# 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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### When we talk about the helicity of a particle, why are we (apparently) ruling out the cases of the spin being perpendicular to the motion direction?

If I assume the (linear) momentum is in the direction of the X-axis, could I NOT find the spin-up in the positive Y-(or Z-) axis and the spin-down in the negative Y-(or Z-) axis?
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### A question for "Scattering amplitudes for all masses and any spins" : What is the exact two-component expression of the high spin wave function?

Now I'm studying the spinor helicity formalism with several liturature. I could understand roughly how to calculate the $n$-point on-shell amplitudes for massless particles very efficiently and ...
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### How to apply MHV rules to QED?

I've been working lately with scattering amplitudes using spinor helicity formalism. I understand how amplitudes with all positive (or negative) $A_n[\pm\pm\dots\pm]=0$ while working with gluons and ...
1 vote
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### Chirality vs Helicity in Top Quark Decay

I still have not had a good explanation of how a Right Handed Top Quark decays. As I understand it, helicity and chirality are both a part of spin. Does this mean that if either is left handed, the ...
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### Noether Charge in ideal fluid

I'm having some problems with the definition of the action in Euler fluid and in the Bose-Einstein condensate. In particular in this article https://arxiv.org/abs/1708.01526 the author apply the ...
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### Helicity for massless particles

The little group for massless particles is $ISO(2)$, with the following Lie algebra: $$[A,B]=0, \; [J^3,A]=iB, \; [J^3,B]=-iA,$$ where $A,B$ generate translations and $J^3$ generates rotations. To ...
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### Physical meaning of the magnitudes of a spinor helicity variable

I wonder if the magnitude (or its square) of a spinor helicity variable has any physical meaning. Let us assume that the spinor helicity variables are defined as follows: \begin{equation} \lambda^\...
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### Why does a gluon soft factor depend only on the adjacent legs?

The gluon soft factor can be obtained by taking the soft gluon limit in the $n$-point MHV amplitude $$A(1,2,\cdots, n)=\frac{\langle i,j\rangle^4}{\langle 1,2\rangle\cdots\langle n, 1\rangle},$$ to be ...
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### $\omega$ vs $\rho ^0$ meson decay into $e^+e^-$

The decay $\omega \rightarrow e^+e^-$ has a partial width that is around 10 times smaller than the partial width for the decay $\rho \rightarrow e^+e^-$. Why is this the case? The only difference I ...
1 vote
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### Spin sign for antiparticle

I have this problem with the sign of the spinor for the antiparticle. In the chiral basis, a spinor is represented by $\psi =(\psi_{L},\psi_{R}$). Now, we consider a particle with mass = 0, so Dirac's ...
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### How to prove the helicity operator is a Casimir operator for massless particles?

I am reading the classification of irreducible unitary Poincaré group representations. It seems that for massless particles, the momentum operator and the Pauli-Lubanski operator get aligned so the ...
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### Find operator that produce a given amplitude

While I was studying spinor helicity formalism, in many articles, it gives the operators that produce a given amplitude. Is there a generic method of finding operators that produce a given amplitude? ...
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### Chirality/helicity of anti-particles (again)

Questions (very) closely related to this one have been posted dozens of times, but the joint information (including even lecture notes and books) is incredibly contradictory, so I state my specific ...
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### Is an Anti-positron actually only a relativistic effect on the observed helicity of an electron?

I have read quite recently this article here: https://www.quantumdiaries.org/2011/06/19/helicity-chirality-mass-and-the-higgs/ in which it is explained that an Anti-positron is actually a different ...
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