2-(\vec p_1 c+\vec p_2 c)^2=(m_1c^4+m_2c^4)^2=s[/math]

[math]{\mathbf P^2}=({\mathbf P_1}+{\mathbf P_2})^2=(E_1+E_2)^2-(\vec p_1 +\vec p_2 )^2=s[/math] (with c=1)

For incoming electrons moving only in the z-direction, we can write

[math]{\mathbf P_1}+ {\mathbf P_2}= \left( \begin{matrix}E_1+E_2\\ 0 \\ 0 \\ p_{1z}+p_{2z}\end{matrix} \right)={\mathbf P}[/math]

We can perform a Lorentz transformation to the Center of Mass frame, with zero total momentum

[math]\left( \begin{matrix}E^*_{1}+E^*_{2}\\ 0 \\ 0 \\ 0\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^* \gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}E_{1}+E_{2}\\ 0 \\ 0 \\ p_{1\ z}+p_{2\ z}\end{matrix} \right)[/math]

Without knowing the values for gamma or beta, we can use the fact that lengths of the two 4-momenta are invariant

[math]s={\mathbf P^*}^2=(E^*_{1}+E^*_{2})^2-(\vec p_{1}\ ^*+\vec p_{2}\ ^*)^2[/math]

[math]s={\mathbf P}^2=(E_{1}+E_{2})^2-(\vec p_{1}+\vec p_{2})^2[/math]

[math]\Longrightarrow m_{1}=m^*_{1}\ ; m_{2}=m^*_{2}[/math]

Setting these equal to each other, we can use this for the collision of two particles of mass m₁ and m₂. Since the total momentum is zero in the Center of Mass frame, we can express total energy in the center of mass frame as

[math](E^*_{1}+E^*_{2})^2-(\vec p^*_{1}+\vec p^*_{2})^2=s=(E_{1}+E_{2})^2-(\vec{p_{1}}+\vec p_{2})^2[/math]

[math](E^*)^2-(\vec p^*)^2=(E_{1}+E_{2})^2-(\vec{p_{1}}+\vec p_{2})^2[/math]

[math](E^*)^2=(E_{1}+E_{2})^2-(\vec{p_{1}}+\vec p_{2})^2[/math]

[math]E^*=[(E_{1}+E_{2})^2-(\vec{p_{1}}+\vec p_{2})^2]^{1/2}[/math]

[math]E^*=[E_{1}^2+2E_{1}E_{2}+E_{2}^2-\vec p_{1} . \vec p_{2} -\vec p_{1} . \vec p_{1} -\vec p_{2} . \vec p_{1} -\vec p_{2} . \vec p_{2} ]^{1/2}[/math]

[math]E^*=[(E_{1}^2- p_{1}^2 )+(E_{2}^2-p_{2}^2 )+2E_{1}E_{2}-\vec p_{1} . \vec p_{2} -\vec p_{2} . \vec p_{1} ]^{1/2}[/math]

[math]E^*=[(E_{1}^2- p_{1}^2 )+(E_{2}^2-p_{2}^2 )+2E_{1}E_{2}-p_{1} p_{2}\cos(\theta) - p_{2} p_{1}\cos(\theta) ]^{1/2}[/math]

[math]E^*=[m_{1}^2+m_{2}^2+2E_{1}E_{2}-p_{1} p_{2}\cos(\theta) - p_{2} p_{1}\cos(\theta) ]^{1/2}[/math]

[math]E^*=[m_{1}^2+m_{2}^2+2E_{1}E_{2}-2p_{1} p_{2}\cos(\theta) ]^{1/2}[/math]

[math]E^*=[m_{1}^2+m_{2}^2+2E_{1}E_{2}-2p_{1} p_{2}\cos(\theta) ]^{1/2}[/math]

Using the relations [math]\beta\equiv \vec p/E\Longrightarrow \vec p=\beta E[/math]

[math]E^*=[m_{1}^2+m_{2}^2+2E_{1}E_{2}(1-\beta_{1}\beta_{2}\cos(\theta))]^{1/2}[/math]

where [math] \theta [/math] is the angle between the particles in the Lab frame.

In the frame where one particle (m_{2 Lab}) is at rest

[math]\Longrightarrow \beta_{2}=0[/math]

[math]\Longrightarrow p_{2}=0[/math]

which implies,

[math] E_{2}=[p_{2}^2+m_{2}^2]^{1/2}=m_{2}[/math]

[math]E^*=(m_{1}^2+m_{2}^2+2E_{1} m_{2})^{1/2}=(.511MeV^2+.511MeV^2+2(\sqrt{(11000 MeV)^2+(.511 MeV)^2})(.511 MeV))^{1/2}\approx 106.030760886 MeV[/math]

where [math]E_{1}=\sqrt{p_{1}^2+m_{1}^2}\approx 11000 MeV[/math]

Inspecting the Lorentz transformation to the Center of Mass frame:

[math]\left( \begin{matrix}E^*_{1}+E^*_{2}\\ 0 \\ 0 \\ 0\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^* \gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}E_{1}+E_{2}\\ 0 \\ 0 \\ p_{1\ z}+p_{2\ z}\end{matrix} \right)[/math]

For the case of a stationary electron, this simplifies to:

[math]\left( \begin{matrix} E^* \\ p^*_{x} \\ p^*_{y} \\ p^*_{z}\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^*\gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}E_{1}+m_{2}\\ 0 \\ 0 \\ p_{1\ z}+0\end{matrix} \right)[/math]

which gives,

[math]\Longrightarrow\begin{cases} E^*=\gamma^* (E_{1}+m_{2})-\beta^* \gamma^* p_{1\ z} \\ p^*_{z}=-\beta^* \gamma^*(E_{1}+m_{2})+\gamma^* p^*_{1\ z} \end{cases}[/math]

Solving for [math]\beta^*[/math], with [math]p^*_{z}=0[/math]

[math]\Longrightarrow \beta^* \gamma^*(E_{1}+m_{2})=\gamma^* p_{1\ z}[/math]

[math]\Longrightarrow \beta^*=\frac{p_{1}}{(E_{1}+m_{2})}[/math]

Similarly, solving for [math]\gamma^*[/math] by substituting in [math]\beta^*[/math]

[math]E_{CM}=\gamma_{CM} (E_{1 Lab}+m_{2 Lab})-\frac{p_{1 Lab}}{(E_{1 Lab}+m_{2 Lab})} \gamma_{CM} p_{1z Lab}[/math]

[math]E^*=\gamma^* \frac{(E_{1}+m_2)^2}{(E_{1}+m_{2})}-\gamma^*\frac{(p_{1\ z})^2}{(E_{1}+m_{2})}[/math]

Using the fact that [math]E^*=[(E_{1}+E_{2})^2-(\vec p_{1}+\vec p_{2})^2]^{1/2}[/math]

[math]E^*=\gamma^* \frac{E^*^2}{(E_{1}+m_{2})}[/math]

[math]\Longrightarrow \gamma^*=\frac{(E_{1}+m_{2})}{E^*}[/math]

This gives the momenta of the particles in the center of mass to have equal magnitude, but opposite directions

Using the relation

[math]\left( \begin{matrix} E^*_{1}+E^*_{2} \\ p^*_{1\ x}+p^*_{2\ x} \\ p^*_{1\ y}+p^*_{2\ y} \\ p^*_{1\ z}+p^*_{2\ z}\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^* \gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}E_{1}+m_{2}\\ 0 \\ 0 \\ p_{1\ z}+0\end{matrix} \right)[/math]

[math]\Longrightarrow \left( \begin{matrix} E^*_{2} \\ p^*_{2\ x} \\ p^*_{2\ y} \\ p^*_{2\ z}\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^* \gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}m\\ 0 \\ 0 \\ 0\end{matrix} \right)[/math]

[math]\Longrightarrow\begin{cases} E^*_{2}=\gamma^* (m_{2}) \\ p^*_{2\ z}=-\beta^* \gamma^* (m_{2}) \end{cases}[/math]

[math]\Longrightarrow\begin{cases} E^*_{2}=\frac{(E_{1}+m_{2})}{E^*} (m_2) \\ p^*_{2\ z}=-\frac{p_{1}}{(E_{1}+m_2)} \frac{(E_{1}+m_{2})}{E^*} (m_{2}) \end{cases}[/math]

[math]\Longrightarrow\begin{cases} E^*_{2}=\frac{(11000 MeV+.511 MeV)}{106.031 MeV} (.511 MeV) \approx 53.0129177 MeV\\ p^*_{2\ z}=-\frac{11000 MeV}{106.031 MeV} (.511 MeV) \approx -53.013 MeV \end{cases}[/math]

[math]\Longrightarrow \left( \begin{matrix} E^*_{1}\\ p^*_{1\ x} \\ p^*_{1\ y} \\ p^*_{1\ z}\end{matrix} \right)=\left(\begin{matrix}\gamma^* & 0 & 0 & -\beta^* \gamma^*\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ -\beta^* \gamma^* & 0 & 0 & \gamma^* \end{matrix} \right) . \left( \begin{matrix}E_{1}\\ 0 \\ 0 \\ p_{1\ z}\end{matrix} \right)[/math]

[math]\Longrightarrow\begin{cases} E^*_{1}=\gamma^* (E_{1})-\beta^* \gamma^* p_{1\ z} \\ p^*_{1\ z}=-\beta^* \gamma^*(E_{1})+\gamma^* p_{1\ z} \end{cases}[/math]

[math]\Longrightarrow\begin{cases} E^*_{1}=\frac{(E_{1}+m_{2})}{E^*} (E_{1})-\frac{p_{1\ z}}{(E_{1}+m_2)} \frac{(E_{1}+m_{2})}{E^*} p_{1\ z} \\ p^*_{1\ z}=-\frac{p_{1}}{(E_{1}+m_2)} \frac{(E_{1}+m_{2})}{E^*}(E_{1})+\frac{(E_{1}+m_{2})}{E^*} p_{1\ z} \end{cases}[/math]

[math]\Longrightarrow\begin{cases} E^*_{1}=\frac{(11000 MeV+.511 MeV)}{106.031 MeV} (11000 MeV)-\frac{11000 MeV}{106.031 MeV} 11000 MeV \approx 53.013 MeV\\ p^*_{1\ z}=-\frac{11000 MeV}{106.031 MeV}(11000 MeV)+\frac{(11000 MeV+.511 MeV)}{106.031 MeV} 11000 MeV \approx 53.013 MeV \end{cases}[/math]

[math]p^*_{1} =\sqrt{p^*_{1\ x}^2+p^*_{1\ y}^2+p^*_{1\ z}^2}\Longrightarrow p^*_{1}=p^*_{1\ z}[/math]

[math]p^*_{2} =\sqrt{p^*_{2\ x}^2+p^*_{2\ y}^2+p^*_{2\ z}^2}\Longrightarrow p^*_{2}=p^*_{2\ z}[/math]

[math] p^*_{1}=p^*_{2}\approx 53.013 MeV[/math]

[math]E^*_{1}= E^*_{2}\approx 53.013 MeV[/math]

Moller electron Lab Frame

Finding the correct kinematic values starting from knowing the momentum of the Moller electron, [math]'''p_{(m)'''Lab}[/math] , in the Lab frame,

Using [math]\theta=\arccos \left(\frac{p_{z(m) Lab}}{p_{(m)Lab}}\right)[/math]

[math]\Longrightarrow {p_{z(m) Lab}=p_{(m)Lab}\cos(\theta)}[/math]

Checking on the sign resulting from the cosine function, we are limited to:

[math]0^\circ \le \theta \le 60^\circ \equiv 0 \le \theta \le 1.046\ Radians[/math]

Since,

[math]\frac{p_{(m)zLab}}{p_{(m)Lab}}=cos(\theta)[/math]

[math]\Longrightarrow p_{(m)zLab}\ should\ always\ be\ positive[/math] Similarly, [math]\phi=\arccos \left( \frac{p_{x(m) Lab}}{p_{xy(m) Lab}} \right)[/math]

where [math]p_{xy(m) Lab}=\sqrt{p_{x(m) Lab}^2+p_{y(m)Lab}^2}[/math]

[math]p_{xy(m) Lab}=(p_{x(m) Lab}^2+p_{y(m) Lab}^2)^2[/math]

and using [math]p_{(m)Lab}^2=p_{x(m) Lab}^2+p_{y(m) Lab}^2+p_{z(m) Lab}^2[/math]

this gives [math]p_{(m)Lab}^2=p_{xy(m) Lab}^2+p_{z(m) Lab}^2[/math]

[math]\Longrightarrow p_{(m)Lab}^2-p_{z(m) Lab}^2=p_{xy(m) Lab}^2[/math]

[math]\Longrightarrow p_{xy(m) Lab}=\sqrt{p_{(m)Lab}^2-p_{z(m) Lab}^2}[/math]

which gives[math]\phi = \arccos \left( \frac{p_{x(m) Lab}}{\sqrt{p_{(m)Lab}^2-p_{z(m) Lab}^2}}\right)[/math]

[math]\Longrightarrow p_{x(m) Lab}=\sqrt{p_{(m)Lab}^2-p_{z(m) Lab}^2} \cos(\phi)[/math]

Similarly, using [math]p_{(m)Lab}^2=p_{x(m) Lab}^2+p_{y(m) Lab}^2+p_{z(m) Lab}^2[/math]

[math]\Longrightarrow p_{(m)Lab}^2-p_{x(m) Lab}^2-p_{z(m) Lab}^2=p_{y(m) Lab}^2[/math]

[math]p_{y(m) Lab}=\sqrt{p_{(m)Lab}^2-p_{x(m) Lab}^2-p_{z(m) Lab}^2}[/math]

Checking on the sign from the cosine results for [math]\phi[/math]

[math]-\pi \le \phi \le \pi\ Radians[/math]

[math]For\ 0 \le \phi \le \frac{-\pi}{2}\ Radians[/math]

x=POSITIVE

y=NEGATIVE

[math]For\ 0 \le \phi \le \frac{\pi}{2}\ Radians[/math]

x=POSITIVE

y=POSITIVE

[math]For\ \frac{-\pi}{2} \le \phi \le -\pi\ Radians[/math]

x=NEGATIVE

y=NEGATIVE

[math]For\ \frac{\pi}{2} \le \phi \le \pi\ Radians[/math]

x=NEGATIVE

y=POSITIVE

Moller electron Center of Mass Frame

Relativistically, the x and y components remain the same in the conversion from the Lab frame to the Center of Mass frame, since the direction of motion is only in the z direction.

[math]p_{x(m) CM}\Leftrightarrow p_{x(m) Lab}[/math]

[math]p_{y(m) CM}\Leftrightarrow p_{y(m) Lab}[/math]

[math]p_{z(m) CM}=\sqrt{p_{(m)CM}^2-p_{x(m) CM}^2-p_{y(m) CM}^2}[/math]

[math]\Longrightarrow E_{(m) CM}=\sqrt{\frac{m(m+E_{(e^-) Lab})}{2}}[/math]

Electron Center of Mass Frame

Relativistically, the x, y, and z components have the same magnitude, but opposite direction, in the conversion from the Moller electron's Center of Mass frame to the electron's Center of Mass frame.

[math]p_{x(m) CM}= -p_{x(e^-) CM}[/math]

[math]p_{y(m) CM}= -p_{y((e^-) CM}[/math]

[math]p_{z(m) CM}=-p_{z((e^-) CM}[/math]

where previously it was shown

[math]p_{(e^-) CM}\approx 53.013 MeV[/math]

[math]E_{(e^-) CM}\approx 53.013 MeV[/math]

Electron Lab Frame

We can perform a Lorentz transformation from the Center of Mass frame, with zero total momentum, to the Lab frame.

[math]\left( \begin{matrix}E_{(e^-) Lab}+E_{(m) Lab}\\ p_{(e^-)z Lab}+p_{(m)z Lab} \\p_{(e^-)z Lab}+ p_{(m)z Lab} \\ p_{(e^-)z Lab}+p_{(m)z Lab}\end{matrix} \right)=\left(\begin{matrix}\gamma_{CM} & 0 & 0 & \beta_{CM} \gamma_{CM}\\0 & 1 & 0 & 0 \\ 0 & 0 & 1 &0 \\ \beta_{CM} \gamma_{CM} & 0 & 0 & \gamma_{CM} \end{matrix} \right) . \left( \begin{matrix}E_{(e^-) CM}+E_{(m) CM}\\ p_{(e^-)x CM}+p_{(m)x CM} \\ p_{(e^-)y CM}+p_{(m)y CM} \\ p_{(e^-)z CM}+p_{(m)z CM}\end{matrix} \right)[/math]

[math]p_{x(e^-) CM}= p_{x(e^-) Lab}[/math]

[math]p_{y(e^-) CM}= p_{y((e^-) Lab}[/math]

[math]\Longrightarrow\begin{cases} E_{Lab}=\gamma_{Lab} (E_{ CM})+\beta_{Lab} \gamma_{Lab} p_{z CM} \\ p_{z Lab}=\beta_{Lab} \gamma_{Lab}(E_{ CM})+ \gamma_{Lab}p_{z CM} \end{cases}[/math]

[math]\Longrightarrow\begin{cases} \gamma_{Lab} = \frac{E_{ Lab}}{E_{CM}} \\ \beta_{Lab} =\frac{p_{zLab}}{E_{LAB}} \end{cases}[/math]

Without knowing the values for gamma or beta, we can use the fact that lengths of the two 4-momenta are invariant

[math]s={\mathbf P}_{CM}^2=(E_{(e^-) CM}+E_{(m) CM})^2-(\vec p_{(e^-) CM}+\vec p_{(m) CM})^2[/math]

[math]s={\mathbf P}_{Lab}^2=(E_{(e^-) Lab}+E_{(m) Lab})^2-(\vec p_{(e^-) Lab}+\vec p_{(m) Lab})^2[/math]

Setting these equal to each other, we can use this for the collision of two particles of mass m₁ and m₂. Since the total momentum is zero in the Center of Mass frame, we can express total energy in the center of mass frame as

[math](E_{1 CM}+E_{2 CM})^2-(\vec p_{1 CM}+\vec p_{2 CM})^2=s=(E_{1 Lab}+E_{2 Lab})^2-(\vec{p_{1 Lab}}+\vec p_{2 Lab})^2[/math]

[math](E_{CM})^2-(\vec p_{CM})^2=(E_{1 Lab}+E_{2 Lab})^2-(\vec{p_{Lab}})^2[/math]

[math](E_{CM})^2=(E_{Lab})^2-(\vec{p_{Lab}})^2[/math]

[math](E_{CM})^2+(\vec{p_{Lab}})^2=(E_{Lab})^2[/math]

[math]E_{Lab}=\sqrt{(E_{CM})^2+(\vec{p_{Lab}})^2}[/math]

Since momentum is conserved, the initial momentum from the incident electron and stationary electron is still the same in the Lab frame, therefore [math]p_{Lab}=11000 MeV[/math]

[math]E_{Lab}=\sqrt{(106.031 MeV)^2+(11000 MeV)^2}\approx 11000.511 MeV[/math]

This is the same as the initial total energy in the Lab frame, which should be expected since scattering is considered to be an elastic collision.

Since,

[math]E_{Lab}\equiv E_{(e^-)Lab}+E_{(m)Lab}[/math]

[math]\Longrightarrow E_{(e^-)Lab}=E_{Lab}-E_{(m)Lab}[/math]

Using the relation

[math]E^2\equiv p^2+m^2[/math]

[math]\Longrightarrow p_{(e^-)Lab}^2=E_{(e^-)Lab}^2-m_{(e^-)Lab}^2=E_{(e^-)Lab}^2-(.511 MeV)^2[/math]

Using

[math]p^2 \equiv p_x^2+p_y^2+p_z^2[/math]

[math]\Longrightarrow p_{(e^-)z Lab}=\sqrt{p_{(e^-)Lab}^2-p_{(e^-)xLab}^2-p_{(e^-)Lab}^2}[/math]

DV_RunGroupC_Moller#Momentum distributions in the Center of Mass Frame

DV_MollerTrackRecon#Calculations_of_4-momentum_components