- #1
Dilatino
- 12
- 0
Assuming that a massive spin-1 particle has momentum only in the z-direction, the polarization vectors are given by
[tex]\varepsilon_{\mu}(J_z = +1) = (0,-\frac{1}{\sqrt{2}},-\frac{i}{\sqrt{2}},0 )[/tex]
[tex]\varepsilon_{\mu}(J_z = 0) = (\frac{p}{m},0,0, \frac{E}{m})[/tex]
[tex]\varepsilon_{\mu}(J_z = -1) = (0,\frac{1}{\sqrt{2}},-\frac{i}{\sqrt{2}},0 )[/tex]
The so-called spin-sum is the claimed to be
[tex]
\sum\limits_{J_z = -1,0,+1} \varepsilon_{\mu}\varepsilon_{\nu}^* = g_{\mu\nu} + \frac{p_{\mu}p_{\nu}}{m^2}
[/tex]
I absolutely don't understand how this spin-sum is evaluated.
What does [itex]\varepsilon_{\mu}\varepsilon_{\nu}^*[/itex] even exactly mean? Is it a scalar product between two of the three above polarization vectors, or is it a "tensor-product" between the components of a single polarization vector which results in a 4x4 matrix and one has finally to sum all such matrices for the three possible values of [itex]J_z[/itex]?
I would really appreciate it if somebody can explain to me what this spin-sum exactly means and how it is evaluated step-by-step.
[tex]\varepsilon_{\mu}(J_z = +1) = (0,-\frac{1}{\sqrt{2}},-\frac{i}{\sqrt{2}},0 )[/tex]
[tex]\varepsilon_{\mu}(J_z = 0) = (\frac{p}{m},0,0, \frac{E}{m})[/tex]
[tex]\varepsilon_{\mu}(J_z = -1) = (0,\frac{1}{\sqrt{2}},-\frac{i}{\sqrt{2}},0 )[/tex]
The so-called spin-sum is the claimed to be
[tex]
\sum\limits_{J_z = -1,0,+1} \varepsilon_{\mu}\varepsilon_{\nu}^* = g_{\mu\nu} + \frac{p_{\mu}p_{\nu}}{m^2}
[/tex]
I absolutely don't understand how this spin-sum is evaluated.
What does [itex]\varepsilon_{\mu}\varepsilon_{\nu}^*[/itex] even exactly mean? Is it a scalar product between two of the three above polarization vectors, or is it a "tensor-product" between the components of a single polarization vector which results in a 4x4 matrix and one has finally to sum all such matrices for the three possible values of [itex]J_z[/itex]?
I would really appreciate it if somebody can explain to me what this spin-sum exactly means and how it is evaluated step-by-step.