# Questions tagged [helicity]

In particle physics, helicity is the projection of the angular momentum onto the direction of momentum. For massless spin-1⁄2 particles, helicity is equivalent to the chirality operator multiplied by $\hbar/2$, so may be used for related chirality questions as well.

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### Prove Spin of a massless particle $S_z=\pm1$

Quote from Introduction to High Energy Physics Edition 4 by Donald H. Perkins chapter 3.3.1 "It can be proved as a consequence of relativistic invariance that for any massless particle of spin $s$, ...
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### Analogue of helicity in higher dimensions and concrete formula

Consider Poincare group $ISO(1,d-1)$ in some dimension $d>4$. There are two Casimirs. Let's look at massless one-particle states: the little group is $ISO(d-2)$, and if we restrict to finite ...
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### Parity of photon helicity state

This question is fundamental enough and I probably should know the answer at this point, but for some reason I am confused. I know that helicity states should go into each other under parity transform ...
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### Can photon helicity change in interactions in QED?

I'm wondering if a photon's helicity can change in QED. For example in Compton scattering could the exiting photon have a different helicity than the incoming photon?
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### Density Matrix for the Relativistic Spin Tensor

In quantum we have a density matrix for the spin states. The density matrix allows us to specify both polarized states, but also various levels of polarization. The relativistic version of the ...
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### Transverse polarizations of a massless spin 1 particle

Physical polarization vectors are transverse, $p\cdot{\epsilon}=0$, where $p$ is the momentum of a photon and $\epsilon$ is a polarization vector. Physical polarization vectors are unchanged under a ...
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