# 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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### Link between photon helicity and polarization of $A^\mu$ electromagnetic potential

From Wigner theorem we know that the irreducible unitary representation of the Poincarè group for massless and spin 1 particle is labelled by the momentum $p_\mu$ and the two possible helicity $+1,-1$ ...
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### 3-Particle Kinematics and Parity of Operators

Recall that if the momentum of scattering amplitudes is taken to be complex and from little group scaling that the 3-particle interaction for massless particles of any spin is given as ...
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### Elastic scattering and conservation of spin

I am trying to understand conservation of spin in QED elastic scattering in these nice notes (VJ Martin, Particle Physics, Spring 2012, University of Edinburgh): https://www2.ph.ed.ac.uk/~vjm/Lectures/...
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### Scalar QED amplitudes with BCFW Recursion Relation

(This question comes from exercise 3.5 of Elvang's and Huang's "Scattering amplitudes in Gauge Theory and Gravity" book. This is not for a class, this is to learn a new technique; albeit I ...
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### Confusion on the helicity formalism

I'm studying group representation theory from a more mathematical point of view but I don't understand the link between helicity formalism and the "classical one". They should be both ...
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### Spin/Helicity-Density Matrices in Particle Physics

I am searching for literature explaining the Spin/Helicity-Density Matrix formalism in order to compute "spin-dependent" matrix elements and cross-sections in particle physics. I would ...
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### Questions on Lorentz generators in Spinor-Helicity formalism

I have read the following PSE posts on Lorentz generators in Spinor-Helicity formalism: Total Angular Momentum Operator in Spinor-Helicity formalism Derivation of conformal generators in spinor ...
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### Questions on Britto-Cachazo-Feng-Witten (BCFW) recursion method

I am reading about amplitude techniques, such as the spinor-helicity formalism: more specifically, the BCFW recursion relations. The lecture notes I am reading are titled as Scattering Amplitudes, and ...
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### Are the massive spinors (left and right Weyl spinors) helicity eigenstates?

For massless spinor, with the help of Dirac equation, I can show that both left-handed and right-handed Weyl spinors are helicity eigenstates with eigenvalues +1 and -1 (respectively). I am unsure ...
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### Spin 1 particle, vector and photon

In the statement: "The rotation of the spin of a photon through 360 degrees brings it back to its originial state, and hence it is a spin 1 particle, in other words its spin is a vector" ...
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### Accounting for photon spin angular momentum in general relativity for two (anti-)parallel photons

General Relativity tells us that the paths of parallel photons propagating in free space should be unaffected by each other, while the paths of anti-parallel photons should bend towards each other. I'...
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### Spinor-helicity variables and gauge transformations of the polarization vector

I am reading Clifford Cheung's 2017 TASI Lectures on Scattering Amplitudes. In section 2, "Crash Course in Kinematics", the spinor-helicity variables are introduced and discussed. In terms ...
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### Spin quantum states and spin observables of photons (spin 1 particles with 0 rest mass)

I was reading Goldbring's article on the implications of $\text{MIP}^\ast=\text{RE}$ which contains a crash course-like overview of (classical, non-relativistic, non-QFT) quantum mechanics. This made ...
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### Charged fermions have both chiralities, with the same mass. Shouldn’t neutrinos, also massive, have both chiralities, though with different masses?

Thanks to answers a previous question, I have realised the difference between helicity, a non-Lorentz-invariant quantity, and the Lorentz invariant chirality. Let me summarise what I understand, ...
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### A thought experiment about neutrinos

I don't understand all the details of Dirac mass, Majorana mass, and many other "deep" notions. I have in mind a very simple thought experiment. Because of neutrino oscillations we know ...
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### Peskin & Schroeder's QFT on page 166

I have two points not clear in Peskin & Schroeder's QFT on page 166. On Figure 5.6, "Since helicity is conserved, a unit of spin angular momentum is converted to orbital angular momentum&...
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### Helicity conservation during nuclear resonance in Goldhaber Experiment

Within the context of the Goldhaber Experiment. If a polarised photon of helicity $\pm 1$, is used to excite a $^{152}Sm$ Samarium nucleus to an excited state $^{152}Sm^*$, that then, by spontaneous ...
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### $A(\phi\phi^*\gamma \gamma)$ with Spinor Helicity Formalism

I am calculating $A_4(\phi\phi^*\gamma\gamma)$ with the spinor helicity formalism (Exercise 2.16). I am following the conventions defined in Elvang's notes Such computation requires three diagrams, ...
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### Confused about helicity suppression in the decay $\pi^- \rightarrow \mu^- \overline{\nu}_\mu$

I know this question has been asked a few times before here in various ways, but I haven't found answers which helped me a lot. For one, the class I am in is not using any of the underlying math, so ...
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### Regarding mass insertion and helicity (?) flip

I am reading Grozin's book "Lectures on QED and QCD: Practical Calculation and Renormalization of One- and Multi-loop Feynman Diagrams" and specifically, I want to understand a paragraph on ...
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I am reading these notes on helicity. One of the definitions (see page 5) that is used is the following (here $J^{a}=\bar{\psi} \gamma^{a} \psi$ is the probability current and \$K_{a}=\bar{\psi} \...