# 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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### Colour-ordering formula of QCD amplitudes (tree-level)

I have been studying colour-ordered amplitudes and spinor helicity formalism for a while. It is now apparent to me that I do not fully understand the 'master' formula which allows us to relate the ...
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### Angular distribution of neutrinos in $e^-u\rightarrow\nu_ed$ scattering

What does the angular distribution of the neutrinos look like in the scattering $e^-u\rightarrow\nu_ed$? The tree-level Feynman diagram looks like this and since all of the fermions couple to a $W$ ...
51 views

### Helicity combinations in $gg\rightarrow t\bar{t}$

So, I am trying to understand what helicity combinations can occur in the outgoing top-antitop pair in the tree-level scattering $gg\rightarrow t\bar{t}$. There are 3 diagrams to consider (see below), ...
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### Confusion about $1/|\vec{p}|$ in helicity operator

The helicity operator is defined as $$h = \frac{1}{|\vec{p}|} \vec{\sigma} \cdot \hat{\vec{p}}$$ One of the first exercises into QED is to check whether this commutes with the Dirac Hamiltonian. ...
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### What is the value of $W_\mu W^\mu$ for massless particles?

What is the value of the quantity $W_\mu W^\mu$ for massless particles where $W^\mu$ is called Pauli-Lubanski vector defined as $W^\mu=\frac{1}{2}\epsilon^{\mu\nu\alpha\beta}P_\nu J_{\alpha\beta}$. ...
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### Difference between left- and right-handed, helicity and chirality

What is the difference? I know there is the (almost) same question What's the difference between helicity and chirality? but when a particle is given as left-handed. Is it helicity or chirality?
### $z$ component of angular momentum under Lorentz transformation for massless particle
This question is related to this Helicity states. Suppose we have $k=[\omega,0,0,\omega]$. In Weinberg's book The Quantum Theory of Fields: Volume I he defines the state $|k,\sigma\rangle$ as an ...