Flux of Incoming Particles

Flux of Incoming Particles

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

Using the operator relations

[math]\displaystyle{ E=i\hbar \frac{\partial}{\partial t} \qquad \vec p=i\hbar \nabla }[/math]

We can obtain the relativitic Schrödinger equation, also known as the Klein-Gordon equation.

[math]\displaystyle{ -\frac{\partial^2 \phi}{\partial t^2}+\nabla^2 \phi=m^2 \phi }[/math]

[math]\displaystyle{ \frac{\partial}{\partial t}\left [i \left (\phi^*\frac{\partial \phi}{\partial t}-\phi \frac{\partial \phi^*}{\partial t} \right ) \right ]+\vec \nabla \cdot \left [-i\left (\phi^*\vec \nabla \phi-\phi \vec \nabla \phi^* \right ) \right ]=0 }[/math]

[math]\displaystyle{ \phi=Ne^{i\vec p \cdot \vec x -iEt} }[/math]

[math]\displaystyle{ \rho=i(-2iE)|N|^2=2E|N|^2 }[/math]

[math]\displaystyle{ \vec j=-i(2i\vec p)|N|^2=2\vec p|N|^2 }[/math]

The number of particles in a beam passing through a unit area per unit time is

[math]\displaystyle{ \frac{Number\ of\ beam\ particles}{time\times Volume}= }[/math] [math]\displaystyle{ \frac{|\vec v_1|2E_1}{V} }[/math]

The number of stationary target particles per unit volume is

[math]\displaystyle{ \frac{Number\ of\ target\ particles}{Volume} }[/math]

[math]\displaystyle{ \frac{2E_2}{V} }[/math]

Initial flux=[math]\displaystyle{ |\vec v_1|\frac{2E_1}{V} \frac{2E_2}{V} }[/math]

[math]\displaystyle{ F=2E_1 2E_2|\vec {v}_1-\vec {v}_2|=4|E_1E_2\vec v_{21}| }[/math]

where [math]\displaystyle{ v_{21} }[/math] is the relative velocity between the particles in the frame where particle 1 is at rest

[math]\displaystyle{ \mathbf P_1 \cdot \mathbf P_2 = E_{1}E_{2}-(\vec p_1 \vec p_2)= E_{1}E_{2} }[/math]

Using the relativistic definition of energy

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

[math]\displaystyle{ \rightarrow \mathbf P_1 \cdot \mathbf P_2 =mE_{2} }[/math]

Letting [math]\displaystyle{ E_{21}\equiv E_2 }[/math] be the energy of particle 2 wiith respect to particle 1, the relativistic energy equation can be rewritten such that

[math]\displaystyle{ |p_{21}^2| =E_{21}^2-m^2=\frac{(\mathbf P_1 \cdot \mathbf P_2)^2}{m^2}-m^2=\frac{(\mathbf P_1 \cdot \mathbf P_2)^2-m^4}{m^2} }[/math]

where similarly [math]\displaystyle{ p_{21} }[/math] is defined as the momentum of particle 2 with respect to particle 1.

The relative velocity can be expressed as

[math]\displaystyle{ v_{21}=\frac{|\vec p_{21}|}{E_{21}} }[/math]

[math]\displaystyle{ F=2E_1 2E_2|\vec {v}_1-\vec {v}_2|=4|mE_{21}\vec v_{12}|=4|mE_{21}\frac{|\vec p_{21}|}{E_{21}}|=4m|\vec p_{21}| }[/math]

The invariant form of F is

[math]\displaystyle{ F=4\sqrt{(\mathbf P_1 \cdot \mathbf P_2)^2-m^4} }[/math]

[math]\displaystyle{ \mathbf P_1 \cdot \mathbf P_2 = E_{1}E_{2}-(\vec p_1 \vec p_2) }[/math]

where in the center of mass frame [math]\displaystyle{ E_1=E_2 }[/math] and [math]\displaystyle{ \vec p_1^*=-\vec p_2^* }[/math]

[math]\displaystyle{ \mathbf P_1 \cdot \mathbf P_2 = E_1^2+\vec p_1\,^{*2} }[/math]

[math]\displaystyle{ F=4\sqrt{(E_1^2+\vec p_1\,^{*2})^2-m^4}=4\sqrt{(\vec p_1 \,^{*2}+m^2+\vec p_1\,^{*2})^2-m^4} }[/math]

[math]\displaystyle{ F=4\sqrt{(2\vec p_1\,^{*2}+m^2)^2-m^4}=4\sqrt{4\vec p_1\,^{*4}+m^4+4\vec p_1\,^{*2}m-m^4} }[/math]

[math]\displaystyle{ F_{cms}=4 \sqrt {4m^2\vec p_1\,^{*2}+4\vec p_1 \,^{*4}} }[/math]

[math]\displaystyle{ F_{cms}=4 \sqrt {\vec p_1\,^{*2}4(m^2+\vec p_1 \,^{*2})} }[/math]

As shown earlier

[math]\displaystyle{ s_{CM}=4(m^2+\vec p_1 \,^{*2}) }[/math]

[math]\displaystyle{ F_{cms}=4 \sqrt {\vec p_1^*\,^2s} }[/math]